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Vulnerability of mangroves, seagrasses and intertidal flats in the tropical Pacific to climate change

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Photo: Christop he Launay
Chapter 6
Vulnerability of mangroves, seagrasses and intertidal ats
in the tropical Pacic to climate change
Michelle Waycott, Len J McKenzie, Jane E Mellors, Joanna C Ellison,
Marcus T Sheaves, Catherine Collier, Anne-Maree Schwarz, Arthur Webb,
Johanna E Johnson and Claude E Payri
‘One of the major challenges in the Pacic Islands region is adjusting to the responses
of coastal ecosystems to climate change.’ (Gilman et al. 2006)i
i Gilman et al. (2006) Pacic Island Mangroves in a Changing Climate and Rising Sea. United Nations
Environment Programme Regional Seas Reports and Studies 179, Nairobi, Kenya.
Published in: Bell JD, Johnson JE and Hobday AJ (eds) (2011) Vulnerability of Tropical Pacic Fisheries
and Aquaculture to Climate Change. Secretariat of the Pacic Community, Noumea, New Caledonia.
298
Contents Page
6.1 Introduction 299
6.2 The nature of mangroves, seagrasses and intertidal ats
in the tropical Pacic 301
6.2.1 Mangroves 301
6.2.2 Seagrasses 303
6.2.3 Intertidal ats 306
6.3 The role of mangroves, seagrasses and intertidal ats
in supporting sheries in the tropical Pacic 308
6.3.1 Mangroves 308
6.3.2 Seagrasses 311
6.3.3 Intertidal ats 313
6.4 Critical requirements for maintaining mangroves,
seagrasses and intertidal ats 315
6.4.1 Solar radiation 315
6.4.2 Temperature 317
6.4.3 Nutrients and salinity 318
6.4.4 Soils/sediments 320
6.5 Recent variation in mangroves, seagrasses and
intertidal ats linked to climate change 321
6.5.1 Mangroves 321
6.5.2 Seagrasses 323
6.5.3 Intertidal ats 323
6.6 Projected vulnerability of mangroves, seagrasses and
intertidal ats to climate change 323
6.6.1 Mangroves 323
6.6.2 Seagrasses 332
6.6.3 Intertidal ats 343
6.7 Integrated vulnerability assessment 345
6.7.1 Mangroves 345
6.7.2 Seagrasses 346
6.7.3 Intertidal ats 348
6.8 Uncertainty, gaps in knowledge and future research 348
6.9 Management implications and recommendations 350
References 352
299
CHAPTER 6
6.1 Introduction
In addition to the extensive coral reef habitats described in Chapter 5, the shallow
subtidal and intertidal zones around the coasts of Pacic Island countries and
territories (PICTs) often support large areas of mangroves and seagrasses. Intertidal
sand and mud atsii with their associated microalgae and infauna are also common
features of most PICTs.
Due to their varying responses to light, temperature and hydrology, and the capacity
of mangroves and seagrasses for dispersal1–4, these three ecosystems usually form
a mosaic. Typically, mangroves are located along the shore, whereas seagrasses and
intertidal ats can extend long distances away from the shoreline in lagoons and
sheltered bays, and often adjoin coral reefs. In many places, coral reefs buffer waves
coming ashore to create suitably sheltered environments for the establishment of
mangroves and seagrasses5.
Mangroves and seagrasses are of special interest to coastal sheries worldwide
because of the role they play in providing nursery areas for commonly harvested sh
and invertebrates6–9. Although the ecology of sh and invertebrates associated with
mangroves and seagrasses in the tropical Pacic is not well understood compared
with other parts of the world, the connectivity among mangroves, seagrasses,
intertidal ats and coral reefs indicates that mangroves and seagrasses throughout
the region provide a similar function to such habitats elsewhere.
In addition to their roles as nursery areas, mangroves, seagrasses and intertidal ats
provide feeding habitats for many species of adult demersal sh, some of which
reside on reefs during the day and forage over seagrasses and intertidal ats at
night10. Seagrasses and intertidal ats are also permanent habitats for several species
of sea cucumbers, the main group of invertebrates targeted as an export commodity
in the region11, and for a wide range of molluscs gleaned for subsistence. Overall,
the range of coastal sheries resources that depend on mangroves, seagrasses and
intertidal ats is extensive, with many of these species important to the food security
and livelihoods of coastal communities in PICTs (Chapters 1 and 9).
The separate values of each ecosystem are surpassed by the productivity that results
when they are inter-connected7,8,12. In particular, movement of nutrients, detritus,
prey and consumers between habitats can have major effects on the structure and
productivity of food webs, with nutrient and detrital subsidies increasing primary
and secondary productivity both directly and indirectly13. In addition to supporting
sheries, mangroves provide protection against wind and wave energy, and stabilise
shorelines; and both mangroves and seagrasses improve water quality by trapping
sediments, nutrients and other pollutants14–18.
ii Includes intertidal areas of sand and mud above mean low tide level, but does not include
intertidal coral reefs or seagrasses.
300
Ecosystems dominated by mangroves and seagrasses are being eroded in some PICTs
due to their proximity to developments in the coastal zone19,20. For example, increases
in the turbidity of coastal waters and higher rates of sedimentation, resulting from
poor land management in the catchments of high islands, are reducing the area and
health of seagrass habitats21,22. The problem is not unique to the region the range
and intensity of anthropogenic effects on coastal habitats have been increasing
worldwide, reducing the extent and quality of mangroves23,24 and seagrasses20,25.
Climate change is expected to exacerbate anthropogenic impacts on mangroves,
seagrasses and intertidal ats3,26,27. Further losses are expected to occur as a result
of greater heat stress, increased sedimentation and turbidity due to higher rates of
runoff, changes in suitable sites for growth of mangroves and seagrasses due to rising
sea levels, and possibly more physical damage from the combination of sea-level rise
and more severe cyclones and storms.
In this chapter, we assess the vulnerability of the mangrove, seagrass and intertidal
at habitats in the tropical Pacic that support coastal sheries. We do this by
examining the effects that changes to surface climate and the tropical Pacic Ocean
(Chapters 2 and 3) are expected to have on the plants that dene these habitats. This
exposure to change is used in the framework described in Chapter 1 to assess the
vulnerability of the habitats under representative low (B1) and high (A2) emissions
scenarios from the Intergovernmental Panel on Climate Change (IPCC) for 2035 and
210028.
We commence by describing the diversity and distribution of mangrove, seagrass and
intertidal at habitats in the tropical Pacic (25°N–25°S and 130°E–130°W), outlining
the role they play in supporting coastal sheries in the region, and summarising
the critical requirements for establishing and maintaining these habitats. Next, we
summarise the limited information on the observed effects of climate change on
mangroves, seagrasses and intertidal ats, and assess the expected vulnerability of
these habitats to the projected changes in solar radiation, air and sea temperatures,
rainfall, nutrients, cyclones and storms, ocean acidity and sea-level rise. For
mangroves and seagrasses, we integrate these assessments to estimate changes in
area under the various scenarios.
We conclude by identifying the uncertainty associated with these assessments, the
important gaps in knowledge, the research required to ll these gaps, and the key
management measures needed to maintain the important roles that the mangroves,
seagrasses and intertidal ats of the region play in supporting coastal sheries.
301
CHAPTER 6
6.2 The nature of mangroves, seagrasses and intertidal ats in the
tropical Pacic
6.2.1 Mangroves
Mangrove forests occur on sediments associated with low-energy shorelines,
between mean low-tide and high-tide levels. Mangroves have evolved to tolerate
saline sediments and inundation by sea water, with different species displaying a
range of tolerances. This variability in tolerance to saline conditions contributes to
patterns of species distribution across the intertidal zone.
The tropical Pacic has an extraordinary diversity of mangroves 31 of the
70 species recognised globally are found in the region, including ve hybrids.
Twenty-three species occur in Papua New Guinea (PNG), making it the country
with the greatest diversity of mangroves in the world29. The diversity of mangroves
decreases progressively from west to east across the region, with only four species
and one hybrid occurring in Samoa (Table 6.1). In French Polynesia, the single species
of Rhizophora is likely to have been introduced30,31 and has proliferated on all the high
islands of the Society archipelago31. The natural absence of mangroves in the eastern
Pacic is likely to be related to propagule dispersion rather than a lack of suitable
conditions.
The area inhabited by mangroves, relative to total land area, is also exceptional in
some PICTs. It is as high as 12% for the Federated States of Micronesia (FSM), about
10% for Palau and around 1–2% for another six PICTs29 (Table 6.1). Although the
area covered by mangroves in PNG is only ~ 1% of total land area, the 4640 km2 of
mangroves represent > 70% of the mangrove area in the region29.
Mangrove species form ecological assemblages, based on similarities in their
morphology, physiology and reproduction strategies. They occur in highly humid to
extremely arid environments, and on soil types that include clay, peat, sand and coral
rubble32. Mangrove communities do differ markedly from each other, however, due to
the variation in tides, wave exposure, river ows and soils associated with different
locations5,33,34. Mangrove trees create extensive and productive forests where conditions
are optimal, but occur as dwarf and scattered shrubs where they are not. Mangrove
communities on high islands also usually differ from those found on atolls, because of
variation in the availability of fresh water, sediments and nutrients from runoff35.
As a result of local conditions and the potential for arrival of mangrove propagules36,
each PICT has a unique combination of mangrove species. Nevertheless, two species
Bruguiera gymnorhiza and Rhizophora stylosa occur in 15 of the 22 PICTs as a result
of their broad environmental tolerances35.
302
Table 6.1 Number of mangrove and seagrass species recorded from Pacic Island countries
and territories (PICTs), together with the estimated area of mangrove and seagrass habitats.
Mangrove assemblages have been classied into seaward, mid and landward
zones, according to where they occur in relation to tidal position37 (Figure 6.1). The
seaward
zone is the outfacing edge of the mangrove forest, which is fully exposed to
all tides and frequent inundation. The soils in this zone are normally soft mud and
sedimentary in origin. Mangrove species inhabiting the seaward zone usually have
aerial roots that anchor and support the plant. The mid zone is subject to less regular
tidal inuences, with the trees generally being exposed only to inundation during
the spring high tides. Soils are also sedimentary but more compacted than those in
the seaward zone. They usually contain carbon and sometimes have inorganic ne
PICT
Total
land area
(km2)
Mangrove Seagrass
References
Species
(hybrid)
Area
(km2)
%
land Species Area
(km2)
%
land
Melanesia
Fiji 18,272 7(1) 424.6 2.32 6 16.5a0.01 29, 54, 260–263
New Caledonia 19,100 15(3) 205 1.07 11 936 5.0 22, 29, 52, 260, 263–267
PNG 462,243 31(2) 4640 1.00 13 117.2 0.03 29, 268–272
Solomon Islands 27,556 17(2) 525 1.90 10 66.3 0.24 21, 22, 29
Vanuatu 11,880 14(3) 25.2 0.21 11 ?a29, 40, 44, 261, 273, 274
Micronesia
FSM 700 15(1) 85.6 12.23 10 44 6.29 21, 29, 260, 263, 274–277
Guam 541 12 0.7 0.13 4 31 5.73 260, 261, 263, 277–280
Kiribati 690 4 2.6 0.37 2** ?b29, 44, 260 , 281
Marshall Islands 112 5 0.03 0.27 3 ?b29, 44, 260, 274
Nauru 21 2 0.01 0.05 0 0 0 29, 282
CNMI 478 3 0.07 0.01 4 6.7 1.40 29, 44, 260, 274, 283, 284
Palau 494 14(1) 47.1 9.53 11 80 16.19 29, 260, 285–289
Polynesia
American Samoa 197 3 0.5 0.26 4 ?c29, 44, 274, 279, 290
Cook Islands 240 0 0 0 0 0 0 29, 291, 292
French Polynesia 3521 1 ?b? 2 28.7 0.82 29, 265, 293, 294
Niue 259 1 0 0 0 0 0 29
Pitcairn Islands 5 0 0 0 0 0 0 295
Samoa 2935 3 7.5 0.26 5 ?b29, 44, 54, 279
Tokelau 10 0*** 0 0 0 0 0 29, 296
Tonga 699 7 13 1.87 4 ?b29, 260, 263, 290
Tuvalu 26 2 0.4 1.54 1* 0 0 29
Wallis and Futuna 255 2 0.2 0 5 24.3 17.0 29, 46, 47, 297
* Local contacts report no seagrass but Ellison (2009)29 noted the presence of one species; ** based on
observations by P Anderson; *** includes one associate species; a = mapping currently in progress;
b = not mapped; c = seagrass not encountered during September 2002 and May 2003 surveys of Tutuila,
Manua Group, Rose Atoll and Swains Island (source: Analytical Laboratories of Hawaii 2004)284.
303
CHAPTER 6
grain-sizes. The landward zone is generally only inundated during the highest
of spring tides, often receiving fresh water from groundwater or land runoff. It is
dominated by mangrove ‘associates’, i.e. plants such as shrubs, vines, herbs and
epiphytes generally found at the back of mangrove communities. Indeed, the
landward zone is usually a narrow strip of vegetation that may transition to a
terrestrial forest37. Diversication of mangrove species can occur within these three
broad habitat zones, for example, due to salinity gradients38.
Figure 6.1 The three zones typical of mangrove habitats in the tropical Pacic, showing
the differences in mangrove species typical of each zone.
6.2.2 Seagrasses
Fourteen species and one subspecies of seagrass have been reported from the tropical
Pacic (Table 6.1). Like mangroves, the greatest number of seagrasses occurs in PNG
and diversity declines to the east (Table 6.1). Seagrasses are absent or unreported
from the Cook Islands, Nauru, Niue, Pitcairn Islands, Tokelau and Tuvalu. However,
the discontinuity of seagrass in the Cook Islands and Tokelau may be the consequence
of limited surveys because both of these PICTs have deep, sheltered lagoons and low-
energy environments suitable for establishment of these plants.
The area of shallow coastal waters where seagrasses occur is extensive in several
PICTs. For example, seagrasses are an important habitat in much of Micronesia,
where they are equal to 16% of land area in Palau, and 5–6% in FSM and Guam
(Table 6.1). Seagrasses are also important habitats in Wallis and Futuna, and New
Caledonia, where they cover areas equivalent to 17% and 5% of land area, respectively
(
Table 6.1
). The area of seagrass is particularly signicant in New Caledonia, where it
covers > 900 km2. Mapping of seagrass habitats has been conducted by eld surveys
in some PICTs (e.g. Solomon Islands) or by remote sensing in others, e.g. New
Caledonia, Wallis and Futuna, Palau, Guam and Commonwealth of the Northern
304
Mariana Islands (CNMI)39. Unfortunately, some seagrass surveys in the region have
not measured the area of habitat (e.g. Vanuatu)40. Mapping of seagrass is currently
underway in Fiji.
Most seagrasses in the tropical Pacic are found in waters shallower than 10 m.
However, there is great variation in the nature of seagrass habitats across the region,
depending on water clarity, nutrient availability and exposure to wave action21,22,40.
Based on the inuence of these factors, ve main categories of seagrass habitat have
been recognised21,22,40 (Figure 6.2). These categories are described below.
Figure 6.2 The ve main habitats where seagrasses occur in the tropical Pacic, together
with the factors limiting growth of seagrasses in each habitat.
6.2.2.1 Bays and lagoons
Calm clear waters and a range of stable sandy substrates in bays, and in lagoons
behind reefs or in atolls, enable a diverse range of seagrass species to establish dense
meadows in both subtidal and intertidal areas. Halodule uninervis often grows well in
such locations, from the intertidal zone to depths of 30 m. In some places, it is patchy
and intermixed with other seagrass species (e.g. Halophila spp.). Another dominant
species in these locations is Syringodium isoetifolium, which often occurs in shallow,
subtidal areas (1–6 m deep) of lagoons behind barrier reefs. Syringodium is more
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CHAPTER 6
tolerant of oxidised substrata than other species and is usually the rst species to
re-establish after a disturbance41. In French Polynesia, Halophila ovalis is a common
species on the shallow sandy substrata of Tuamotu atolls, where this seagrass usually
mixes with macroalgae, forming dense mats.
6.2.2.2 Island fringing reefs
Wide fringing reefs, found where wave action is slight to moderate, provide subtidal
and intertidal areas with stable sediments that support diverse and dense seagrass
meadows. Even so, the seagrasses inhabiting fringing reefs may differ widely
because of variation in exposure to solar irradiation, nutrient availability, wave
action and the associated movement of sediments3,42. Where runoff from high islands
delivers nutrients, seagrasses on the landward edge of fringing reefs can be highly
productive. Pooling of water at low tide on the landward side of fringing reefs allows
the seagrasses there to attain greater leaf heights, and prevents desiccation of the
plants. In contrast, intertidal areas expose seagrasses to damage, particularly when
low tides occur during the day in summer.
Variation in environmental conditions across fringing reefs often results in a
succession of seagrass communities. In particular, seagrasses in the genera Thalassia
and Cymodocea often dominate inshore intertidal areas because they tolerate a
moderate level of disturbance43; Halophila ovalis, Halodule spp. and Syringodium
isoetifolium occur in shallow water on fringing reef platforms44 and Thalassodendron
ciliatum, which has strong woody rhizomes and roots, attaches to rock and coral
rubble banks at depths of 6–8 m on the seaward margin of fringing reefs45. In
Wallis and Futuna, the seagrass meadows on the fringing reef nearest the beach are
dominated by Halodule spp., whereas Halophila ovalis is most common in the middle of
the reef and S. isoetifolium on the seaward edge46,47.
6.2.2.3 Barrier and patch reefs
Physical disturbance from waves and the movement of sediment usually prevent
seagrasses from growing on the windward sides of barrier reefs. However, they
occur on the leeward side of islands, or where the back-reef is large21,22,40. Thalassia
hemprichii is common on barrier reefs because it is able to tolerate shallow sediments,
high temperatures and strong currents. In Solomon Islands, Enhalus acoroides, which
has robust rhizomes and roots, is common on barrier reefs with strong currents in
Malaita. In contrast, seagrasses are not a common feature on the barrier reef in New
Caledonia.
In some very rare situations, Halodule uninervis can form scattered patches
mixed with Halophila ovalis. Conversely, Cymodocea spp., Halodule uninervis and
T. hemprichii form dense beds on lagoonal patch reefs48. Some species of seagrass
(e.g. Halophila decipiens, Cymodocea rotundata and T. hemprichii) also grow on shallow
306
subtidal patch reefs21. These environments have suitable conditions for growth
because there is limited disturbance from wave action, protection from currents by
the reef crest, and availability of coarse carbonate sediments.
6.2.2.4 Estuaries
Seagrasses grow in the lower reaches of estuaries on the high islands of Melanesia.
However, growth is limited in these extreme environments by uctuations in light
and salinity, and scouring by currents49. Seagrass meadows in estuaries are generally
dominated by structurally large species, such as Enhalus acoroides, which are tolerant
of high temperatures and low salinity50, and can withstand partial burial51. Seagrasses
in estuaries have more microalgal and macroalgal epiphytes than seagrasses in other
habitats21.
6.2.2.5 Deep water
Little is known about the few species of seagrass that occur in deeper water in the
tropical Pacic. Halophila decipiens is commonly reported from depths of 60 m in New
Caledonia52, interesting given that this species commonly occupies coral reef habitats
also. This species also occurs in French Polynesia, where it grows on the sandy bottom
of channels and embayments, and the outer reef slope53. It has also been reported from
depths of ~ 40 m in Solomon Islands22, and 10 to 25 m at the Great Sea Reef, Fiji54. In New
Caledonia, a closely related species, Halophila capricorni, is also commonly observed on
the sandy bottom of channels near coral reefs at depths of 20 to 30 m52.
6.2.3 Intertidal ats
In many PICTs, a proportion of the coastal zone between the active sandy beach
margin and mean low tide comprises sandy or muddy intertidal ats. These habitats
are also often associated with the margins of lagoons on atolls and high islands.
Mangroves frequently border the landward margin of intertidal ats, whereas
seagrasses and/or coral reef often occur at the seaward edge. The transition from
intertidal sand or mud ats to mangroves or seagrasses is dictated by comparative
vertical elevation in relation to mean sea level.
There is limited understanding of the role of intertidal ats, and the associated food
webs, in supporting the sh and invertebrates that contribute to subsistence and
small-scale commercial coastal sheries in PICTs (Chapter 9). However, intertidal
locations and shallow marine ecosystems (< 1 m deep) in other parts of the world
yield some of the highest rates of primary production through growth of the benthic
microalgae (BMA) community55 and, globally, the area of intertidal ats is about
three times greater than that of mangrove forests56. Although the areas of intertidal
ats have yet to be mapped for the vast majority of the main islands in the region,
it is clear that they can comprise signicant areas, and support important sheries
(Box 6.1).
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CHAPTER 6
Box 6.1 Importance of intertidal ats
The extent, and ecological and socio-economic roles of intertidal ats in the
tropical Pacic are still poorly understood. However, elsewhere these habitats
(1) play a signicant role in nutrient cycling and primary production in shallow
coastal ecosystems; (2) support high densities and large harvests of burrowing
(infaunal) invertebrates, e.g. arc shells Anadara spp.; and (3) help mediate pollution
(eutrophication) through denitrication by enhanced bacterial processes within the
surface layers of sediment and burrows of infauna.
In the 16 equatorial atolls of the Gilbert’s Group, Kiribati, and in several other Pacic
Island countries and territories (PICTs), the contribution of intertidal ats to the
subsistence of coastal communities is frequently overlooked. These habitats can be
highly productive, and communities often glean molluscs, polychaetes, crustaceans
and echinoderms from these areas at low tide for household food supplies. People
also earn income from harvesting edible species from intertidal ats and selling them
fresh or preserved to urban markets, or as export commodities in the case of sea
cucumbers.
Greater attention must be given to estimating the areas of intertidal ats in PICTs,
their ecological function, and their contributions to food security and livelihoods,
especially as these habitats are highly vulnerable to being lost as a result of projected
sea-level rise. The signicance of intertidal ats to coastal sheries in some PICTs is
illustrated by Tarawa Atoll in Kiribati, where (1) the land area of ~ 31 km2 at high
tide, increases to 52 km2 during low spring tides, expanding the total ‘land’ area by
~ 170%; and (2) total annual harvests of arc shells have been estimated to be as high as
1800 tonnes per year (Chapter 9).
Photo: Art hur Webb
Women gleaning from an intertidal at in Kiribati
308
Although intertidal ats lack conspicuous vegetation, and are often considered to
be ‘unvegetated’ or ‘bare’, they frequently support a rich diversity and abundance of
BMA communities, comprising photosynthetic microalgae (diatoms, dinoagellates
and cyanobacteria) and reducing and oxidising bacteria57,58. The high rates of benthic
primary productivity by these BMA communities in turn support a rich array of
benthic epifauna (animals living on the sediment surface), and infauna (burrowing
organisms). Transient sh species feed on this fauna during high tide, and birds
frequent these habitats at low tide to prey on the wide range of food found there.
6.3 The role of mangroves, seagrasses and intertidal ats in
supporting sheries in the tropical Pacic
The mosaic of mangrove, seagrass and intertidal at habitats plays an important role
in supporting the demersal sh and invertebrates that contribute to the subsistence
and commercial coastal sheries of the region59,60 (Chapter 9). For example, several
important demersal sh species associated with coral reefs use this habitat mosaic
as a nursery area59,61,62, and for feeding when they are adults63. There is also the
possibility that juvenile sh may use mangrove and seagrass habitats in sequence as
they develop, before residing permanently on coral reefs. As adults, several species of
demersal sh venture from reefs into adjacent mangrove or seagrass areas to forage
at night, depending on which habitat is nearby. Mangroves, seagrasses and intertidal
ats are also the primary habitats of important invertebrate species, such as sea
cucumbers, crabs and molluscs64,65 (Chapter 9).
Below, we describe the specic roles played by mangroves, seagrasses and intertidal
ats as habitats for the sh and invertebrates that underpin coastal sheries in the
tropical Pacic, and as shing areas.
6.3.1 Mangroves
A large number of sh and invertebrate species harvested in the tropical Pacic by
subsistence and commercial coastal sheries are associated with mangroves during
their life cycle (Tables 6.2 and 6.3). These species contribute to three of the four
categories of coastal sheries described in Chapter 9: demersal sh, invertebrates
targeted for export commodities, and invertebrates gleaned from intertidal and
shallow subtidal habitats for subsistence (Figure 6.3). In PNG, mangroves are also the
location of recreational sheries for barramundi and black bass66.
Harvesting of sh and invertebrates from mangrove habitats is divided into activities
that capture (1) resident species, like arc shells Anadara spp., oysters Crassostrea spp.,
mangrove crabs Scylla spp. and sea cucumbers Holothuria scabra65–70, or (2) species
of sh and shrimp that use mangroves temporarily during high tide, e.g. banana
prawns Fenneropenaeus merguiensis71.
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CHAPTER 6
Mangroves also contribute to coastal sheries in two other ways. First, they provide
important nursery (feeding and shelter) areas for juvenile sh and invertebrates27,72–77.
Second, they are intermittent feeding areas for adult sh normally harvested from
coral reefs or other habitats70,78. The nursery value of mangroves stems from the refuge
their dense roots provide79, the food resources found there63, and reduced ow rates80.
The larger species of sh that feed within inundated mangrove habitats include
carnivores, such as snappers (Lutjanidae), trevallies (Carangidae) and barramundi
Lates calcarifer, which feed on the juvenile sh sheltering there78; and herbivores-
detritivores, such as mullet (Mugilidae)27, herring (Clupeidae)81 and shrimp27,82.
Table 6.2 The number of species associated with mangrove and seagrass habitats, as
juveniles and/or adults, for families of sh caught by coastal sheries in Pacic Island
countries and territories (PICTs). All information is preliminary due to the lack of
extensive sampling of sh in mangrove and seagrass habitats in many PICTs.
* Families of sh mostly with only one species harvested from either mangrove or seagrass
habitats include Chandidae (milksh), Chirocentridae (wolf herring), Cynoglossidae (tongue
sole), Drepanidae (sicklesh), Elopidae (giant herring), Lacteriidae (snapper), Megalopidae
(tarpon), Monacanthidae (leatherjacket), Scatophagidae (buttersh), Scombridae (Spanish
mackerel), Soleidae (sole), Sparidae (bream) and Toxotidae (archer sh).
Family* Common name Habitat
Mangrove Seagrass Both
Acanthuridae Surgeonsh 3 3 3
Atherinidae Hardy head 5 3 3
Belonidae Long tom 3 3 3
Bothidae Flounders 2 2 2
Carangidae Trevallies 4 2 2
Clupeidae Herring 7 8 7
Dasyatidae Rays 5 5 5
Engraulidae Anchovies 4 4 4
Gerreidae Silver biddies 5 5 5
Haemulidae Grunts 5 5 5
Hemiramphidae Halfbeaks 5 4 4
Labridae Wrasses 3 3 3
Leiognathidae Ponysh 7 4 4
Lethrinidae Emperors 5 5 5
Lutjanidae Snappers 7 4 4
Polynemidae Threadns 3 3 3
Mugillidae Mullet 9 6 6
Mullidae Red mullet 7 5 5
Nemipteridae Threadn bream 2 2 2
Platycephalidae Flatheads 4 2 2
Scaridae Parrotsh 2 2 2
Serranidae Groupers 8 3 3
Siganidae Rabbitsh 4 4 4
Sillaginidae Whiting 4 4 4
Sphyraenidae Barracuda 3 3 3
Synodontidae Lizardsh 3 2 2
Teraponidae Grunters 3 3 3
310
The value of any given area of mangroves for sheries is linked, however, to the
availability of adjacent habitats. Because many parts of the mangrove habitat drain
completely at low tide, sh and shrimp can use these areas only when they are
inundated, and must rely on nearby subtidal habitats for shelter at other stages of
the tidal cycle83. The most commonly used subtidal adjacent habitats are drainage
channels within and beside mangroves, which often contain fallen timber from
mangrove trees (snags) and areas of seagrass84. Thus, mangrove-based food webs are
linked to the attributes of nearby areas (Figure 6.4), and any assessment of the value of
mangroves to sheries species needs to consider the availability of adjacent habitats.
Figure 6.3 The subsistence and commercial coastal shing activities that occur in
mangrove habitats in the tropical Pacic at (a) high tide, and (b) low tide.
a)
b)
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CHAPTER 6
Table 6.3 The number of species associated with mangrove and seagrass habitats, as
juveniles and/or adults, for the main groups of invertebrates caught by coastal sheries in
Pacic Island countries and territories (PICTs). All information is preliminary due to the lack
of extensive sampling of invertebrates in mangrove and seagrass habitats in many PICTs.
Blank spaces indicate that the invertebrate group does not commonly occur in the habitat.
Invertebrate group Common name Habitat
Mangrove Seagrass Both
Holothuridae Sea cucumber 3
Portunidae Crab 5 3 3
Penaeidae Shrimp 6 3
Palinuridae Spiny lobster 1
Thalassinoidea Mud lobster 1
Paguroidea Hermit crab 2 3
Stomatopoda Mantis shrimp 1 2
Cephalopda Octopus 3
Echinoidea Sea urchin 3
Bivalvia Arc shell* 4 4 2
Gastropoda Whelk* 3 2 1
Sipunculida Peanut worm 2 2 2
* Indicates that several species are included under this broad common name.
6.3.2 Seagrasses
Seagrasses are also a vital part of the mosaic of habitats that support many of the
demersal sh and invertebrates harvested by coastal sheries (Chapter 9) (Figure 6.5).
Seagrasses are thought to play a particularly important role in the coastal sheries
of New Caledonia, Palau, FSM, Guam and Wallis and Futuna, where relatively large
areas of this habitat occur (Table 6.1), and in the large lagoons of Solomon Islands
where rabbitsh (Siganus spp.) support subsistence and commercial sheries85,86.
Although the physical structure of seagrasses is not as robust as mangrove roots, the
leaves of many seagrasses (e.g. Thalassia spp., Thalassodendron spp., Cymodocea spp.,
and Syringodium isoetifolium) are relatively tall and at high leaf densities can provide
juvenile sh and invertebrates with much protection from predation87. Many seagrass
meadows also remain submerged at low tide, which means that juvenile sh, shrimp
and crabs can shelter there throughout the tidal cycle, feeding on zooplankton
delivered by currents, and the epiphytes and epifauna on seagrass leaves. The
seagrass leaves themselves also provide food for large numbers of species88. For these
reasons, seagrass beds provide nursery areas for a wide range of sh and invertebrates
that live on coral reefs or in other habitats as adults89–91. Tropical species of seagrass
vary greatly in their structural complexity and therefore do not all provide the same
degree of shelter. Nevertheless, even seagrasses with comparatively low leaf heights
and densities can support high numbers of juvenile sh and invertebrates87.
312
Figure 6.4 The mosaic of mangroves, seagrasses and coral reefs that occur in the
coastal waters of many Pacic Island countries and territories, showing (a) the
ontogenic movements of sh and invertebrate species among habitats as they grow; and
(b) the foraging movements of adult sh and invertebrates from reefs to mangroves and
seagrasses at night to feed. Note that diagrams depict high tide.
Many sh also visit seagrass meadows as adults to forage for food. The juvenile sh
and invertebrates associated with seagrass attract a range of predatory sh from
nearby coral reefs at night to feed. These species include emperors (Lethrinidae)21 and
a)
b)
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CHAPTER 6
snappers92. Herbivorous and omnivorous sh and invertebrates, such as the barred
halfbeak Hemiramphus far, scribbled rabbitsh Siganus spinus and goatsh Barberinus
sp. have been observed within seagrass beds in Solomon Islands85. Spiny lobsters
(Palurinidae) also feed frequently on seagrass epiphytes and seagrass leaves93.
In addition to the sh and invertebrate species that use seagrass meadows as nurseries
or intermittent feeding areas, a wide range of valuable invertebrates live permanently
in seagrass habitats, especially sea cucumbers11,65,85, sea urchins, other bivalves and
octopus85,93–95. These species are harvested during low tide96,97, and are often gleaned
by women and children for household food (Chapter 9).
6.3.3 Intertidal ats
The species of sh and invertebrates caught over intertidal ats and the methods
used to catch them differ depending on geographic location, the tide and the season.
The capture of demersal sh using gill nets, seine nets and hand lines occurs during
high tide, whereas the gleaning of a range of molluscs, crustaceans, sea cucumbers
and polychaetes occurs at low tide98–100 (Figure 6.6). Furthermore, the harvest of some
species (e.g. mantis shrimp) relies on rst identifying and marking burrows at low
tide, and then returning on the incoming tide to capture the shrimp as it emerges to
hunt. The responses of sh and invertebrates to phases of the moon and seasons are
understood by coastal communities and used to harvest some species over monthly
or annual cycles70,74.
The diversity and abundance of sh of species occurring on intertidal ats at high
tide depends on the proximity of mangroves, seagrasses and/or coral reefs, and
estuaries. This is not the case for infaunal invertebrates, which have limited mobility.
For burrowing molluscs, crustaceans and polychaete worms, the intertidal sediments
provide shelter from predation by birds, and desiccation at low tide. However, some
species emerge at low tide, e.g. ddler crabs, which feed on the BMA community
that thrives in these shallow, sunlit habitats101. Carnivorous sh, such as bonesh
Albula spp., rays (Dasyatidae) and trevallies (Carangidae) use the ats during high
tide to prey on infauna. Herbivorous/detritivorous mullet and milksh Chanos chanos
also visit intertidal ats at high tide to feed directly on the BMA community63,72.
Our preliminary understanding of intertidal ats in the tropical Pacic, like similar
habitats in other parts of the world, indicates that their assemblages of BMA and
infauna directly and indirectly support a wide variety of species102.
The diverse range of shing and gleaning activities that occur on intertidal ats make
important contributions to food security and livelihoods in many PICTs. For example,
in Kiribati, harvests of arc shells from intertidal ats at low tide have exceeded
3000 tonnes per year in the western Gilberts Group (Chapter 9). These bivalves comprise
a regular and preferred part of the daily diet and can be a particularly important food
for people in urban areas who cannot regularly catch or purchase fresh sh or other
foods.
314
Figure 6.5 Subsistence and commercial coastal shing activities in seagrass habitats in
the tropical Pacic at (a) high tide, and (b) low tide.
a)
b)
315
CHAPTER 6
Polychaete worms are also harvested and dried in rural areas of Kiribati and shipped
to urban South Tarawa. These harvests provide an important source of income for
people in rural areas. Recreational sheries for bonesh (Albula spp.) in Cook Islands,
Kiribati and New Caledonia also centre on productive intertidal sand ats.
6.4 Critical requirements for maintaining mangroves, seagrasses
and intertidal ats
6.4.1 Solar radiation
Like all plants, mangroves and seagrasses need light for photosynthesis. However,
light is rarely limiting for mangroves within the tropical and subtropical habitats
where they occur their light requirements for maximal photosynthesis are
considerably less than the amount of light available on cloudless days. On the other
hand, excessive irradiance can raise leaf temperatures and predispose mangroves
to photoinhibition103,104. Elevated levels of ultraviolet radiation in the tropics can
also inhibit growth of mangroves by disrupting protein synthesis and depressing
photosynthesis via stomatal closure.
Mangroves have adapted to their environment by developing ‘sun’ and ‘shade’ leaves.
Shade leaves are larger and thinner than sun leaves, with a higher volume-to-surface
ratio and fewer stomata103,105. Also, leaves developing in high light intensity have
more adaptations to guard against desiccation than those developing in low light
intensity103,106. In general, there are two broad groups of mangroves (1) species which
are somewhat shade-tolerant both as seedlings and adults (e.g. Rhizophora stylosa and
Bruguiera parviora), and (2) those that are shade intolerant (e.g. B. gymnorhiza and
Laguncularia racemosa).
In contrast, light often limits the distribution, species composition, biomass and
growth rates of seagrasses107–110. These owering plants have high minimum light
requirements compared with other marine primary producers111 because they
(1) have a high respiratory demand to support a large non-photosynthetic biomass
of roots and rhizomes, (2) can only use a restricted spectral range of light, and
(3) must regularly oxygenate their root zone to compensate for anoxic sediments. The
high minimum light requirement restricts seagrasses to shallow coastal areas where
sunlight can penetrate. There are, however, differences in light requirements among
species. For example, Halophila spp. generally have a low minimum light requirement
and can grow at greater depths than other species112.
The BMA communities which inhabit the upper few centimetres of intertidal
ats typically have high turnover rates and are composed of a diverse range of
heterotrophic and autotrophic species which include photosynthetic algae57,58.
316
Figure 6.6 The subsistence and commercial coastal shing activities that occur over
intertidal sand and mud ats in the tropical Pacic at (a) high tide, and (b) low tide.
The greater proportion of the BMA population is usually conned to the upper
10 mm or so of the sediment surface113–115 and primary productivity within the BMA
community, derived from autotrophic species, such as cyanobacteria and diatoms, is
the main source of in situ organic matter production in this habitat. Intertidal BMA
species may also migrate vertically within the upper few centimetres of sediment.
a)
b)
317
CHAPTER 6
This may be a response to avoid excessive heat, light, desiccation and/or predation114.
Benthic microalgal assemblages can also adapt physiologically to ambient light
conditions by varying the amount of chlorophyll they contain; this photo-adaptation
response can occur in, for example, turbid estuary conditions116. Given the exposed
nature of intertidal BMA communities at low tide, and shallow water depths during
high tide, light is not generally considered to be a signicant limiting factor to BMA
productivity in the intertidal zone.
6.4.2 Temperature
Temperature is a critical factor for the survival and growth of plants because the
enzymes involved in most cellular processes operate most efciently within specic
temperature ranges. Mangroves need warm temperatures for photosynthesis,
respiration and energy processes involving salt regulation and extrusion, water
uptake and growth. As a result, the northern and southern limits of most mangrove
species coincide with the 20°C winter isotherm117. The heat tolerance of mangroves
is less well studied, although the optimum temperature for photosynthesis of
mangroves is < 35°C, and little or no photosynthesis occurs at 40°C118,119. Also, the
rates of assimilation and water evaporation from plant pores are more efcient at leaf
temperatures of 25–35°C, and decline sharply with increases above 35°C120,121.
Tropical seagrasses require water temperatures of 25–35°C, otherwise the energy
created from photosynthesis may not exceed the energy used for respiration122. Where
water temperature rises to 35–40°C, photosynthesis declines due to the breakdown
of photosynthetic enzymes123 and the carbon balance of the plants goes into decit
because respiration continues to increase. Under such circumstances, growth rates of
some tropical seagrasses can decrease because they have a limited capacity to store
carbohydrates. These temperature thresholds vary among species122 and seasons124.
Thus, tropical seagrasses usually occur only in intertidal habitats where maximum
summer temperatures are < 40°C122,123. They can survive higher temperatures for short
periods, but prolonged exposure leads to the ‘burning’ of leaves or plant mortality.
The BMA communities of intertidal ats often live in extreme physiochemical
conditions, especially during periods when low tide corresponds to midday sun
exposure (high temperatures and UV exposure) or heavy rainfall (large changes
in ambient salinity). Studies of temperature-related control over BMA community
composition and productivity are limited mainly to temperate climates125–127, where
temperature appears also to be linked to changes in light128. Increases in temperature
within the optimum range in these temperate environments marginally increase
microalgal photosynthetic rates and nutrient uptake129. However, the effects of higher
average temperatures on BMA communities within the environments of the tropical
Pacic are unknown.
318
6.4.3 Nutrients and salinity
Growth and production of mangroves also depends on adequate supplies of
macronutrients and micronutrients. Key plant macronutrients are: nitrogen, phosphorus,
sulphur, magnesium, calcium and potassium. Micronutrients required for plant
growth include: iron, manganese, copper, zinc, nickel, boron chloride, sodium, silicon,
cobalt, selenium and aluminium. Sulphur, magnesium, potassium, boron, chloride
and sodium are rarely limiting because they occur naturally in sea water. The critical
need for nitrogen and phosphorus by mangroves has been demonstrated repeatedly
through nutrient addition experiments130,131, with most studies reporting limitation of
one or both nutrients. However, the importance of nitrogen and phosphorus varies
with location and position in the mangrove forest131,132.
Mangroves have adaptations that allow them to tolerate high levels of salinity –
membranes in cells at the root surface exclude most of the salt. The salt that does
enter the plant is either excreted via the leaves or stored in leaves until they die and
are shed, depending on the mangrove species. Because of the limited availability of
fresh water in the soils of the intertidal zone, mangrove plants have also developed
ways of limiting the amount of water they lose through their leaves. They can restrict
the opening of their stomata, and have the ability to vary the orientation of their
leaves to reduce evaporation during the harsh midday sun133.
The most important macronutrients for seagrasses are carbon, nitrogen and
phosphorus. Seagrasses do not grow at their full capacity unless these macronutrients
are available in sufcient quantities134,135. The carbonate sediments found in reef
environments typically bind phosphorus, reducing its availability for seagrasses,
whereas sediments derived from the land are limited in nitrogen136. This general
pattern can vary, however, depending on local nutrient inputs and sediment
properties137. The levels of nutrients in some sediments in the tropical Pacic,
e.g. those at Dravuni in Fiji, are among the lowest recorded for seagrass ecosystems138.
Where nutrients are added to seagrass meadows that are normally nutrient-limited,
the plants generally show an increase in growth and biomass139,140. However, excessive
nutrients can lead to proliferation of phytoplankton, macroalgae or algal epiphytes on
seagrass leaves and stems, reducing the amount of light reaching the seagrass141,142.
Eutrophication of shallow estuaries and lagoons can also lead to the proliferation of
bloom-forming ‘ephemeral’ macroalgae, which can shade and eventually displace
seagrasses143. The impacts of nutrients on seagrasses in the tropical Pacic are usually
localised to small bays, areas near human settlements21,22, or areas adjacent to activities
such as shrimp aquaculture, which have damaged some mangroves or seagrasses
in New Caledonia144. Seagrasses generally grow best at salinities of 35 practical
salinity units (PSU), although seagrasses have been observed growing in salinities of
4 to 65 PSU, with some species being more tolerant of extremely low salinity145.
319
CHAPTER 6
Benthic microalgae communities at the sediment surface obtain nutrients from
the water column and interstitial water below the illuminated upper sediment
layer146. However, due to tight coupling between mineralising bacteria and BMA
production within the upper sediment layers, nutrients from the water column
can play a relatively unimportant role147. The BMA community also intercepts and
assimilates nutrients which may otherwise be uxed from sediments to the overlying
water column and may limit pelagic primary productivity in this way58,148. Indeed,
signicant concentrations of ammonium (NH4+), nitrate (NO3-), silicate (SiO2) and
phosphate (PO43-) may be intercepted and prevented from entering the overlaying
water column by BMA communities149–151.
Benthic microalgae communities have relatively ready access to nutrients in
sediments, which results in an inconsistent response to addition of nutrients to the
water column. Nutrient loading has been shown to either increase BMA biomass
and productivity125,152 or have little effect150,153. Also, ‘blooms’ of BMA that occur in
response to nutrient loading, seasonal change and removal of BMA grazers154,155 are
controlled by the two-dimensional nature of the BMA habitat. The ready supply of
organic matter associated with productive BMA communities, and the mineralisation
of this material by bacteria, are also associated with important processes such as
denitrication150,151,156. Just as cyanobacteria are associated with nitrogen xation57,
denitrifying bacteria in the lower anoxic layer of sediments can contribute to the
substratum being a net sink for dissolved inorganic nitrogen and its removal via
denitrication as nitrogen gas151,157. Net rates of denitrication are enhanced by the
presence of burrowing infauna157,158. Thus, it is likely that the typically bioturbated
intertidal ats of the tropical Pacic may play an important mediating role in nutrient
processing and productivity.
Women collecting arc shells ('palourde') in New Caledonia Phot o: Johann Bell
320
In nutrient-limited systems typical of the more pristine coastal habitats of the tropical
Pacic, the productivity of BMA is likely to be a more important contributor to primary
production than phytoplankton (Chapter 4). In fact, BMA communities can be a major
source of organic carbon input into shallow coastal ecosystems via the assimilation
of sediment-born nutrients57,113,156. This in turn supports benthic epifauna and infauna
populations, which contribute directly and indirectly to subsistence and commercial
coastal sheries and other biogeochemical processes that mediate or enhance the
release of nutrients157–159. The vital role of BMA can change, however, in shallow areas
subject to eutrophication, sustained high turbidity and/or intense physical disturbance.
Under such conditions, BMA productivity may be greatly depressed and phytoplankton
productivity in the upper water column can become more dominant160.
6.4.4 Soils/sediments
Mangroves grow in various combinations of sand, silt and clay, which are often rich in
organic matter (detritus). Sandy soils are porous and facilitate water percolation and
aeration during low tide, while clays are less well aerated. Differences in soil types can
have an effect on the distribution of mangrove genera, e.g. Avicennia and Sonneratia
thrive in sandy areas, Rhizophora are found in peat soils and contribute to the formation
of peat161, and Bruguiera
favour heavy clays containing little organic matter162. The subsoils
in mangrove ecosystems are typically waterlogged, have little aeration and a heavy
load of organic material decomposing at a slow rate. The oxygen content of only
the rst few millimetres of soil is replenished by the circulation of tidal water and
exchange with the atmosphere. Below that, the organic load and ne particle size
result in anoxic conditions. A supply of oxygen to the roots is vital for plant growth
and nutrient uptake134. Consequently, mangroves in many locations rely on internal
transport of gases to meet their oxygen requirements. The metabolic costs involved in
this process reduce the rate of plant growth163.
Sediment characteristics are also important in determining the growth, germination,
survival and distribution of seagrasses164–166. In particular, sediment texture affects
levels of nutrients and diffusion of oxygen167. Sandy sediments have lower fertility and
diffuse oxygen more readily167,168. Conversely, ner-textured sediments usually have
higher fertility and greater levels of anoxia because pore water has less interaction
with the overlying water column169. The effects of anoxia on seagrass are complex
anaerobic conditions can stimulate germination in some species170 but can also result
in elevated sulphide levels, which inhibit production of leaf biomass in mature
plants171,172. Sulphide is also toxic to seedlings of some species173. Overall, however, there
is still insufcient information to identify the ‘ideal’ sediment types for seagrass169.
Low-energy intertidal environments (i.e. lagoon sands and mud ats) generally yield
the greatest BMA production55. Comparatively larger BMA communities are found in
sandy sediment rather than ne silt and mud due to limited penetration of light into
the ner sediments (e.g. light penetration into quartz sands may be more than twice
as deep as into mud)174. Sandy sediments also tend to allow enhanced movement of
321
CHAPTER 6
interstitial water and therefore dissolved gases (e.g. oxygen and carbon dioxide) and
nutrients. This provides a deeper habitat with favourable conditions for the BMA
community58.
Benthic microalgae communities can also inuence the physical properties of
sediments by enhancing the stability of the sediment surface via the secretion of
mucous threads that bind sediment grains. In some cases, this results in formation
of continuous mats over the sediment surface58,114,175. These mats greatly reduce
resuspension of sediments due to wave action and water movement, and nutrient ux
due to interstitial sediment ushing. This effectively regulates nutrient release into
the water column from comparatively nutrient-rich deeper sediment zones58,156,176.
6.5 Recent variation in mangroves, seagrasses and intertidal ats
linked to climate change
6.5.1 Mangroves
Mangroves are sensitive to even minor transitions in coastal conditions, such as
altered drainage patterns, saltwater intrusion, accretion or erosion in response
to changes in sea level35. The response of mangroves to these changes can be seen
through variations in the composition and relative abundance of plant species within
the mangrove habitat23,177,178. Although the responses may be gradual, particularly
in undisturbed systems, the alterations in coverage and composition of species can
be used to assess the effects of climate change and other environmental impacts
on mangrove habitats. This can be demonstrated through palaeo-environmental
reconstruction178, geographic information systems (GIS)179, or ecosystem monitoring180.
Examples of recently observed changes in mangrove ecosystems in the tropical
Pacic associated with sea-level rise include (1) gradual retreat of mangrove zones
in southern PNG in response to rates of sea-level rise similar to those projected
globally178,181 (Figure 6.7); and (2) GIS analysis of shoreline change over four decades
in three mangrove areas in American Samoa, where there was landward movement
of seaward margins of 25, 64, and 72 mm per year during sea-level rise of ~ 2 mm
per year179. A study from the Caribbean also demonstrates the response of mangrove
ecosystems to changes in sea level the largest area of mangroves in Bermuda has
been reduced by 26% due to retreat of the seaward edge, owing to inundation stress
caused by sea-level rise of 2.8 mm per year182,183. In parts of Micronesia, mangrove
sediment accretion rates are also not keeping pace with current rates of sea-level
rise184,185.
The success of owering and ‘seed set’ of three species of mangroves in Fiji has been
inuenced by rainfall patterns186,187. Higher success was found on the west coast of
Viti Levu relative to the dry coast, and in normal years relative to drought years.
322
In general, detection of changes in mangrove cover and health in the tropical Pacic
has been limited and difcult because adequate baseline data and monitoring are
usually lacking29. A shortage of meteorological, hydrological, hydro-geological and
water quality data in many PICTs188 compounds the problem. In addition, the limited
data on the physical, chemical and biological processes in catchments, including
soil erosion, loss of biodiversity and land clearing189 (Chapter 7), make it difcult
to separate the effects of coastal development and land use practices on mangrove
habitats from any effects of climate change.
Figure 6.7 Replacement of the mangrove Bruguiera by Rhizophora within the past
3000 years in the Tipoeka Estuary, Papua New Guinea (based on sedimentary rock strata
and pollen data), demonstrating gradual landward retreat of high island mangroves
during sea-level rise rates of 0.7 mm per year (source: Ellison 2008)181.
20 40 60 80
Percentage of pollen count
Sediment depth (cm)
20 40 60 80
0
100
200
300
400
500
600
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CHAPTER 6
6.5.2 Seagrasses
Estimates of recent changes in seagrass habitats across the tropical Pacic are
difcult to make because (1) seagrass meadows are known to uctuate seasonally,
and change from year to year190,191; and (2) maps of the distribution of seagrass
area and biomass are either limited or imprecise. Isolated instances of seagrass
burn-off (blackened dying leaves) have been observed on fringing reefs in Fiji.
Although this burning is caused by exposure to elevated temperatures122, there are
insufcient data to conclude that these events are occurring at increasing rates. In
other parts of the world, temperature-related seagrass losses have been recorded
(e.g. southern Australia), and high temperatures have caused large-scale diebacks of
Amphibolis antarctica and Zostera muellerii192.
6.5.3 Intertidal ats
There has been little research on the impacts of climate change on intertidal at
habitats and their associated BMA communities and infauna in the tropical Pacic.
The most likely impacts are from rising sea levels. However, the gradual rates of sea-
level rise since the beginning of the industrial era, and the effects of atmospheric
pressure and El Niño-Southern Oscillation (ENSO) on sea level (Chapter 3), make
any effects difcult to detect, particularly against the background of high natural
variability. The task is made all the more difcult because where intertidal ats are
close to urban areas any effects of climate change would be confounded by direct
human impacts, such as pollution, extraction of sand and coral for construction,
coastal development, over-harvesting of infauna and changes in water quality.
6.6 Projected vulnerability of mangroves, seagrasses and intertidal
ats to climate change
6.6.1 Mangroves
6.6.1.1 Solar radiation
Exposure and sensitivity
Mangrove habitats in much of the tropical Pacic are expected to be exposed to
reductions in light as a result of the increase in the percentage of cloudy days due to
intensication of the hydrological cycle (Chapter 2). Conversely, in New Caledonia,
projected decreases in rainfall of 5–10% by 2035 and 5–20% by 2100, and in cloudy
days, are expected to increase solar radiation.
Because the requirements of mangroves for light are lower than the average levels
of solar radiation in the region, mangroves are not expected to be sensitive to the
projected changes in levels of solar radiation caused by a more intense hydrological
324
cycle. During periods of high solar radiation, however, the absorption of light
translates into heat energy, which can be expected to exacerbate the effects of higher
temperature on water loss (Section 6.6.1.2).
Potential impact and adaptive capacity
The potential impact of altered solar radiation on mangroves is expected to be low,
except where mangroves have high exposure to solar radiation combined with
limited freshwater supply. These conditions occur, for example, on the leeward side
of high islands such as Viti Levu and Vanua Levu in Fiji, and on the west coast of
New Caledonia where total rainfall is projected to decline (Chapter 2). If slow rates
of sea-level rise were to occur they may enhance the adaptive capacity of mangroves
to increased exposure to light by increasing tidal ushing and freshwater supply.
However, such slow rates are not expected and thus limited adaptive capacity is
expected for mangroves which are exposed to high levels of solar radiation.
Vulnerability
Relative to other factors, the vulnerability of mangroves to projected changes in solar
radiation is low, except in areas of combined high radiation and restricted runoff and
tidal inundation, where vulnerability is expected to be moderate.
6.6.1.2 Temperature
Exposure and sensitivity
Mangroves in the tropical Pacic will be exposed to projected increases in air
temperature and sea surface temperature (SST) of 0.5–1.0°C in 2035 for the B1
and A2 emissions scenarios, 1.0–1.5°C for B1 in 2100 and 2.5–3.0°C for A2 in 2100
(Chapters 2 and 3).
The sensitivity of mangroves to increased surface air temperature and SST is not well
known193 but is likely to be moderate. For example, Rhizophora mangle develops more
silt roots per unit area when subjected to a 5°C increase in water temperature and
produces more but signicantly smaller leaves194. Also, young seedlings of a species
of Avicennia are killed by water temperatures between 39°C and 40°C, although
established seedlings and trees are not affected16,180. On the other hand, mangroves
growing near coastal power stations show little or no visible effects from warmer
efuent water195.
Potential impact and adaptive capacity
Mangroves have a high degree of tolerance to heat stress compared with other
plants196. Thus, even for the A2 scenario in 2100, the projected increases in air
temperature are not expected to have substantial effects on the growth and survival
of mangroves because the projected increases are below those known to cause
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CHAPTER 6
detrimental effects. Respiration (CO2 efux) from plants and microbial communities
in sediments approximately doubles with every 10°C increase in temperature, so that
on hot days there would be reduced net carbon gain, increased methane emissions
and decreases in soil carbon storage26. In addition, mangroves have a range of
adaptations, such as reducing the apertures of their stomata, to cope with water loss
induced by increased evaporation under heat stress103,180.
Vulnerability
Mangroves are expected to have very low vulnerability to the projected increases
in air temperature and SST. However, an indirect vulnerability to increases in SST
may result from the projected decreases in coral cover due to thermal bleaching
(Chapter 5), which are expected to reduce sediment supply to mangroves on low
islands, and increase exposure to wave action.
6.6.1.3 Rainfall
Exposure and sensitivity
In equatorial areas of the Pacic, rainfall is expected to increase by 5–15% for the B1
emissions scenario and 5–20% for the A2 scenario in 2035, and by 10–20% in 2100
for both emissions scenarios (Chapter 2). In the subtropics, rainfall is projected to
decrease by 5–10% for B1 in 2035, and by 10–20% for A2 in 2035 and for both scenarios
in 2100 (Chapter 2). Extremes in wet and dry periods are likely to become more
extreme, and droughts associated with the projected changes in rainfall are expected
to be more intense due to the increase in temperature (Chapter 2).
Photo: Nicol as Petit
Mangrove habitats can be important shing areas
326
Mangroves are expected to be moderately sensitive to these changes because soil
salinity along the intertidal gradient is affected by the interaction of tidal inundation
and rainfall. At locations with low rainfall and high evaporation, soil salinity in the
upper intertidal gradient may be high, even though inundation is infrequent. On the
other hand, where rainfall greatly exceeds evaporation, for example, in Kosrae, FSM197,
salinity levels do not build up in the soil, and soil salinity is negatively correlated
with distance from the seaward edge of the mangrove habitat.
Potential impact and adaptive capacity
The effects of lowered salinity associated with increases in rainfall are likely to benet
mangrove ecosystems in equatorial areas, but are expected to be negative in the
subtropics where decreases in rainfall (increases in salinity) are projected. Reduced
runoff from catchments in New Caledonia may decrease the delivery of sediment
to mangrove habitats near estuaries, making it more difcult for the trees at the
seaward margins to accumulate sediment and adapt to rising sea levels35. Increased
drought conditions may also reduce the owering and fruiting of mangroves186,187,
and perhaps increase the areas of upper intertidal salt ats currently found in the
drier areas of the region, such as the leeward side of Viti Levu in Fiji.
Depending on environmental conditions, mangroves can minimise water loss
and maximise growth by using water more efciently and reducing transpiration
rates. Such physiological plasticity is one reason why mangroves are so successful
across the intertidal seascape and these attributes may assist them to adapt to drier
conditions. Too much fresh water also poses problems for mangroves. In stagnant
ooded soils, roots of many mangroves develop a very thin, slightly oxidised zone
that can effectively isolate the actively growing root area198. Seedlings without well-
developed aerial roots would suffer more in this situation than mature trees.
Vulnerability
Mangroves are expected to have low to moderate vulnerability to the projected
changes in rainfall, and subsequently salinity, under both scenarios in 2035, with
some benets to plant growth possible from increasing rainfall in equatorial areas.
However, as rainfall changes are magnied over time, the vulnerability of mangroves
will increase to moderate in 2100 under both scenarios, particularly in areas of the
Pacic that experience declining rainfall.
6.6.1.4 Nutrients
Exposure and sensitivity
The projected changes in rainfall outlined above are expected to alter runoff patterns
and the delivery of nutrients to mangrove habitats. Future changes in nutrient supply
are hard to quantify because they will be related to the intensity of rainfall. However,
increases in nutrients derived from runoff are expected in equatorial areas of the
Pacic, and decreases in New Caledonia.
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CHAPTER 6
Nutrient enrichment enhances vertical accretion and surface elevation of mangrove
forests through increased deposition of roots161. Where nutrients are limited,
the responses of mangroves are complex; they differ across different types of
mangrove forests or locations, depending on the availability of the various nutrients
required132,133. For example, Rhizophora mangle in Belize is limited to different degrees
by nitrogen and phosphorus, depending on the zone in which it occurs131,199. Below-
ground decomposition is generally enhanced by additional phosphorus but not
additional nitrogen131. In contrast, both nitrogen and phosphorus are limiting for
mangroves in Florida, USA133.
Potential impact and adaptive capacity
In equatorial areas, the addition of nitrogen and phosphorus is likely to increase
plant productivity by altering both tree growth and nutrient dynamics, with the
magnitude and pattern of response differing for different nutrients131,132. In general,
increased nutrients may benet mangroves, or assist them to adapt to rising sea
levels161,200. But changes in nutrient delivery, when coupled with low rainfall, have
the potential to affect mangroves negatively. For example, projected decreases in
rainfall (e.g. New Caledonia) may be expected to increase mangrove mortality where
nitrogen concentrations increase201. Ultimately, community composition could be
affected, with different mangrove species surviving at different rates, depending on
their requirements for nitrogen and phosphorous131,133.
Because mangroves have large nutrient and carbon stores in soils and plant
biomass202,203, small changes in nutrients alone are not likely to have signicant effects.
However, when a decrease in nutrients is coupled with increases in temperature and
atmospheric CO2 (and associated increases in respiration), negative effects on plant
tissue balance may occur204 (Section 6.6.1.6).
Photo: Gary Bell
Mangrove roots provide shelter for sh at high tide
328
The adaptive capacity of mangroves to changes in nutrient delivery will mostly be
at the community level, with different species dominating under different nutrient
conditions, and community composition shifting accordingly. This will have
implications for the diversity and structure of mangrove habitats204, and the services
they provide to sh and invertebrate species harvested by coastal sheries.
Vulnerability
The effects of the projected increases in nutrient delivery on mangroves around high
islands in the equatorial Pacic are likely to be positive. In contrast, mangroves in
New Caledonia are expected to be negatively affected by the projected decreases
in availability of nutrients. The vulnerability of mangroves in New Caledonia is
assessed as low, however, due to their inherent adaptive capacity.
6.6.1.5 Cyclones and storms
Exposure and sensitivity
Although global climate models do not project an increase in the frequency of
cyclones in the tropical Pacic, there is the possibility that cyclones and storms will
become more intense within the cyclone belt over the remainder of this century. In
particular, wind speeds associated with cyclones may increase by 1–8% for every 1°C
rise in SST (Chapter 2).
Mangroves are sensitive to strong winds associated with cyclones and storms, which
damage foliage, desiccate plant tissues, and increase evaporation rates and salinity
stress35. The landward margin of mangroves is particularly prone to high evaporative
loses and drying-out of the substrate. Increased wave surge during cyclones erodes
sediments in the seaward mangrove zone and reduces the stability of plants normally
provided by their root systems64,80. On the positive side, stronger winds may facilitate
pollination of species such as Rhizophora and Excocaria, and the dispersal of seeds.
Potential impact and adaptive capacity
Under prolonged and severe wind conditions, evaporative losses may result in
die-back of mangroves. Stronger wave surges are also likely to remove mangroves
from the seaward edge of mangrove habitats. While the logs from fallen trees may
provide some shelter for juvenile sh if washed into subtidal areas, losses in primary
productivity can be expected to exceed such benets in many places. The movement
of large, woody debris in mangrove areas during high tide can also disturb
establishment of seedlings.
After a cyclone, there is usually a narrow zone of damage to mangroves along the
coast due to storm surge, and complete defoliation in the path of the storm. Mangrove
species have different tolerances to cyclone damage205. Rhizophoraceae have low
tolerance and cannot resprout from dormant buds, whereas species of Avicennia can
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CHAPTER 6
resprout. Mortality of mangroves as a result of storms has led to collapse of peat soils
and changed hydrological conditions206. In general, mangroves grow new leaves
after cyclones and storms unless there is structural damage to the trees or burial of
the roots by sediments. Over time, recruitment of seedlings occurs from adjacent
undamaged areas, and the mangrove habitat is re-established. This natural adaptive
capacity can be enhanced and accelerated by replanting programmes.
Vulnerability
Mangrove habitats in the tropical Pacic are considered to have moderate
vulnerability to the effects of more intense cyclones. Damage is expected to occur
during these high-energy events, but the trees should eventually recover from the
effects of wind and waves, prolonged inundation and sediment deposition, where the
physical conditions required for growth and survival are restored.
6.6.1.6 Carbon dioxide
Exposure and sensitivity
For the B1 and A2 emissions scenarios, atmospheric concentrations of CO2 are
projected to be ~ 400 ppm in 2035. By 2100, CO2 levels are expected to be 450–500 ppm
for B1, and 750–800 ppm for A2207. The projected levels of CO2 are also expected
to increase the acidity of the ocean, and reduce the availability of carbonate ions
(Chapter 3).
The few studies on the impacts of elevated CO2 on mangroves suggest that primary
production of mangroves is likely to be enhanced under future climate change
scenarios. In situations of increased moisture stress, enhanced CO2 may also partially
reduce the negative effects of reduced humidity and rainfall208. Increased levels of
CO2 may also change the patterns of species dominance and accelerate mangrove
encroachment into adjacent inland brackish and freshwater environments. However,
when increases in CO2 are combined with higher temperature and nutrient levels,
there may be negative effects on plant tissue balance (Section 6.6.1.4).
Potential impact and adaptive capacity
The projected increases in atmospheric CO2 are expected to increase productivity of
mangroves, provided that salinity and humidity are also conducive to tree growth.
The increased acidication of the ocean is not likely to affect mangrove habitats
greatly, although the process by which dissolved calcium from dead shells makes
some brackish waters alkaline may be weakened as acidication increases. Even if
soil acidity increases, however, mangroves are not expected to be affected adversely,
because many mangrove soils are neutral to slightly acidic due to sulphur-reducing
bacteria and the presence of acidic clays162. In Malaysia, mangroves occur in very
acidic brackish waters, probably due to the aeration of soil sulphates, forming
sulphuric acid.
330
A common plant adaptation to elevated CO2 concentrations is decreased nitrogen
investment in leaves and a concomitant increase in the carbon:nitrogen ratio of plant
tissues209. If mangroves respond in this way, the changes in plant tissue balance will
have knock on effects for food webs210, and on nutrient cycling211.
An indirect impact of increased ocean acidity on mangrove systems could be
reduction in the supply of carbonate sediment, expected to result from reduced rates
of calcication by corals (Chapter 5). This may reduce the ability of mangroves on low
islands to adapt to sea-level rise.
Vulnerability
Mangroves are unlikely to suffer negative effects as a result of increased atmospheric
CO2 alone. Rather, they are expected to grow faster and become carbon sinks in
some places. There may also be increased allocation to below-ground biomass with
elevated CO2, resulting in greater gains in soil surface elevation and stability under
sea-level rise212. In some locations, synergies with increased temperature and altered
nutrient delivery may result in negative effects on plant tissue balance. In such places,
mangroves are likely to have a very low to low vulnerability to elevated CO2.
6.6.1.7 Sea level
Exposure and sensitivity
The conservative projections for sea-level rise made in the IPCC Fourth Assessment
Report (IPCC-AR4) of ~ 10 cm for the B1 and A2 emissions scenarios in 2035,
~ 20–40 cm for B1 and ~ 20–50 cm for A2 in 2100, have now been increased
substantially. More recent estimates are 20–30 cm for the B1 and A2 scenarios in 2035,
70–110 cm for B1 and 90–140 cm for A2 in 2100 (Chapter 3).
Mangroves grow between mean sea level and mean high water, and the zonation
of mangrove species (Figure 6.1) is determined by inundation frequency controlled
by the tides. If the tidal conditions under which mangroves grow are altered, the
growth and survival of the trees are affected. In experiments to simulate the effects
of inundation due to sea-level rise on the growth of Rhizophora mangle, for example,
seedlings maintained under conditions where an increase of 16 cm was imposed on
normal tidal water levels were 10–20% smaller than control plants after 2.5 years213.
Potential impact and adaptive capacity
The projected rise in sea level could potentially have a powerful effect on mangroves.
However, where mangroves can continue to accumulate sediments at appropriate
rates, the effects are likely to be less severe. The capacity of mangrove forests to
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CHAPTER 6
resist sea-level rise is likely to depend on the source of sediment, and the rate of
sedimentation, which in turn is inuenced by rainfall, tidal amplitude, coastal
currents and wave energy214. Biogenic processes, particularly root growth rates, will
also be important in the response of mangroves to sea-level rise161.
Sedimentation is expected to be slower in areas of natural subsidence, such as
southern PNG, American Samoa and western Viti Levu in Fiji178,179,215. Mangroves
on low islands may be able to compensate for low rates of sea-level rise through
accumulation of peat161,182. Most continental and high island mangroves are expected
to adapt if the rate of sediment deposition exceeds the rate of sea-level rise. However,
various surface and subsurface processes, such as sediment accretion and erosion,
biotic contributions, below-ground primary production, sediment compaction,
uctuations in water-table levels and pore water storage, make sedimentation rates
alone a poor indicator of mangrove responses to rising sea level216,217.
The potential impact of sea-level rise on mangroves will be greatly reduced in those
locations where they can migrate landward217. The scope for migration will depend
on the rates of sea-level rise and accumulation of sediments, and changes in elevation.
Historical records show mangrove die-back under accelerated rates of sea-level rise,
followed by re-establishment as sea level falls (Figure 6.8). Landward migration
will, however, be constrained in many locations by barriers such as coastal roads
and settlements, and where steep terrain occurs behind mangroves. In addition, the
projected acceleration in the rate of sea-level rise after 2050207 is expected to make it
difcult for mangroves to re-establish and reach reproductive maturity before their
intertidal elevation envelope is reduced again. PNG, Solomon Islands and FSM have
freshwater swamp forest or marsh on the landward margin of mangroves that could
become mangrove habitat with rising sea level.
Thus, establishment of mangroves in new landward areas is only likely where (1) the
topography is suitable for colonisation, (2) the rate of sea-level rise is compatible with
the life cycles of mangrove species, (3) the hydrology and sediment composition is
suitable, and (4) there is limited competition with non-mangrove species214,217.
Vulnerability
The vulnerability of mangroves to projected sea-level rise is high for both scenarios in
2035, particularly in locations where the coastline is subsiding and sedimentation rates
are low. Vulnerability is expected to be very high for both B1 and A2 scenarios in 2100
where landward migration is blocked by infrastructure, where there is intensive land
use and steep gradients, and as the magnitude of sea-level rise increases later in the
century.
332
Figure 6.8 Sedimentary evidence of the extent of mangroves at Folaha, Tongatapu,
Tonga, 7000–5500 years ago when forests growing 1.5–2.5 m below present sea level were
exposed to accelerated sea-level rise (1.2 mm per year). The mangroves died back to create
a lagoon, ultimately re-establishing after a fall in sea level181.
6.6.2 Seagrasses
6.6.2.1 Solar radiation
Exposure and sensitivity
Seagrass habitats in the tropical Pacic are expected to be exposed to reductions in
light as a result of climate change. The projected increases in rainfall (Section 6.6.1.3)
(Chapter 2), are likely to reduce the availability of light by < 1–15% in 2035 and 5–20%
in 2100 due to (1) increased turbidity of coastal waters from higher levels of runoff
from high islands (Chapter 7); (2) greater growth of phytoplankton and epiphytic
333
CHAPTER 6
algae from the associated nutrients; and (3) a possible increase in the percentage
of cloudy days due to intensication of the hydrological cycle (Chapter 2). No
reduction in availability of light is expected for the large areas of seagrass habitat
in New Caledonia due to the projected decreases in rainfall of 5–10% by 2035 and
5–20% by 2100 (Chapter 2). Indeed, increases in the number of cloud-free days are
likely to occur there in winter.
The seagrasses found in the tropical Pacic have varying tolerances to low levels of
light, and grow at different depths (Figure 6.9). These species are sensitive to reduced
levels of light because the resulting decreases in photosynthesis affect growth rates. In
extreme cases where carbon reserves are depleted and respiration demand outstrips
photosynthesis, plants will die111,142,218–223. Light limitation, caused by suspended
sediment and excess nutrients, has a major impact on seagrass meadows218,224.
Figure 6.9 Estimated light requirements and maximum depth limit of various seagrass
species in the tropical Pacic (source: Collier and Waycott 2009)4.
The mechanisms that seagrasses use to recover from periods of reduced light are
species-specic43,225–227 and vary due to differences in the morphological plasticity,
storage products, life-form and growth rates of species109. In general, morphologically
large and slow-growing species, such as Thalassia spp., tolerate prolonged periods
40
30
20
10
0
0 5 10 20 30 40 50
Maximum colonisation depth (m)
Light requirement (% surface irradiance)
334
of low light but are slow to recover if severely affected. In contrast, small rapidly-
growing species, such as Halophila spp., cannot tolerate extended periods of low light
because of limited storage reserves. They can recover from such impacts quickly if
conditions improve, however, by regenerating through seed production and rhizome
extension221.
The response of seagrasses to reduced light is rapid. When seagrasses from the
Pacic were exposed to low light levels (< 14% incident light), the rate of leaf
extension changed within 7 days, and signicant losses of leaves per shoot occurred
after 14 days221,228,229. After 46 days, shoot density was reduced and complete loss of
shoots was predicted after 100 days.
Seagrasses may also be affected by over-exposure to UV irradiance230. High levels
of UV reduce production of chlorophyll a and enhance production of anthocyanins
(vacuolar pigments), causing ‘reddening’ of plant leaf tissues230.
Potential impact and adaptive capacity
Changes in solar radiation are expected to have profound effects on seagrasses in
the tropical Pacic, ranging from changes in the relative abundance and species
composition of species, including loss of large, slow-growing species where exposure
to low light levels are severe and prolonged, to changes in leaf colour where exposure
to UV increases. Signicant losses to the area of seagrass meadows are expected to
occur where light availability is reduced for long periods (~ 100 days)221. Possibly up
to 20% of seagrass area in the region could be lost by 2100 due to light reduction
alone. Such losses are expected to occur mainly in locations with signicantly higher
rainfall, where the resulting turbid conditions persist for months.
Seagrasses are able to respond to shorter-term (days to weeks) reductions in light
through a range of morphological and physiological adjustments2. When the factors
limiting light are removed, and if the seagrasses have not completely drained their
reserves, they can recover from vegetative fragments left in the meadows. If whole
meadows have been lost, then recovery can only occur through recruitment of
seedlings. Seagrass communities comprising small species, such as Halophila spp.
and Halodule spp., have a greater capacity for recovery, because they produce copious
quantities of seed and have rapid colonisation rates due to their growth form4,43.
Vulnerability
The seagrasses expected to be most vulnerable to changes in light conditions are
those that occur in estuaries or in coastal habitats subject to runoff. Vulnerability
is expected to be moderate in 2035 and 2100 for most locations, increasing to
high around islands with large, steep catchments, where runoff remains in bays
and lagoons for long periods. Complete loss of seagrass is expected to occur if
turbidity and light reduction persist at below the minimum light requirements for
periods > 100 days221,229.
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CHAPTER 6
6.6.2.2 Temperature
Exposure and sensitivity
Seagrasses in the tropical Pacic are projected to be exposed to increases in SST in the
range of 0.5–1.0°C in 2035 for the B1 and A2 emissions scenarios, 1.0–1.5°C for B1 and
2.5–3.0°C for A2 in 2100 (Chapters 2 and 3).
Seagrasses are likely to be highly sensitive to increases in SST, whether they occur
as short-term ‘spikes’ in maximum temperature over periods of hours, or as chronic
exposures for weeks or months, because in many locations seagrasses are already
growing at their maximum temperature tolerance122,229. Short-term exposure to
temperatures > 40°C causes death of seagrass leaves. Such effects have been recorded
regularly for the tropical seagrass meadows of the region231. However, there is a wide
range of responses to short-term increases in temperature > 40°C among seagrasses,
with death of leaves occurring more rapidly for the smaller species122,229 (Figure 6.10).
Figure 6.10 Thresholds for survival of seagrass species in the tropical Pacic under
elevated sea surface temperatures (SST) and increasing exposure. Species of Halophila are
the most sensitive to high SST, and Cymodocea rotundata and Syrigodium isoetifolium are the
most tolerant (source: Campbell et al. 2006, Collier unpublished data)122. Note, however,
that all species shown here can co-exist throughout the region and may have similar long-
term (> 30 days) temperature thresholds.
Chronic elevated SST of up to +3°C results in increased respiratory demand and loss
of seagrasses when respiration outstrips photosynthesis229. Once again, responses
are likely to be species-specic, although data on the effects of chronic temperature
stress for tropical seagrasses are limited.
45
40
35
0
0 1 hour 6 hours 24 hours 30 days
Time exposed
Temperature (ºC)
336
Potential impact and adaptive capacity
The projected increases in SST are expected to cause changes in the species
composition, relative abundance and distribution of seagrasses in the tropical Pacic.
Short-term temperature ‘spikes’ are likely to reduce biomass through ‘burn off’,
whereas overall increases in SST are expected to drive the more chronic changes in
species composition (structural complexity) and distribution229. The fact that the roots
and rhizomes of seagrasses are buried in sediments, offers some protection against
the impacts of short-term changes in SST. However, as seagrasses possess high light
requirements111, their ability to adapt to longer-term increases in SST will be limited
by their overall respiration demand229. Thus, the impact of increasing SST will depend
on light availability, with interactions between elevated temperatures and reduced
light levels resulting in greater potential impacts. Where seagrasses are not stressed
by light, temperature may become the primary driver for responses by seagrasses.
However, because seagrasses are typically light-limited, light levels are expected to
continue to dominate their responses to changing environmental conditions.
Vulnerability
Many of the seagrass meadows in the region are expected to have moderate to high
vulnerability to increases in SST. Shallow intertidal seagrass meadows are likely to
be at the greatest risk, particularly where the less robust Halophila spp. and Halodule
spp. dominate. Where seagrasses are already experiencing lower light levels,
meadows will have high vulnerability to increases in SST because their relatively
high respiration demands are expected to exceed their capacity for gaining carbon
through photosynthesis.
6.6.2.3 Rainfall
Exposure and sensitivity
In the equatorial areas of the Pacic, rainfall is projected to increase by 5–15% for
the B1 emissions scenario and 5–20% for the A2 scenario in 2035, and by 10–20% in
2100 for both emissions scenarios (Chapter 2). In the subtropics, rainfall is expected
to decrease by 5–10% for B1 in 2035 and by 10–20% for A2 in 2035 and both scenarios
in 2100 (Chapter 2). Extremes in wet and dry periods are also projected to become
more extreme.
In addition to affecting light, greater runoff from higher rainfall is expected to reduce
salinity and increase the transfer of sediments, nutrients and toxic chemicals from
catchments to seagrass meadows. Strong reductions in salinity inhibit the growth
of seagrasses145. However, the effects of salinity are usually localised, being more
signicant in bays and lagoons where the residence times of water are in the order of
weeks to months.
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CHAPTER 6
Seagrasses are sensitive to the deposition of sediments because physical burial
prevents their ability to grow21,22,49,219,222,232. Modest additions of sediments can benet
seagrass habitats through provision of new substrate and the addition of nutrients.
However, where deposition rates are greater than the ability of seagrass to grow
through the new sediments, plants will die. Movement of sediments can also cause
scouring of seagrass habitats3, with damage being proportional to sediment loads
and the volume of runoff. Scouring of seagrasses has been a problem in Pohnpei233.
Chemical pollutants such as diuron, known to be poisonous to seagrasses, have been
documented to occur in tropical seagrass sediments234. Where chemical pollutants
are present, the effects are generally proportional to sediment loads and the volume
of runoff. The effects of variation in nutrient loads on seagrasses are discussed in
Section 6.6.2.4.
Potential impact and adaptive capacity
The potential effects of changes in rainfall on seagrass habitats are complex, but
expected to be signicant in several PICTs. The greatest impacts are likely to occur
around high islands in the tropics, where runoff and sediment loads are projected
to increase in the future. Intertidal seagrass habitats close to land are likely to be
affected more severely than subtidal meadows because they are directly exposed
to both rainfall and runoff. Particularly signicant impacts are expected where soil
erosion associated with coastal agriculture (e.g. palm oil plantations), land clearing
(e.g. logging and mining) and mine tailing discharge occurs (Chapter 7).
The capacity for seagrass habitats to adapt to reduced salinity will be correlated to the
species that occur in each region. Some species, such as H. ovalis, have broader salinity
tolerances235 and are likely to become more prevalent in lower salinity environments.
Limited adaptive capacity is expected among species that do not tolerate freshwater
ushes, although research is still needed to identify the thresholds and responses
of species. Similarly, seagrasses are unlikely to have much adaptive capacity to
pollution, which is expected to have a cumulative effect on the plants. Low chronic
loads of some toxins have been reported to have a positive effect on plant growth.
However, further research is needed to identify specic plant responses, in particular
the relationship between toxicant loads and seedling germination and growth3.
Vulnerability
The large seagrass habitats in nearshore bays and lagoons around high islands in the
tropics are expected to have a moderate to high vulnerability to reductions in salinity
and increases in sediments and toxic chemicals resulting from increased rainfall and
runoff. Seagrasses in New Caledonia are expected to have low vulnerability to future
patterns of rainfall, providing that future management can ensure that runoff from
mining activities does not damage the plants.
338
6.6.2.4 Nutrients
Exposure and sensitivity
The projected changes in rainfall described immediately above are also expected
to change the availability of nutrients in seagrass habitats because of altered runoff
patterns. As outlined in Section 6.6.1.4, changes in supply of nutrients are hard to
quantify, but increases in equatorial areas, and decreases in New Caledonia, are
expected.
In general, seagrass meadows throughout the tropical Pacic occur in reef-associated,
carbonate-dominated sediments and are phosphorous limited (Sections 6.2.2
and 6.4.3). Delivery of additional phosphorus, nitrogen and other micronutrients
through increases in runoff is expected to enhance seagrass growth. However,
elevated concentrations of nutrients in the water column are also expected to result
in increases in epiphytic algae on seagrass leaves236. These algae block light, retarding
the growth of seagrasses111. They also increase the resistance of leaves to water
movement and can be expected to increase the loss of leaves during storm surge.
Potential impact and adaptive capacity
The height and density of seagrass leaves, and the area of seagrass, could potentially
increase under the inuence of more nutrients where nutrients are limiting130,139,140.
However, where other factors limit seagrass growth, the potential for seagrass
habitats to benet from increased nutrients will not be realised. For example, in
Photo: Andre S eale
A tropical seagrass habitat
339
CHAPTER 6
areas where increased runoff reduces salinity and/or water clarity in coastal waters
for extended periods (i.e. in bays and ‘ponded’ lagoons with high residence times),
e.g. parts of Marovo and Roviana Lagoons in Solomon Islands, seagrass growth is
unlikely to increase. The inhibiting effects of turbidity are expected to be common in
those catchments where agriculture, forestry and mining have not been managed to
minimise runoff (Chapter 7). The potential effects of increased epiphytic algae may
be reduced where herbivorous invertebrates and sh are common. This may rarely
occur, however, because increased nutrient loads are likely to be associated with
areas under active development and shing pressure can also be expected to be more
intense in such places (Chapter 9).
The seagrass habitats of New Caledonia are not expected to benet from increased
nutrients because of projected reductions in rainfall. However, the possibility of more
intense rainfall events and cyclones means that local areas may periodically receive
excessive nutrients (and sediments) due to increased rates of removal of drier topsoil
and reduced catchment vegetation when these events do occur (Chapter 7).
In locations where nutrient concentrations are relatively low, and where light is not
limiting, seagrasses have the capacity to absorb increased nutrient levels and increase
their biomass130. Higher nutrient loads may also increase nutrient concentrations in
tissues140, to the point where the plants are unable to use or store any more nutrients
and where other factors become limiting. When nutrient loads exceed the ability of
seagrasses to use them given the available light, blooms of epiphytic algae occur,
further reducing light availability111.
Vulnerability
Modest increases in availability of nutrients is expected to have a small positive effect
on seagrass habitats in the tropical Pacic. Where levels of runoff and nutrients are high,
the potential benets will be over-ridden by the adverse effects of low salinities, reduced
light due to turbidity or algal blooms and, in poorly managed catchments, the effects of
chemical pollutants. Such problems are expected to be more pronounced under the A2
scenario in 2100, when seagrasses are likely to have low to moderate vulnerability to the
combined impacts of altered nutrients, salinity and turbidity.
6.6.2.5 Cyclones and storms
Exposure and sensitivity
As outlined in Section 6.6.1.5, cyclones and storms may possibly become more intense
within the cyclone belt over the remainder of this century (Chapter 2). In addition to
increasing sediment loads and nutrient levels, more intense cyclones and storms are
expected to increase the power of waves affecting coastal habitats (Chapter 3).
340
Wave surge strips leaves from seagrasses and often uproots the subsurface rhizomes,
removing the plants from large areas of the intertidal and shallow subtidal zones3.
Reductions in light caused by greater turbidity following cyclones can also be
expected to affect seagrasses219.
Succession in species composition of seagrasses is expected to occur in those areas
where cyclones remove plants4,237. Small species (e.g. Halophila ovalis and Halodule
uninervis) would be expected to dominate initially and then be replaced gradually
by the larger, climax species (e.g. Cymodocea serrulata, Thalassia hemprichii and Enhalus
acoroides). Note, however, that this succession does not usually proceed to a fully stable
community in locations where other factors, such as reduced light, limit the colonisation
by structurally large species. Such moderate levels of disturbance generally result in
seagrass communities being dominated by smaller species of seagrass4.
Potential impact and adaptive capacity
Severe storms can devastate seagrass habitats through the combined effects of
physical disturbance, reductions in light and salinity, and movement of sediments.
Such impacts are expected to be greatest in shallow, subtidal and intertidal areas
because they receive the full force of wave energy (Chapter 3). However, the degree
of change to seagrass habitats due to the more moderate effects of cyclones and
storms depends on the species composition of the meadow. Small seagrasses, such
as Halophila spp. or Halodule uninervis, are likely to suffer more damage than larger
species with rhizomes buried deeper into the sediment, such as T. hemprichii.
Vulnerability
Intertidal and shallow subtidal seagrasses are expected to be highly vulnerable to
any increase in cyclone intensity. The effects of wave surge on seagrass habitats in
the path of a cyclone are likely to be devastating, except for seagrasses growing in
relatively deep water (Section 6.2.2.5). In addition, scouring by mobile sediments
associated with high energy water movements would signicantly affect seagrass
meadows. Small species of seagrass are expected to be more vulnerable than large
species in areas where the physical effects of cyclones and storms diminish away
from the trajectory of the storm. However, because these species also have the
capacity to recover rapidly4, the effects may only be short-term provided propagules
are available to re-establish the meadows.
6.6.2.6 Carbon dioxide
Exposure and sensitivity
Future emissions of CO2 are projected to reduce the pH of the tropical Pacic Ocean by
0.1 units by 2035, and by 0.2 to 0.3 units by 2100 for the A2 emissions scenario
(Chapter 3). All seagrass meadows will be directly exposed to these declines in pH.
341
CHAPTER 6
However, the pH in seagrass meadows can vary by up to 0.9 units over diurnal cycles
as a direct result of carbon uptake by seagrasses and other autotrophs (including
epiphytic algae). The largest changes in CO2 concentrations occur during the day,
when photosynthesis is at a maximum, and in shallow water238–241.
The most critical effect of increases in CO2 concentration and reductions in pH for
seagrasses relate to changes in the availability of dissolved inorganic carbon, and
the rate at which the plants take up dissolved inorganic carbon. Seagrasses use both
HCO3- and CO2, with HCO3- requiring conversion to CO2 at some stage, either external
to the leaf (within the boundary layer) or after uptake235. Seagrasses obtain about 50%
of their dissolved inorganic carbon from HCO3-; the remainder coming from direct
CO2 uptake242–244. The photosynthetic rates of seagrasses are currently limited by the
availability of CO2 at the present-day average pH of 8.2 but higher concentrations
of CO2 at lower pH result in faster photosynthetic rates242,243,245,246. In the temperate
seagrass Zostera marina, higher photosynthetic rates at lower pH for one year are
translated into increased productivity and reproductive output247. These ndings are
consistent with the evolution of seagrasses at a time of higher CO2 concentration224.
If changes in dissolved inorganic carbon alone are considered, seagrasses could
benet from projected increases in CO2 concentrations248. However, there are
differences among seagrass species in their uptake mechanisms and sensitivity to
higher CO2 concentrations246. These differences could affect the species of seagrass
within meadows and the value of the habitat.
Calcifying epibiota growing on seagrass leaves, including foraminifera, bryozoa,
spionid polychaetes and algae, are expected to be sensitive to changes in pH, with
reductions in calcication and growth occurring at reduced pH249,250. Although the
abundance of these calcifying organisms may be reduced on seagrass leaves, elevated
CO2 could enhance photosynthetic rates in non-calcifying epiphytes249 and the total
coverage of epibiota may not be altered signicantly.
Potential impact and adaptive capacity
The most likely effects of elevated CO2 on seagrasses will be increases in their
productivity, biomass and reproductive output247. Higher CO2 can also reduce the
amount of light-saturated photosynthesis required to meet daily carbon budgets245.
This should allow seagrasses to colonise deeper areas with lower light. However,
differences in the sensitivity of seagrass species to elevated CO2246 could result in
some seagrasses beneting more than others. Any notable effect on seagrasses of
changes in the epibiota on their leaves caused by increased CO2 concentrations is
unlikely.
The greater projected productivity of seagrasses, and the changes in their species
composition, under higher levels of CO2 are expected to ow-on to increase the
ecosystem services provided by seagrasses in places where other impacts are
342
minimised. In particular, the richness and productivity of food webs supported by
seagrasses may increase, and the shelter that the plants provide for juvenile sh and
invertebrates (Section 6.3.2) may be enhanced.
Vulnerability
Seagrasses are not expected to be vulnerable to increasing concentrations of CO2.
Instead, the effects of such increases on seagrass meadows in the tropical Pacic are
expected to be higher photosynthetic rates, and greater productivity, biomass and
reproductive output.
6.6.2.7 Sea level
Exposure and sensitivity
The most recent estimates for future sea-level rise are considered to be 20–30 cm for
the B1 and A2 emissions scenarios in 2035, 70–110 cm for B1 and 90–140 cm for A2 in
2100 (Chapter 3). Typically, seagrass habitats are limited by light availability on their
deeper edges and should be sensitive to projected sea-level rise, with the increased
depth likely to reduce light to the point where some of the deepest plants may not
survive3.
Surveys of seagrasses throughout the tropical Pacic have not generally estimated
the proportion of meadows likely to be limited by light at the deeper edges of their
distributions. As a result, we cannot determine the percentage of habitat exposed to
sea-level rise. However, some seagrass species, e.g. Halophila decipiens, have a greater
Photo: Len Mc Kenzie
Seagrasses provide important habitats for small sh
343
CHAPTER 6
tolerance for lower light conditions and so the species composition, or relative cover
of species at the deeper margins of meadows, i.e. their lower depth limit, may shift in
favour of such species as sea level rises49.
Potential impact and adaptive capacity
The expected rises in sea level are likely to result in the loss of seagrass cover or
changes in species composition along the seaward edges of deeper meadows. Species
growing on the deeper margins of seagrass habitats are likely to be at the limit of
their light tolerance range and unable to adapt to further reductions3 in light. In
other parts of the meadows, however, the structure of the seagrass canopy is likely
to change through the varying responses of species to reduced light2,3,221. This shift
in composition is expected to be towards species with lower biomass. The effects of
such changes on the ecosystem services provided by seagrasses to coastal sheries
have not been studied in the tropical Pacic, although the inuence of changes to
leaf height and density of seagrasses on sh and invertebrate communities have been
described for temperate areas251.
Seagrasses are capable of growing both vertically and horizontally and are expected
to adapt to rising sea levels by growing landward in pace with their upper depth
limit, provided the newly inundated sediments are suitable3. Coastal developments,
such as rock walls or groynes, would prevent the potential colonisation of suitable
habitat. Colonisation of newly inundated habitat will also be limited where the
accretion of sediments favours establishment of mangroves.
Vulnerability
Seagrass meadows are estimated to have moderate vulnerability to sea-level rise
where their depth is limited by light, and where expansion landward is blocked.
Elsewhere, seagrass habitats are expected to have low vulnerability to sea-level rise.
6.6.3 Intertidal ats
The aspect of projected climate change of greatest relevance to intertidal at habitats
is sea-level rise. As outlined in Chapter 3, intertidal ats are expected to be exposed
to rises in sea level of 20–30 cm for the B1 and A2 scenarios in 2035, 70–110 cm for
B1 in 2100, and 90–140 cm for A2 in 2100. Intertidal ats are likely to be highly
sensitive to these changes where this habitat cannot expand landward, or where rates
of sedimentation do not keep pace with sea-level rise. Indeed, considerable losses of
intertidal ats are expected to occur as a result of permanent inundation.
The potential impacts of the exposure of intertidal ats to sea-level rise are
permanent changes to BMA communities and the associated epifauna and infauna.
Many intertidal species preferentially inhabit vertical zones corresponding to subtle
changes within the intertidal area above or below mean sea level58. The relationship
344
between sediment surface height and average sea level is expected to be disturbed
by ongoing sea-level rise. Consequently, gradual shifts in composition and/or
abundance of intertidal BMA communities, epifauna and infauna are expected. In
turn, this is likely to have signicant knock on effects on the sh and invertebrates
harvested from intertidal ats. In particular, permanent inundation of intertidal ats
will allow continuous access by demersal sh species and exclude species that forage
at low tide (e.g. birds and crabs). Benthic microalgae communities and fauna now
common in the subtidal zone can be expected to gradually colonise permanently
submerged intertidal areas. The new shallow subtidal zones created by rising sea
levels may be ecologically challenging environments, with low water exchange and
large temperature and salinity uctuations.
A sea-level rise of 50 cm is expected to permanently inundate intertidal ats in
PICTs with micro-tidal conditions (e.g. parts of Cook Islands), whereas a rise
of ~ 1 m would be needed to permanently inundate the greater proportion of
existing intertidal ats in PICTs with larger tidal ranges (e.g. central Pacic
atolls). Although this is a simplistic analysis, species which have a strong
dependence on a functioning intertidal at habitat are expected to be gradually
forced landward until they can no longer migrate. The burrowing crabs
Uca spp., which feed when they emerge from their burrows at low tide, provide an
example of the expected effects of sea-level rise on intertidal species. These crabs
are restricted to upper intertidal ats where there is adequate time between tides for
them to emerge and feed on sediments with the necessary moisture content Uca
spp. scrape the upper layers of sediment, lter BMA, meiofauna and detrital material,
and then deposit balls of cleaned’ sediment 3–4 mm in diameter. Changes in the
appropriate levels of moisture in the sediment, and the time between falling and
rising tide due to sea-level rise, will probably displace these species.
Photo : Tony Falklan d
Intertidal ats, Tarawa Island, Kiribati
345
CHAPTER 6
Progressive replacement of species dependent on intertidal ats may occur due
to colonisation by species adapted to permanent submergence, but a loss of
biodiversity is also expected. The impacts of such losses on subsistence shing
communities in the region will vary based on their dependence on these habitats.
In some PICTs, intertidal gleaning is one of the main ways that low income
urban and rural families secure dietary protein because they do not have the
equipment or skills to catch sh. In other locations, collection of intertidal species
is less important, because they are not a traditional component of the diet,
or because harvests are already reduced from over-exploitation or pollution
(Chapter 9).
6.7 Integrated vulnerability assessment
6.7.1 Mangroves
The projected changes in solar radiation, temperature, rainfall, nutrients and CO2 are
expected to have minimal effects on mangrove habitats in the tropical Pacic and,
in principle, could work together to increase growth and productivity. However,
these potential benets are likely to be negated by the adverse effects of sea-level
rise (Table 6.4). The projected rates of sea-level rise are expected to cause mangroves
on the seaward fringes of their habitats to retreat180 because they are unlikely to be
able to accumulate sediments or produce sufcient root biomass to contribute to soil
volume at the same rate as the rise in sea level29 (Section 6.6.1.7). Even where rates of
sedimentation are high, there is no guarantee that mangroves will survive because
many species are intolerant of rapid sedimentation252. Thus, mangroves are likely to
incur inundation stress in low intertidal positions, leading to reduced productivity,
mortality and reduced forest area.
Mangroves have the potential to adapt in many areas by migrating landwards179,180
(Section 6.6.1.7) but the maintenance of mangrove habitats through this process
will depend on the rate of sea-level rise. If the rate accelerates, as projected
(Chapter 3), migrating mangroves are unlikely to be able to escape the stress of
inundation. Mangroves located where sedimentation rates are low, e.g. in places
remote from river discharge, are expected to be particularly vulnerable. Mangroves in
New Caledonia may be more vulnerable than those elsewhere in the region because
not only will the projected decreases in rainfall reduce the supply of sediment, the
lower precipitation may also increase salinity stress.
When the effects of changes to all the various features of the environment are
integrated, mangroves are expected to have moderate vulnerability to climate change
for the B1 and A2 emissions scenarios in 2035, increasing to a high vulnerability for
B1 in 2100, and a very high vulnerability for A2 in 2100 (Table 6.4).
346
Table 6.4 Summary of the projected effects of climate change variables on mangrove and
seagrass habitats in the tropical Pacic for the B1 and A2 emissions scenarios in 2035 and
2100 (based on the information in Sections 6.6.1 and 6.6.2), together with an assessment of
the overall vulnerability of mangrove and seagrass habitats by integrating these effects.
The likelihood and condence associated with the integrated vulnerability assessments
are also indicated. Note that the projected effects of each climate change variable can be
negative (-) or positive (+); nil = no projected effect.
Scenario Variable Integrated
vulnerability
Light Temp. Rainfall Nutrients CO2Cyclones Sea level
Mangroves
B1/A2 2035 Low
(-)
Very low
(-)
Low
(+/-)
Low
(+/-)
Very low
(+/nil)
Moderate
(-)
High
(-)
Moderate
B1 2100 Low
(-)
Very low
(-)
Moderate
(-)
Low
(+/-)
Very low
(+/nil)
Moderate
(-)
Very high
(-)
High
A2 2100 Low
(-)
Very low
(-)
Moderate
(-)
Low
(+/-)
Very low
(+/nil)
Moderate
(-)
Very high
(-)
Very high
Seagrasses
B1/A2 2035 Moderate
(-)
Moderate
(-)
Moderate
(-)
Low
(+/nil)
Very low
(+)
Moderate
(-)
Low
(-)
Moderate
B1 2100 Moderate
(-)
Moderate
(-)
Moderate
(-)
Low
(+/nil)
Very low
(+)
Moderate
(-)
Moderate
(-)
Moderate
A2 2100 High
(-)
High
(-)
High
(-)
Moderate
(+/-)
Very low
(+)
High
(-)
Moderate
(-)
High
The effects of sea-level rise are expected to result in losses of around 10% of mangrove
habitat in most PICTs where mangroves are common today by 2035 for the B1 and A2
emissions scenarios (Table 6.5). By 2100, losses are expected to be around 50% for the
B1 scenario and 60% for the A2 scenario in most of these PICTs, with losses of up to
80% possible in some PICTs (e.g. Tonga).
6.7.2 Seagrasses
On balance, the combined changes to the key attributes of the environment for
seagrasses are expected to cause moderate losses of these important sh habitats
(Table 6.4). In intertidal and shallow-water habitats, the projected increases in air
temperature, SST, sediment deposition, turbidity, storm surge and algal overgrowth
from elevated nutrient loads, and decreases in light and salinity from higher runoff
are expected to interact to create more hostile environments for many seagrass species.
Seagrasses growing in estuaries, and in fringing reef and bay or lagoon habitats
adjacent to high islands heavily exposed to increased runoff, are likely to be more
vulnerable than those growing on atolls or on barrier and patch reefs (Section 6.2.2).
Although deepwater seagrasses will be relatively protected from disturbances caused
by stronger waves, reductions in light will affect their survival and productivity.
Unlikely Somewhat likely Likely Very likely Very low Low Medium High Very high
0% 29% 66% 90% 100% 0% 5% 33% 66% 95% 100%
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CHAPTER 6
Sea-level rise is expected to result in the loss of those seagrasses growing in deep
water at their present depth limit. Although there will be opportunities for seagrass
to expand landward in some places, physical barriers or unsuitable substrate will
prevent colonisation in other areas.
Table 6.5 Projected percentage loss in areas of mangrove and seagrass habitats for the B1 and
A2 emissions scenarios in 2035 and 2100 in Pacic Island countries and territories (PICTs)
that have total areas of mangroves and seagrasses > 5 km
2
. These estimates are based on the
expert opinion of the authors because data on the land area to be inundated for the B1 and
A2 scenarios are not yet available to inform a quantitative assessment. The estimates include
the perceived scope for the major areas of the existing mangroves and seagrasses in each
PICT to migrate, or not migrate, landwards as the case may be.
When the effects of changes to all the various features of the environment are
combined, seagrass habitats are expected to have moderate vulnerability to climate
change for the B1 and A2 emissions scenarios in 2035 and the B1 scenario in 2100,
increasing to a high vulnerability under the A2 scenario in 2100 (Table 6.4).
Based on a simple application of observed impacts from such synergistic effects in
present-day seagrass habitats, future losses of seagrasses in the tropical Pacic could
be in the range of 5–20% by 2035 for the B1 and A2 emissions scenarios. By 2100, the
losses could be as great as 5–30% for B1 and 10–35% for A2 (Table 6.5). Seagrass losses
PICT
Mangroves Seagrasses
B1/A2
2035
B1
2100
A2
2100
B1/A2
2035
B1
2100
A2
2100
Melanesia
Fiji 10 50 60 < 5 5–10 10–20
New Caledonia 10 50 60 5–10 5–20 10–25
PNG 10 50 60 5–20 5–30 10–35
Solomon Islands 10 50 60 5–20 5–30 10–35
Vanuatu 10 50 60 5–20 5–30 10–35
Micronesia
FSM 10 50 60 < 5–10 5–25 10–30
Guam 10 60 70 5–20 5–35 10–50
CNMI 30 70 80 < 5–10 5–25 10–35
Palau 10 50 60 < 5–10 5–25 10–35
Polynesia
French Polynesia 10 50 60 < 5 5–10 10–20
Samoa 10 50 60 5–20 5–35 10–50
Tonga 30 70 80 5–10 5–20 10–20
Unlikely Somewhat likely Likely Very likely Very low Low Medium High Very high
0% 29% 66% 90% 100% 0% 5% 33% 66% 95% 100%
Unlikely Somewhat likely Likely Very likely Very low Low Medium High Very high
0% 29% 66% 90% 100% 0% 5% 33% 66% 95% 100%
348
are not expected to be as great in subtropical areas because the projected decreases in
rainfall (Chapter 2) are likely to limit impacts. Nevertheless, reductions in seagrass
habitat of 5–10% may occur in these parts of the region for the B1 and A2 scenarios in
2035, increasing to potential losses of 5–20% for B1 and 10–25% for A2 in 2100.
6.7.3 Intertidal ats
The intertidal at habitats of the region are expected to be primarily vulnerable to sea-
level rise, so that any conclusions about the integrated effects of climate change are
essentially the same as those presented in Section 6.6.3. As sea level rises, it is highly
likely that intertidal ats will be lost around many high islands due to steep terrain
or infrastructure barriers. There will also be limits on the landward progression of
intertidal lagoonal habitats on atolls with narrow land areas.
Intertidal ats are expected to have low to moderate vulnerability to climate change for
the B1 and A2 emissions scenarios in 2035, increasing to high for the B1 and A2 scenarios
in 2100. Projections are being developed based on currently available topographic
mapping to estimate the percentage loss of intertidal sand and mud ats from the
expected rises in sea level for the B1 and A2 emissions scenarios in 2035 and 2100.
6.8 Uncertainty, gaps in knowledge and future research
There are still major gaps in knowledge of the distribution, diversity and coverage of
mangrove and seagrass habitats, and the areas of intertidal ats, across the tropical
Pacic. Indeed, caution is needed in interpreting the information on coastal sh
habitats presented here because much of it is outdated, or based on limited surveys.
In many cases, the areas of mangroves, seagrasses and intertidal ats are likely to be
(often gross) underestimates. The best estimates are for mangroves and seagrasses in
New Caledonia due to the extensive research efforts there. Reasonable estimates are
also available for mangroves in some other PICTs, such as FSM, American Samoa,
Tonga, and Wallis and Futuna, despite the fact that assessing changes in mangrove
area over time in the tropical Pacic is difcult253.
Systematic mapping of mangroves, seagrasses and intertidal ats for all PICTs,
including habitat area, plant density and species composition, is a research priority.
In the case of seagrass habitats, mapping is also needed to show the depths to
which existing meadows extend. This information will (1) raise awareness among
coastal planners about the locations and scale of these important sheries habitats;
and (2) provide a baseline for monitoring changes in the area, density and species
composition of mangroves and seagrasses, and the area of intertidal ats.
The species composition and relative abundance of mangroves and seagrasses are
relatively well known in most PICTs (Table 6.1), but this is not the case for the epifauna
and infauna associated with these habitats. Faunal studies are needed at a basic level,
349
CHAPTER 6
followed by comparisons of biodiversity, relative abundance and size composition
of fauna among mangroves, seagrasses and intertidal ats, and between different
mosaics of these habitats. Movements of animals among these habitats and between
them and coral reefs, in terms of life history development and foraging behaviour
described for other parts of the world7–9,12, remain poorly understood in the tropical
Pacic. Such research will greatly improve our understanding of food webs and the
other ecosystem services provided by mangroves, seagrasses and intertidal ats to
coastal sheries. It will also allow knowledge to progress from the limited range of
examples of habitat roles to an understanding of the processes underpinning these
roles.
More reliable data on sea-level rise and sedimentation rates are needed throughout
the region to enable more accurate predictions of the responses of mangroves and
intertidal ats and the possible mitigating effects of sedimentation254. Reliable
data on sea-level rise are now being collected in many PICTs with instrumentation
installed through the South Pacic Sea Level and Climate Monitoring Project
(Australian Bureau of Meteorologyiii). However, the time-series is not long enough to
elucidate a trend and recordings must continue for many years. Caution will also be
needed in interpreting these data for the reasons outlined in Chapter 3, and because
the tectonics of coastlines within PICTs vary215. It will also be important to improve
the resolution of topographic maps so that the areas projected to be inundated, which
may be suitable for colonisation by mangroves and seagrasses, can be estimated more
accurately and protected.
Even with a concerted effort to address the large gaps in knowledge outlined above,
it will be difcult to separate the effects of climate change on coastal habitats from
other natural and human impacts. Despite the need to understand the potential
effects of climate change, it is imperative that existing research on the effects of
changes to catchments and shorelines on these habitats continues to receive the
necessary funding. The possible impacts from agriculture, forestry and mining, for
example, are likely to be much greater than those from climate change in the short
to medium term. Understanding the processes behind such impacts, and the most
appropriate measures to ameliorate the adverse effects, is also likely to build the
resilience of coastal sh habitats to climate change. Ideally, monitoring programmes
should be designed that enable managers to separate the effects of climate change
from local stressors. Managers can then identify adaptations needed to maintain the
habitat mosaic in the face of climate change5,217,255, and assess the effectiveness of these
adaptations.
The need to downscale global climate models to provide more accurate assessments of
projected changes in surface climate and features of the tropical Pacic Ocean at scales
more relevant to PICTs is also important for integrated coastal zone management in
iii www.bom.gov.au/pacicsealevel
350
PICTsiv (Chapters 2 and 3). When combined with the results of the research outlined
above, this information will enable managers to identify (1) the mangrove areas and
seagrass meadows most at risk from local impacts, e.g. higher levels of runoff; and
(2) the locations where management effort should be focused.
The substantial research list described above is beyond the capacity of most PICTs.
Many will need to form partnerships with scientic institutions within and outside
the region, and seek the assistance of the communities who live adjacent to these
habitats, to ll the gaps. Location-specic studies at representative sites across the
region are especially needed, because most of the present knowledge and assessments
are extrapolated from other parts of the world. The Seagrass-Watchv programme
offers a model for providing useful and spatially extensive environmental monitoring
data for some of the necessary surveys in PICTs where resources are limited.
6.9 Management implications and recommendations
The high level of connectivity among coral reefs, mangroves, seagrasses and
intertidal ats (the coastal habitat mosaic) means that the loss of one habitat could
have implications for the other components of the mosaic. Therefore, an over-riding
priority for management should be to secure connectivity among all these habitats
to enhance the resilience of coastal ecosystems7 and help safeguard coastal
sheries production6 in the face of future climate change (Chapter 9). The practical
management measures for maintaining this mosaic are summarised below.
¾Improve integrated coastal zone management to reduce existing impacts on
mangroves, seagrasses and intertidal ats from agriculture, forestry, mining
and road construction in catchments, and sand mining and construction on the
coast. These measures will help maximise the natural potential of these habitats
to adapt, and will be particularly important in reducing the synergistic effects of
terrestrial pollution and climate change on coastal habitats2,21,256–259.
¾Strengthen governance and legislation to ensure the sustainable use and
protection of vegetated coastal habitats29. There is a continuing need to build the
capacity of management agencies to improve (1) the ability of staff to understand
the threats to coastal sh habitats; (2) the networks for transferring this knowledge
to communities through co-management or community-based management
arrangements; (3) the national regulations needed to underpin effective protection
for mangroves and seagrasses; and (4) local and national systems to achieve
compliance with regulations.
iv This work is now being done progressively for the tropical Pacic by the Australian Bureau of
Meteorology and CSIRO, and partners, under the Pacic Climate Change Science Programme; see
www.cawcr.gov.au/projects/PCCSP
v www.seagrasswatch.org/about.html
351
CHAPTER 6
¾Implement and facilitate interventions that are likely to support coastal sh
habitats to adapt to climate change. For example, plan to allow for landward
migration of mangroves, seagrasses and intertidal ats where possible. This will
involve placing infrastructure on higher ground and removing existing barriers
in low-lying areas.
¾Initiate regular high-level discussions between planners and sheries managers
to ensure that barriers to adaptation of coastal sheries habitats are minimised as
plans are developed to assist all sectors respond to climate change.
¾Engage local communities in sustainable management of coastal habitats, such as
supporting community-based conservation areas where local committees manage
or restrict use of areas based on the state of resources29. This requires regular
transfer of research and monitoring information to communities to build local
capacity.
¾Promote community-based co-management approaches, where management
is carried out primarily by local stakeholders in close cooperation with relevant
local and national government institutions and non-government organisations.
Adaptive co-management makes optimum use of social capital, such as existing
(or assigned) resource rights, local governance, traditional knowledge, self-
interest and self-enforcement capacity. It is increasingly seen as an effective way
to implement conservation and management measures where customary tenure
exists, e.g. throughout much of Melanesia.
352
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... These ecotones form a complex mosaic of habitats (Adams, 2017), featuring diverse array of substrates such as sand, mud, and coral rubble, along with biological structures, such as seagrass meadows, oyster reefs, adjacent coral reefs, and mangroves. They are prone to natural extremes, such as tidal regime, temperature, salinity, and nutrient and sediment loading from interconnected shorelines and watersheds (Amos et al., 2013;Gao, 2019; R. R. Carlson et al., 2021;Teneva et al., 2016;Waycott et al., 2011). These conditions result in dynamic physiochemical environments conducive to the establishment of a mosaic of benthic habitats, which collectively shape the structure and function of flats ecosystems. ...
... Coastal flats and their associated fisheries face increasing threats from climate change (Danylchuk et al., 2023;Waycott et al., 2011) and are further compounded through localized human-induced stressors, such as overfishing, which can al-ter both food web dynamics and disrupt habitat bioengineering processes through the removal of herbivorous fish and megafauna (Jackson et al., 2001). Additional cumulative pressures include coastal development and poor water quality and management practices that lead to large scale regime shifts in seagrasses and seascape structure (Danylchuk et al., 2023;Hall et al., 2016;Santos et al., 2020). ...
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Flats ecosystems are dynamic, shallow, nearshore marine environments that are interconnected and provide immense ecological and socio-economic benefits. These habitats support a diversity of fish populations and various fisheries, yet they are increasingly threatened by anthropogenic stressors, including overfishing, habitat degradation, coastal development, and the cascading effects of climate change. Effective habitat management and restoration are essential but are often missing for flats ecosystems. Despite navigating a landscape of imperfect knowledge for these systems, decisive action and implementation of habitat protection and restoration is currently needed through policy and practice. We present a comprehensive set of 10 strategic guiding principles necessary for integrating habitat management and restoration for the conservation of interconnected flat ecosystems. These principles include calls for comprehensive ecosystem-based ­management, integrating adaptive strategies that leverage diverse partnerships, scientific research, legislative initiatives, and local and traditional ecological knowledge. Drawing on successes in other environmental management realms, we emphasize the importance of evidence-informed approaches to address the complexities and uncertainties of flats ecosystems. These guiding principles aim to advance flats habitat management and restoration, promoting ecological integrity and strengthening the socio-economic resilience of these important marine environments.
... These tropical arboreal structures, particularly along the fringe, provide several ecosystem services that provide benefits to organisms living along coastlines worldwide as well as regulating processes with a global impact. Ecological benefits from mangrove forests include water quality improvement, wave amelioration, habitat creation, and carbon sequestration (Lin and Dushoff 2004;Waycott et al. 2011;Alongi 2014;Barreto et al. 2015;Whitfield 2016). These functions of mangrove forests not only contribute to mitigating climate change but foster an environment that can be inhabited by a variety of marine biota that benefit the forest. ...
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... Moreover, tidal hydrodynamic processes significantly impact soil carbon accumulation (Spivak et al. 2019;Yang et al. 2021). Unlike mudflats directly exposed to wind and seawater, mangroves and seagrasses benefit from more stable environmental conditions due to their dense vegetation cover (Waycott et al. 2011;Sasmito et al. 2020). Consequently, the comparatively stable environmental conditions in mangroves and seagrasses promote higher soil carbon buildup levels than in mudflats. ...
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... The dashed gray line represents the biological C pump. Similarly, in mangrove ecosystems, sea level rise decreases tree cover, thus reducing C uptake and increasing the erosion potential of sedimentary C stores (Waycott et al., 2011). As plant mortality and erosion increases, more stored C may be released to the atmosphere or surrounding water as CH 4 or CO 2 (Reddy et al., 2022). ...
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... Seagrasses are one of the most seriously endangered ecosystems globally (Waycott et al. 2009), and their status in the Pacific Islands Countries and Territories, including New Caledonia, is becoming compromised under increasing threats from anthropogenic activities, further exacerbated by pressures related to climate change (Waycott et al. 2011). The current state of seagrass in New Caledonia is not currently monitored, but McKenzie et al. (2021) found no particular trend in their regional analysis (one site in New Caledonia). ...
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... The prevalence of T. hemprichii at Mead-2 can be ascribed to the specific environmental conditions present at this site. Previous studies (Waycott, 2011;Kilminster et al., 2015) have suggested that this species struggles to endure prolonged sun exposure and is susceptible to the impacts of rainwater. The discrepancies in species abundance observed among the sites have led to the development of distinct community structures, consequently influencing the potential for carbon sequestration and storage within the seagrass meadows. ...
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... intensive seaweed farming), coastal mining (e.g. sand extraction), coastal development, and poor water quality, with climate-change exacerbating these impacts (Waycott et al. 2011). The dynamics of tropical seagrasses mean that while ocean warming may limit growth, elevated CO 2 concentrations can increase thermal tolerance (Zimmerman 2021) and seagrass have some inherent resilience to future pressures (McKenzie and Yoshida 2020). ...
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Partitioning of the nitrogen stock in a Fijian seagrass bed dominated by Syringodium isoetifolium (Ascherson) Dandy and in an adjacent area bare of macrophytic vegetation was assessed to evaluate the effect of the presence of seagrass on coral sediment. Concentrations of major nutrients, such as nitrogen and phosphate, were as low in the water column at the seagrass bed and the bare area as they were in the open ocean. Concentrations of ammonium and dissolved organic nitrogen, however, were higher in the water within the seagrass canopy than they were in other waters. In sediments at the seagrass bed and the bare area, interstitial nitrogen, such as nitrate and dissolved organic nitrogen, was a minor component of the total nitrogen (0.3-0.05%). On the other hand, concentrations of total organic nitrogen in seagrass-bed sediment (about 70% of which was in the form of amorphous organic nitrogen and the rest of which came from living and dead seagrass) were more than three times higher than those in bare-area sediment. Concentrations of organic carbon from amorphous organic materials in seagrass-bed sediment showed no large change with depth, resulting in an apparent decrease in the carbodnitrogen atom ratio from 60 to 10. These results suggest some mechanisms to minimize the loss of nitrogen stock from the sediment of tropical seagrass beds.
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Mangrove forests are one of the world's most threatened tropical ecosystems with global loss exceeding 35% (ref. 1). Juvenile coral reef fish often inhabit mangroves, but the importance of these nurseries to reef fish population dynamics has not been quantified. Indeed, mangroves might be expected to have negligible influence on reef fish communities: juvenile fish can inhabit alternative habitats and fish populations may be regulated by other limiting factors such as larval supply or fishing. Here we show that mangroves are unexpectedly important, serving as an intermediate nursery habitat that may increase the survivorship of young fish. Mangroves in the Caribbean strongly influence the community structure of fish on neighbouring coral reefs. In addition, the biomass of several commercially important species is more than doubled when adult habitat is connected to mangroves. The largest herbivorous fish in the Atlantic, Scarus guacamaia, has a functional dependency on mangroves and has suffered local extinction after mangrove removal. Current rates of mangrove deforestation are likely to have severe deleterious consequences for the ecosystem function, fisheries productivity and resilience of reefs. Conservation efforts should protect connected corridors of mangroves, seagrass beds and coral reefs.
Chapter
Involving a wide range of scientists working on intertidal sediments, this 1997 book is of importance to all environmental scientists. Individual chapters explore the underlying biogeochemical processes controlling the behaviour of carbon, the nutrients nitrogen and phosphorus, and contaminants such as toxic organics, trace metals and artificial radionuclides in intertidal environments. The biogeochemistry of these environments is critical to understanding their ecology and management. All of the chapters include both a comprehensive review and the results of recent research. The authors are active researchers in this diverse and ecologically important environment. This book is mainly for researchers and managers working on these environments, but it will also serve as a valuable advanced undergraduate and graduate reference text in environmental chemistry, environmental science, earth science and oceanography.
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Rhizophora mangle L., the predominant neotropical mangrove species, occupies a gradient from low intertidal swamp margins with high insolation, to shaded sites at highest high water. Across a light gradient, R. mangle shows properties of both “light-demanding” and “shade-tolerant” species, and defies designation according to existing successional paradigms for rain forest trees. The mode and magnitude of its adaptability to light also change through ontogeny as it grows into the canopy. We characterized and compared phenotypic flexibility of R. mangle seedlings, saplings, and tree modules across changing light environments, from the level of leaf anatomy and photosynthesis, through stem and whole-plant architecture. We also examined growth and mortality differences among sun and shade populations of seedlings over 3 yr. Sun and shade seedling populations diverged in terms of four of six leaf anatomy traits (relative thickness of tissue layers and stomatal density), as well as leaf size and shape, specific leaf area (SLA), leaf internode distances, disparity in blade–petiole angles, canopy spread: height ratios, standing leaf numbers, summer (July) photosynthetic light curve shapes, and growth rates. Saplings showed significant sun/shade differences in fewer characters: leaf thickness, SLA, leaf overlap, disparity in bladepetiole angles, standing leaf numbers, stem volume and branching angle (first-order branches only), and summer photosynthesis. In trees, leaf anatomy was insensitive to light environment, but leaf length, width, and SLA, disparities in bladepetiole angles, and summer maximal photosynthetic rates varied among sun and shade leaf populations. Seedling and sapling photosynthetic rates were significantly depressed in winter (December), while photosynthetic rates in tree leaves did not differ in winter and summer. Seasonal and ontogenetic changes in response to light environment are apparent at several levels of biological organization in R. mangle, within constraints of its architectural baiiplan. Such variation has implications for models of stand carbon gain, and suggest that response flexibility may change with plant age.
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Introduite de maniere intentionnelle pour des raisons economiques dans les archipels eloignes d’Oceanie (Hawaii et la Societe), ou elle etait naturellement absente, la mangrove a colonise en quelques decennies de grandes portions de zone littorale. Cette formation vegetale est devenue un element banal du paysage insulaire, mais reste un sujet de controverse en raison de ses impacts ecologiques reels ou potentiels. Rejetee par la Societe dans l’archipel hawaiien, avec le renouveau culturel polynesien et les efforts de preservation de la biodiversite, elle est, en Polynesie francaise, du fait de conflits d’interets generateurs d’inaction et d’une certaine indifference, en voie d’assimilation. La representation negative de la mangrove, commune aux deux archipels, est en rupture avec l’image habituelle de cette vegetation, celle du milieu nourricier et regulateur des equilibres naturels en zone cotiere tropicale.