MARINE ORGANISMS RESPONSE TO CLIMATE CHANGE EFFECTS IN THE WESTERN INDIAN OCEAN

Abstract

There is widespread consensus among climate scientists today that global climate change is real and has anthropogenic roots. Marine species, for example, are exposed to a large array of abiotic stressors, such as warming and ocean acidification, that are linked directly to anthropogenic climate change. The general view on whether natural populations can adapt to anthropogenic change is that many species will fail to adapt to rapid climate change effects. This accelerating trend has profound effects on marine species and ecosystems, globally and the Eastern African region is no exception. Here, we provide a review that is the result of the workshop "Marine Organisms Response to Climate Change Effects-Adaptation or Extinction?" among ocean scientists actively studying the impact of climate change in the Western Indian Ocean. The review examines several key marine organisms and ecosystems in the region of the Western Indian Ocean, how these may be impacted by climate change, and the way forward for successful conservation and management.

Full text

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MARINE ORGANISMS RESPONSE TO CLIMATE CHANGE EFFECTS IN THE WESTERN
INDIAN OCEAN
Hollander J.1, Linden O.1, Gudka M.2, Duncan M I.3,19, Obura D.2, James N.3,4, Bhagooli R.5
31
,22, Nyanapah J.6,
Onyango C.7, Duvane J.8, Louis Y.5, Ngotho D., Mvungi E.10, Mamboya F.11, George R.12, Hamad H.13, Issa
Hamisi M.14, Adeleke B.7, Ngoa E.15, Harlay J.23, Oduor, N.17, Fondo E.17, Wambiji N.17, Raharinaivo L.18,
Winkler A.19, Okemwa G.17, Karisa J.17, Madi Bamdou M.21, Mtaki K.20, Randrianandrasana J.18
*Corresponding author: World Maritime University | Global Ocean Institute, 211 18 Malmö, Sweden. E-mail
address: johan.hollander@wmu.se (J. Hollander).
32
Keywords: Western Indian Ocean, coral, seagrass, mangrove, ocean acidification, fish, blue economy, climate
change, anthropogenic, adaptation
Abstract
There is widespread consensus among climate scientists today that global climate change is real and has
anthropogenic roots. Marine species, for example, are exposed to a large array of abiotic stressors, such as
warming and ocean acidification, that are linked directly to anthropogenic climate change. The general view on
whether natural populations can adapt to anthropogenic change is that many species will fail to adapt to rapid
climate change effects. This accelerating trend has profound effects on marine species and ecosystems,
globally and the Eastern African region is no exception. Here, we provide a review that is the result of the
workshop “Marine Organisms Response to Climate Change Effects – Adaptation or Extinction?” among ocean
scientists actively studying the impact of climate change in the Western Indian Ocean. The review examines
several key marine organisms and ecosystems in the region of the Western Indian Ocean, how these may be
impacted by climate change, and the way forward for successful conservation and management.
31
The Biodiversity and Environment Institute, Reduit, Republic of Mauritius
32
Facts and opinions published in the Journal of Indian Ocean Rim Studies (JIORS) express solely the opinions of the respective
authors. This in no way represents the views of IORA. The Authors are responsible for their citing of sources and the accuracy of
their references and bibliographies. The editors cannot be held responsible for any lacks or possible violations of third parties’ right.

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Introduction
Global climate change has emerged as a major problem for humanity, challenging many aspects of human life
today and compromising the life of future generations. The repercussions expand beyond human welfare, with
most ecological communities on land and in the oceans, in polar, temperate and tropical areas affected. In the
marine environment, climate change alters the physio-chemical properties of the ocean, with changes in
temperature, oxygen concentrations and pH having impacts on all marine organisms (Somero 2010). The
Intergovernmental Panel on Climate Change (IPCC) predicts an increase of temperature of between 3–5
degrees by the end of the century. Such extreme change will unquestionably expose coastal and marine
ecosystems to environmental stress at dangerously high rates. The question is to what extent natural populations
can adapt to anthropogenic changes. Unfortunately, many scientists today predict that key species will fail to
adapt to such rapid climate change, and that many ecosystems will change fundamentally resulting into
significant loss of biodiversity (IPCC 2018).
In the Western Indian Ocean (hereafter, WIO), these problems are real and very often acute (Wilkinson et al.,
2000; Obura et al., 2002; Mahongo 2009; Ateweberhan & McClanahan 2010; Watkiss et al., 2012; George et al.
2018). Increasing human populations have also changed historical land use patterns, in the form of urbanization
of rural areas and increasing the area of cultivated land. These changes will continue to reduce the quality and
quantity of drainage water with impacts on coastal ecosystems of the WIO (e.g. IOC-UNESCO, 2016; Okuku
2007; Mvungi & Pillay 2019). Additionally, cyclone strength and frequency are increasing, influenced by
increasing Sea Surface Temperatures (SST) that add energy to the atmosphere (Mahongo et al. 2014).
Ramirez et al. (2017) identified the WIO as one of six hotspots of marine biodiversity globally that are particularly
affected by global warming, with the list of consequences growing longer each year (George et al. 2018;
Mahongo; 2011; Mahongo et al., 2014; Mvungi & Pillay, 2019). This text will focus on a few of the most important
coastal ecosystems and organism groups in the WIO which provide essential ecosystem services in the region,
and which are now facing serious stress from climate change; fish, coral reefs, seagrasses, and mangroves.
The article will describe and discuss the results of past and ongoing climate change related research. Finally,
the article will try to widen the scope and discuss climate change effects on the whole in the WIO region,
conceptual and/or infrastructural improvements, and research avenues that have been overlooked or need
further exploration. However, providing a complete review of climate change impacts is beyond the scope of this
article.
The ideas and concepts presented below are the result of discussions at the workshop “Marine Organisms
Response to Climate Change Effects – Adaptation or Extinction?” among experts, scientists and researchers
actively studying the impact of climate change in the WIO. The workshop was funded by WIOMSA and organised
in collaboration with Lund University and the World Maritime University, in Mombasa, October 2018. In relation
to this special issue on Blue Economy in the Indian Ocean, this article focuses on how the responses of marine
organisms and ecosystems to climate change may affect their ability to provide and sustain ecosystem services
that are the foundation of the blue economy. Biodiversity and ecosystem functions generate the products and
services used by people (UNECA 2016, Attri 2016, Obura 2018), so the ability of organisms and ecosystems to
adapt to or withstand climate change will fundamentally affect the level and stability of service provision into the
future.
Corals
Key ecosystem characteristics, features and distribution
Coral reefs are perhaps the most biodiverse of ecosystems globally and among the most productive, hosting up
to one quarter of marine species at some point in their life cycle (ref). In the WIO, they are one of the dominant
shallow marine ecosystems, fringing the coasts of mainland East Africa and the islands of the region (Obura
2015). Their high productivity provides fish and other food, protects the coastlines from storms and generates a

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range of additional ecosystem services that support rural and urban population, tourism and recreation sectors
and biomedical sciences (Eliff & Silva 2017).
There are some 400 species of reef-building coral in the WIO (Obura 2012, Veron et al. 2015), and their
symbiosis with intracellular dinoflagellates, zooxanthellae, drives the coral reef ecosystem. Photosynthesis by
the zooxanthellae generates excess energy used by the coral to grow and rapidly secrete their calcium carbonate
skeleton, which over multiple generations of corals growing on the skeletons of their ancestors builds the reef
system. Corals and their symbiotic algae need clear warm tropical waters and are restricted to shallow (< 30 m)
rocky bottoms not too close to rivers that discharge turbid water and silt into the sea. The symbiosis is, however,
vulnerable to high temperatures. Whitening, or bleaching, of the coral occurs in response to exposure to
excessively high temperatures (as well as other stress factors) (Obura 2009). If the stress is not excessive the
bleached coral may recover, but if excessive, mortality occurs. Thus, corals are highly vulnerable to climatic
warming of the last decades (Hughes et al. 2018,), and WIO coral reefs have gone through two major bleaching
events in 1998 and 2016, as well as smaller events in between (Obura et al. 2017, Gudka et al. 2019,
McClanahan et al. 2014).
All countries in the WIO, both mainland and island, contain coral reefs within their territorial waters. The most
common type of coral reefs are fringing reefs which generally contain patch reefs in adjacent shallow lagoons.
There are also isolated atoll reefs around some small islands as well as barrier reefs notably in Toliara,
Madagascar and the region also has large submerged reef banks in the Mascarene Plateau (Ahamada et al.
2002). Discharge of freshwater and sediments from major rivers into shallow waters generally creates conditions
unsuitable for corals to live, leading to natural breaks in extensive coral reef formations in several countries.
Comoros Islands have a total of 430 km2 of coral reefs with at least 195 reef-building coral species (Ahamada
et al. 2008; Obura 2012). Mayotte’s lagoon covers 1500 km2 and harbours at least 249 reef-building coral
species. Kenya’s coral reefs cover an area of approximately 639 km2, and are differentiated into two regions:
the more-diverse (239 species) southern reef is an almost continuous fringing reef system from Malindi to
Tanzania and the northern lower diversity discontinuous patchy and fore reef slopes from Lamu to the border
with Somalia. Madagascar has 3450 km of coral reefs, comprising 1130 km of fringing reef, 502 km of barrier
reef, 557 km of coral banks and 1711 km of submerged reef (Cooke 2012). The northern part of Mozambique is
characterised by extensive, highly-biodiverse coral formations fringing the coastline and the surrounding island
groups. The southern half of Mozambique and the extreme north-eastern part of South Africa (approximately 40
km² reef area) consist of marginal, non-accreting coral reefs generally dominated by soft corals. Réunion, the
youngest island, has short stretches of narrow fringing reefs along southwestern coasts (Turner & Klaus, 2005)
with a total length of 25 km and an area of 12 km2. Coral reefs cover an area of approximately 1,690 km2 in the
Seychelles, with most reef areas located in the outer islands. Tanzania’s fringing coral reefs consist of at least
273 species are located along about two thirds of Tanzania’s continental shelf and surround most of the islands
(Obura 2017). Mauritius Island consists of a discontinuous fringing reef, small barrier reef and large lagoon patch
reefs and Rodrigues Island has nearly continuous fringing reefs with both islands having approximately 240 km2
of shallow reef habitats (Turner & Klaus, 2005).
Description of benefits
Coral reefs are home to and support close to a quarter of the ocean biodiversity including fish. In this way, they
support small-scale artisanal fisheries essential for the supply of protein for numerous people in the WIO. Reefs
also generate income from tourism, and they protect the coastline by reducing wave energy and wave height,
hence protecting coasts from erosion. Unfortunately, corals are sensitive to elevated temperatures and many
experts including the IPCC predict that they will be the first ecosystem to effectively become extinct as a result
of ocean warming (1.5 C IPPC report).
The impacts on coral reefs as a result of ocean warming have been studied extensively in the WIO region (Obura
2005; Gudka et al., 2019; McClanahan et al., 2007; Obura et al. 2018). Although ocean temperature increases
occur globally, in the tropics and subtropics, the rate of rising temperature can vary across regions and at local
scales (Hoegh-Guldberg et al. 2007; Hughes et al. 2017), contributing to variation in the resilience of coral reefs

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to cope with temperatures and temperature spikes (McClanahan et al. 2007). Many factors affect local variation
in temperatures and the responses of corals and their symbiotic algae, potentially imposing directional selection
and local adaptation or phenotypic plasticity (West & Salm 2003, Obura 2005, Obura 2009, McClanahan et al,
2014; Pratchett et al, 2013).
Ocean acidification causes negative impacts on calcifying species like phytoplankton, molluscs, corals and
coralline algae (Harvey et al. 2013). For species with a calcified skeleton, the lower pH will impede building a
calcium carbonate (CaCO3) structure (Doney et al. 2009). Still, these effects will be most pronounced in colder
regions than in the tropics, since warmer temperatures stimulate faster growth and development by a higher
metabolism (Byrne 2011) which may counteract the negative effects of acidification (McNeil et al. 2004; Kleypas
and Yates 2009). At the moment, the impact of sea level rise is not considered a major threat to coral reefs in
the region, as the progression of sea level rise may be within the growth rate of most corals (Anthony & Marshall
2009).
Other than direct stressors, there are several synergistic indirect stressors that are influencing corals negatively.
For example, a stressed coral reef will be more susceptible to coral diseases, which encompasses pathogenic
fungi, bacteria or viruses that may reduce growth rate, impair reproduction and cause direct mortality (Harvell
2007). Many of these diseases are still relatively unknown, but today we see strong correlations between the
appearance of such pathogens after acute stress events such as coral bleaching and tropical cyclone damages
(van Hooidonk & Huber, 2006; Randall & van Woesik, 2015; 2017). There are also processes that are often not
immediately associated with global climate change effects such as eutrophication and pollution from various
sources, that accentuate the pressure on corals and associated species. Currently in the WIO, warmer sea
surface temperatures and El Ninõ-Southern Oscillation effects are generating stronger and more frequent
cyclones and heavier storms, which also affect new geographical areas that previously have been spared from
such events (Fitchett & Grab 2014). The region also experiences greater variation in precipitation, with the
resulting run-off from land carrying solid waste such as plastics, nutrient pollution (mostly nitrogen and
phosphorus), suspended sediment and chemicals as well as metals from industry and agriculture. Finally,
increased temperatures and alterations in ocean circulation and currents cause changes in cold and warm water
areas and allow new and sometimes deleterious species to migrate and invade new areas.
Future Research Areas
Previous research in the region has largely focused on the genus Acropora (Baird et al. 2009; Meyer et al. 2011;
Mattan-Moorgawa et al. 2012; Louis et al. 2017). Certainly, more studies on additional species are required, in
particular comparative studies on temperature response, both in the laboratory and in the field. A few key genera
that have been suggested for such studies are Porites and Favia. Nevertheless, for better management of
coastal ecosystems including coral reefs, research must also focus on rare and endemic species in the region
(e.g. Anomastrea sp.) that possess a high conservation value, and need protection of their habitats, particularly
species that live in critical habitats. Better knowledge about the sensitivity and robustness of different taxa and
populations to climate warming, possible acclimatization to changing conditions and the possible existence of
unique genetic makeups with a stronger resilience towards warming are essential aspects to be considered. A
relevant question is if priority should be placed on species that are ecosystem engineers, those with higher
resistance to extreme conditions, or rare or endemic species.
Moreover, adaptations in corals to climate change is a complex process, but we know variations exist in traits
and that selection occurs. In order to understand local morphological adaptations, behavioural changes and
variation in gene expressions, long-term monitoring programs are required because adaptations can vary among
species and across populations. This research should include a focus on the symbiotic dinoflagellates, now
known to be highly variable at high levels of classification (LaJeunesse et al. 2018). This particular field provides
several new and important research directions in terms of understanding coral resilience. For example, there
may be specific micro-organisms or certain microbe communities that facilitate corals resistance against
bleaching or invasive microorganisms which can drive shifts in these unique symbiotic microbial communities,
obstructing selection towards adaptations or even make corals more vulnerable to bleaching.

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Opportunities for future work
In the WIO, there are several research groups focusing on coral research. These research groups are either
affiliated with universities, governments or various NGOs. Most of the research commenced after the serious
bleaching event that hit the region in connection with the 1997/98 El Nino event when sea surface temperatures
increased by several degrees killing up to 90% of corals on many reefs of the region (Obura, 1999; Wilkinson et
al., 1999; Goreau et al., 2000; but also CORDIO Reports 1999, 2000, 2002, 2005 and 2008). However, research
in the WIO region is challenging since there is very little local funding available. In addition, the region is large
and comprises both mainland countries and island nations, which make logistics and management demanding.
Even if it is today possible to forecast bleaching events (Donner et al. 2017), as they until now has been
associated with El-Nino-events, and substantial research has been accomplished to understand coral responses
to elevated temperatures (Hoegh-Guldberg et al. 2007; Hughes et al. 2017), there is a lack of contingency
funding which can be mobilized in the event of a new mass bleaching event. In addition, there is little knowledge
of species-specific responses to rising temperatures.
To achieve such essential knowledge, new research is required in several fields. Taxonomic research, for
example, has unfortunately been underfinanced for many years in all areas, not only in the marine realm. Still,
museums around the world provide funding for larger sampling campaigns required to identify and classify new
species. Such research is of key importance also when it comes to forecasting the ability of different species to
survive the various impacts of climate change. Without correctly identifying which organisms comprise an
existing healthy coral microbial community, the understanding of invasive/introduced species dynamics, or even
the identification of unique genetic constitutions of certain coral populations will be impossible. This would
obviously entail substantially more man hours of field research to map coral reefs around the WIO region in
detail, but also to conduct laboratory and mesocosm experiments, as well as to conduct targeted meta-
population molecular studies.
Coral reefs are complex habitats, and a change in one end of the system will cascade and produce a multitude
of consequences elsewhere. To try and understand the various changes occurring to coral reefs, the WIO needs
formal, coordinated research groups and consortiums of laboratories, which can combine and expand efforts.
One important focus of such research groups would be to prepare a strategic monitoring plan for any impending
major bleaching events in the region, as these are likely to become more common in the next few years. This
would require a funded regional contingency plan, core-funding secured for long-term monitoring, and a plan to
study pre-determined measurement indicators at sentinel sites; during pre-, peak- and post-mass bleaching
events.
Similarly, there is a need to develop research and monitoring related to ocean acidification. The Global
Observation Ocean Acidification Network (GOA-ON) and OceAn pH Research Integration and Collaboration in
Africa (ApHRICA) are pilot projects to deploy ocean pH sensors in the WIO for the first time. The initiative aims
to improve knowledge and to provide selected teams with training and basic observation equipment, the “OA”
toolkit. However, only a few research teams up to now have received these toolkits and initiated the first
observations. Sustainable Oceans, Livelihoods and food Security Through Increased Capacity in Ecosystem
research in the Western Indian Ocean (SOLSTICE-WIO) is a project funded by the UK Global Challenges
Research Fund (GCRF) that raises awareness and transfers knowledge on climate change issues to local
researchers and students. The long-term objective of all this work is to create a local research community of
practise for ocean acidification and other stressors on the marine environment and relate the ocean acidification
observation to species and ecosystems of socioeconomic importance to make the science relevant for society.

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Table 1. Description of marine conservation biology actions and their putative impacts on key
organisms in the WIO region.
Actions
Taxa
Impacts
Assess local adaptations
and resilience
Fish, corals
Predictive accuracy of species responses to
climate change
Contingency funding
Corals
forecast bleaching events
Species-specific responses
Corals,
seagrasses,
mangroves, fish
Identify vulnerable or robust taxa
MPAs
Corals,
seagrasses,
mangroves, fish
Protection of species that withstand extreme
conditions, and rare endemic species
Improved molecular labs
Corals,
seagrasses,
mangroves, fish
To identify population structures, species and
rare allele combination, gene expressions, and
traits
Focus on sympodium
dinoflagellates and
associated microbes
Corals
Symbiotic microbial communities can drive coral
adaptations and resilience
Taxonomic research
Corals
Invasive species can obstruct selection
Establish baseline
information
Corals,
seagrasses,
mangroves, fish
Improve comparative studies before and after
extraordinary events
Improve ecosystem-based
services
Corals,
seagrasses,
mangroves, fish
Mitigates global climate change. Simultaneously
highlights and protect key species or
ecosystems, and improve ocean conservation
biology and biodiversity
Standardize research and
sampling methods
Corals,
seagrasses,
mangroves, fish
Improve validation and statistical power across
the region
Support for long term
monitoring studies
Corals, HABs, pH
To enable forecasting future extreme events
Develop core lab facilities
for molecular techniques
Corals,
seagrasses,
mangroves, fish
Assure the availability of expensive molecular
equipment, and support specialist training in the
region

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Seagrass and mangroves
Key ecosystem characteristics and features
Seagrasses in the WIO are intimately associated with coral reef habitats, growing on sandy as well as hard-
substrate bottoms, and dominating the shallow lagoons and some shallow reef slopes throughout the region.
They may also grow sub-tidally in protected bays, estuaries and semi-enclosed sites, closely associated with
mangroves that dominate the intertidal zone along these coasts. Mangroves and seagrasses are the only two
groups of vascular plants that have adapted to life in saline water. The general ecology of seagrass ecosystems
and mangroves has been well covered elsewhere (see for example: Linneweber and Drude de Lacerda 2002;
Green et al. 2003; Hemminga and Duarte 2008; Hogarth 2015). For that reason, we will mainly focus our efforts
to discuss conditions specific to the WIO.
Distribution
Seagrass beds and mangrove forests are found throughout the WIO region. Of the estimated 1 million hectares
of mangroves in the WIO region, about 90% is found in Kenya, Tanzania, Mozambique and Madagascar
(UNEP/Nairobi Convention 2009). Thirteen seagrass species are reported along 12 000 km of the region’s
coastline (Gullström et al. 2013).
Description of benefits
Historically, less research has focused on the ecology and physiology of seagrasses compared to coral reefs
and mangroves. However, today the situation is changing, and research related to the ecology of seagrasses
and their role in the lifecycle of other marine organisms has increased dramatically (e.g. George et al. 2018;
Lyimo et al. 2018; Bouma & Infantes 2019; Meysick et al. 2019). Important research has also focussed on the
ability of seagrasses to adapt to climate change and the role of seagrasses in the carbon cycle e.g. the long-
term uptake of atmospheric carbon, referred to as blue carbon (Duarte et al. 2005, 2010; Kennedy et al. 2010;
Mcleod et al. 2011; Fourqurean et al. 2012; Alongi et al. 2016; Lyimo et al. 2018; Gullström et al. 2018).
Mangrove forests and seagrass meadows constitute multifunctional, high-productivity ecosystems that serve as
spawning and/or nursery habitats for a wide variety of fish and invertebrates (de la Torre-Castro & Rönnbäck
2004; Hughes et al. 2009; de la Torre-Castro et al. 2014; Nordlund et al. 2018), thus are key fishing grounds (de
la Torre-Castro & Rönnbäck 2004; Unsworth & Cullen 2010; Nordlund & Gullström 2013). In addition, some
birds, reptiles and even mammals depend on both habitats for their survival. For example, researchers have
identified 40 different fish species, 125 sessile species and 72 species of attached microalgae associated with
seagrass meadows (Moksnes et al. 2008). Mangroves and seagrasses are accordingly keystone habitats
providing ecosystems with high biodiversity (Duffy 2006; Blaustein 2008). Furthermore, these types of vegetation
along the coast can reduce wave height and energy (Lacy & Wyllie-Echeverria 2011; Manca et al. 2012) and
enhance the accumulation of sediment (Potouroglou et al. 2017). Sediment accumulation in seagrass meadows
is highly beneficial as it keeps sediment from leaving the shore (Lacy & Wyllie-Echeverria, 2011; Manca et al.,
2012; Duarte et al. 2013; Spalding et al. 2014), hence counteracting coastal erosion. The root systems of
mangroves and seagrasses bind the sediment and the shoots, while stems and leaves slow down water
movement (Ginsberg & Lowenstam 1958; Fonseca et al 1983; Saenger 2002), and protect shores from erosion.
Mangroves and seagrass meadows also have the ability to reduce turbidity, making the water clearer by lowering
the horizontal transport of suspended sediments and thus moderating sedimentation impacts on adjacent coral
reefs (Duke & Wolanski 2000; Schaffelke et al. 2005; De Boer 2007). However, human activities have increased
run-off from land and elevated water turbidity including the levels of nutrients, with negative impacts on the plants
and associated organisms (Mohammed 2002 Hamisi et al. 2004; Uku & Björk 2005; Hamisi et al. 2009; Huxham
et al. 2010). Elevated water temperatures caused by global warming have been reported to negatively influence
productivity of tropical seagrasses in the WIO region (George et al. 2018) and these negative effects are
exacerbated by local stressors e.g. eutrophication (Mvungi & Pillay, 2019). Such disturbances have also been

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shown to increase methane emission together with sulphide concentrations in the sediments (Lyimo et al., 2018),
where high levels of sulphide could enhance seagrass mortality due to sulphide toxicity.
Seagrass meadows have also been proposed as ocean acidification refugia because of their ability to respond
positively to changes in carbonate chemistry by storing more CO2 as Blue Carbon (Cyronak et al. 2018). This,
in turn, has the potential to locally reduce ocean acidification by increasing pH and aragonite saturation (e.g.
Yates et al. (2016). Such biogeochemical services provided to the coral reef system by not only seagrass
meadows but also mangrove, are highlighted particularly in Seychelles for the Indian Ocean (Camp et al. (2016)
and provide a strong conceptual framework for conservation efforts.
The protection and management of mangrove forests together with seagrass beds are categorised as one of
the most important elements of blue carbon strategies to mitigate climate change (Laffoley & Grimsditch 2009;
Nellemann et al. 2009; Duarte et al. 2013). Peat formation in mangroves comprises a substantial carbon sink
(Ong 1993), and in Kenya, seagrass beds are 4-6 times higher in sediment carbon compared with un-vegetated
controls (Githaiga et al. 2017).
Unfortunately, these habitats are today threatened (Waycott et al. 2009). The main reason for the loss of
mangrove vegetation and seagrass meadows along the coasts of the oceans is overharvesting, reclamation,
eutrophication and the use of destructive fishing techniques (Larkum et al. 1989; Stapel et al. 1996; FAO 2007;
Hamad et al. 2014). Not only does seagrass function as nurseries for a large diversity of marine species,
seagrass act as essential fish habitats contributing towards key fishing grounds (de la Torre-Castro & Rönnbäck
2004; Unsworth & Cullen 2010; Nordlund & Gullström 2013). Seagrass-based fisheries are extremely important
for coastal livelihoods as traditional small-scale fishing activities support millions of people (Barnes-Mauthe et
al. 2013; de la Torre-Castro et al. 2014). However, fishing methods such as bottom trawls, beach seines,
explosives, poisons and rakes are destructive to seagrass habitats (Nordlund et al. 2018).
With elevated sea levels, mangroves and seagrass meadows may, in suitable areas, be instrumental to mitigate
the effects of strong winds, tidal waves, sea surges, tsunamis, and shoreline erosion (Mazda et al., 2007; Godoy
& Lacerda, 2015). However, with sea-level rise, mangrove forests may be flooded and seagrass beds silted
over. On the other hand, with sufficient space, mangrove forests and seagrass beds, as well as other natural
habitats such as saltmarshes and the supralittoral zone with sand dunes may be able to migrate landward.
Unfortunately, today, the expansion of transformed urban and agricultural land further into coastal areas has led
to a coastal squeeze of these important natural habitats (Doody 2004). Consequently, in most cases the
mangrove and seagrass habitats will disappear, together with a number of associated ecosystem goods and
services.
Future Research Areas
As elevated sea levels, temperature, altered nutrient supply, and rainfall change at rapid rates, there is a great
risk of the biogeochemical cycles (for example the water, carbon and nitrogen cycle, etc.) being altered in these
natural systems. Pore-water or submarine groundwater discharge may play a major role in the exchange and
export of dissolved inorganic and organic carbon (DIC and DOC), nutrient and micronutrient availability at inland
bays and wetlands (Burnett et al. 2003). Understanding how these potential changes in hydrological,
geochemical and biochemical processes associated with changes in climate may affect mangrove and seagrass
biomass and productivity is starting to become particularly relevant (Maher et al. 2013).
Organisms have adapted to changing environments for millions of years in order to obtain a good phenotype-
environmental match. However, the present environmental change in combination with global climate change is
probably taking place at a higher pace than any time before, which requires a much faster response (e.g.
phenotypic plasticity) from marine organisms. To develop better and more science-based coastal management
procedures when, for example, allocating space for various human activities and marine protected areas
(MPAs), we need to understand the adaptive capacity among different populations within species, as well as
potential differences between species e.g. mangrove trees and seagrasses plant species. With a better
knowledge about evolvability and adaptive capacity, it would be possible to develop models and carry out
simulations in order to more accurately forecast future changes in species distribution and community changes.

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Efforts need to be made to understand both current taxonomy and microevolutionary processes if we are to
adequately model the specific responses of continually evolving subpopulations.
Opportunities for management and conservation
For the WIO region, it is important to remember that mangroves and seagrasses provide a wide range of coastal
ecosystem services but are also highly threatened arguably by an equally extensive list of issues. This
emphasizes the urgent need for improved management and a clear idea about the road forward.
Global climate change is contributing to elevated sea levels at an accelerating trend globally and is having
profound effects on coastal and urban areas in all continents, and the WIO region is no exception (Ngusaru et
al. 2000; Masalu 2002; Almström & Larsson 2008; Shaghude et al. 2015; Shaghude et al. 2018). In order to
prevent or mitigate the impacts of coastal erosion, various methods have been tried. In most cases hard ‘grey’
structures such as groins and seawalls have been constructed (Shaghude et al. 2015). However, such structures
may result in additional problems by shifting erosion alongshore and eventually eroding sandy beaches and
dune landscapes and causing siltation in other areas. During the last few decades, various forms of “green”
infrastructure have been trialled to manage coastal erosion effectively, with the added benefit of providing
numerous ecosystem services (Jones et al. 2012; Duarte et al. 2013; Munang et al. 2013; Spalding et al. 2014;
Temmerman et al. 2014). Mangrove vegetation and seagrass meadows are an integral part of these natural
shoreline protection solutions.
One area which has lacked in-depth research previously, is the natural interactions that exist between the marine
and terrestrial environment. Seashores with a diverse set of ecosystems including seagrass, mangroves as well
as rich beds of different algae and coastal forests, lead to a higher diversity and larger biomass of different plants
and insects in the coastal area (the supralittoral) (McLachlan & Brown, 2006). Rich shoreline and shallow water
plants and algal communities create a food web that connects the marine environment with the adjacent land. It
is therefore not only an exchange of sand and sediment but also a considerably exchange of particulate and
dissolved organic matter when plants and algae are washed up onto land and when animals from both habitats
migrate between the beach and the dunes searching for food (Defeo et al., 2009; Mellbrand et al., 2011). These
synergies contribute to conserving and/or further increasing biodiversity but have been largely under-studied at
local levels in the WIO.
Mangroves and seagrass beds in the WIO offer similar research-based management challenges as discussed
for coral reefs and later for fish. Probably the single most important improvement to enhance the quantity and
quality of research in the region would be to facilitate even stronger networks and communication among
research groups across countries in the WIO. With closer interactions among researchers, knowledge transfer
would improve, and research groups could agree upon standardized methods for basic field work procedures,
such as harmonised sampling designs to enable comparisons between location and/or among regions. Such
studies also need to be transferred, preferably to an open source regional database, where geographical
coordinates together with detailed GIS maps (Geographic Information System) can be stored and viewed for
any user. CORDIO’s Marine Spatial Atlas for the Western Indian Ocean (http://cordioea.net/outputs-
results/maspawio/) is a good example of how such service and interface could appear.
Fish
Key ecosystem characteristics and distribution
The coastline of the WIO is characterized by a wide diversity of habitats and biogeographic regions. The southern
extent of the WIO encompasses South Africa’s warm-temperate coastline. Here, habitats for fishes include salt
marsh and seagrass (in estuaries) as well as rocky reefs and mixed rocky and sandy shorelines. South Africa’s
east coast is classified as sub-tropical and, in this region, the southernmost mangroves and coral reefs are
found. This region is also the southernmost distribution limit for many tropical Indo Pacific fish species. The
coastlines of northern Mozambique, Tanzania, Kenya, Somalia and the island states of Comoros, Madagascar,
Reunion and Mayotte, Mauritius and Seychelles are tropical, with fringing coral reefs, extensive mangrove
forests and seagrass beds. The WIO supports a diverse fish assemblage with approximately 2200 fish species

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recorded (van der Elst et al. 2005; Kulbicki et al. 2013), representing 15% of the marine fishes found globally
(Smith & Heemstra 1986, van der Elst et al. 2005). The variety of habitats, geomorphological histories and
biogeographic regions in the WIO also gives rise to different structures of reef fish assemblages a high level of
endemism, particularly off the warm-temperate coastline of South Africa, with 13% of fish species endemic to
the WIO (van der Elst et al. 2005; Samoilys et al. 2019). Over 50% of the fish species here have an Indo-Pacific
origin (tropical), while 29% are deep sea species, 4% cosmopolitan and 3-4% Atlantic species (van der Elst et
al. 2005).
Description of benefits
The coastal zone in the WIO is rather densely populated, with an estimated three million people directly
dependent on fishing for their livelihood in this region (van der Elst et al., 2005; Temple et al. 2017). Overfishing
can reduce the adaptability of fish to climate variability and climate change (Hsieh et al. 2006) although very little
work has been done on the impact of climate change on the fish communities of the WIO, with the most data
available from South Africa (e.g. James et al., 2013, Potts et al., 2015).
A large proportion of WIO counties’ population live near the coastal zone with fisheries contributing to
employment and protein (van der Elst et al. 2005). Most of the fishers in the region are considered artisanal or
subsistence, rather than industrialized (van der Elst et al. 2005) thus generating accurate estimates of fisheries
importance is difficult but localized studies indicate their importance for livelihoods is substantial (McClanahan
et al. 2015). The high reliance on fisheries resources means that the effects of climate change on fish populations
can transcend resource productivity and result in serious food security and livelihood compromises (McClanahan
et al. 2015, Daw et al. 2009). There is thus an urgent need to accurately predict how climate change will affect
fisheries so adaptive management of the resource and the people that rely on it can be optimized (McClanahan
et al. 2015).
Impacts/threats of climate change
The effects of climate change on fish populations can be either direct or indirect (Breeggemann et al. 2016).
Direct effects of climate change on fish populations are physical changes in the ocean environment that influence
internal patterns and processes which modulate behaviour, demographic and distribution responses (Rijnsdorp
et al. 2009, Brander 2010). Changes in ocean temperature, oxygenation, and acidification are considered the
three most important direct physical environmental drivers on fish populations, and responses to these stressors
will determine resilience to future climate change (Gruber 2011, Pörtner 2012). Although direct effects of physical
environmental drivers on fish populations can often interact (e.g., Munday et al. 2009), the overarching
environmental driver is considered to be temperature because of the pervasive effect of temperature on
ectotherm physiological rates which regulate performance (Holt & Jørgensen 2014, Potts et al. 2015, Hoey et
al. 2016). Given that very little research has been conducted on the effects of climate change of fish and fisheries
within the WIO, the impacts of climate change on the fish of the WIO can mainly be inferred from global and
regional studies (Samoilys et al. 2019).
Indirect effects influence fish populations is via changes in the productivity or structure of ecosystem processes
that have cascading effects throughout ecosystem networks (Brander 2010). For example, in the northern
hemisphere, climate warming is changing the abundance and spatial distribution of zooplankton species in the
North Sea (Helaouët & Beaugrand 2007), which in turn will exert a bottom-up effect on Atlantic cod recruitment
within the same area (Beaugrand et al. 2003). In the northern Atlantic, northward movements of a number of
fishes has been recorded during the last couple of decades, involving species like mackerel, herring cod etc
(see e.g. Perry et al. 2005).
Studies investigating the effects of ocean acidification on fish suggest that the early life stages may be the most
vulnerable to ocean acidification (Melzner et al. 2009b, Baumann et al. 2012), although some species are more
tolerant than others and can emerge as “winners” (Ishimatsu et al. 2004, Munday et al. 2011). Deleterious ocean
acidification responses manifest through the negative effects on growth, development, metabolism, behaviour,
and ultimately, mortality and recruitment (Ishimatsu et al. 2008, Munday et al. 2010). In a pioneering ocean
acidification study on a South African linefish, Erasmus (2017) and Edworthy (2017) found reduced growth,
development, metabolism, and survival of post-flexion dusky kob (Argyrosomus japonicus) larvae at low pH

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levels of 7.78 predicted for the end of the century (2100). This reduced larval performance could ultimately result
in complete recruitment failure of the species if no evolutionary adaptation to low pH occurs. Direct effects of
ocean acidification on subadult and adult fishes are, however, believed to be minimal as compensatory
mechanisms can maintain intracellular acid-base balance in an acidic environment (e.g., Melzner et al. 2009a,
Haigh et al. 2015). Low oxygen zones can also affect fish populations through reductions in available habitat or
altered metabolic/physiological processes, that can ultimately result in increased mortality (Stramma et al. 2010).
Temperature and oxygen are intrinsically linked in the ocean, as rising temperatures will reduce the
concentration of oxygen while simultaneously increasing oxygen demand in organism tissues (Deutsch et al.
2015).
Habitat loss associated with climate change can also affect the composition of fish communities. Chabanet
(2002) found that two years after the 1998 coral bleaching event, fish communities on coral reefs in Mayotte
were dominated by herbivores as opposed to browsers of sessile invertebrates. An increase in the frequency
and intensity of extreme events may also result in the loss of essential nursery habitats for coastal fish such as
mangroves, seagrass and salt marsh (James et al. 2013). Mann and Pradervand (2007) in subtropical South
Africa found a close relationship between the abundance of adult fish in the marine environment and the
availability of estuarine nursery habitats, with the abundance of adults decreasing when nursery habitats were
lost.
The response of fish populations to changes in ocean temperature can be broken down into four broad
components; behaviour, phenology, distribution, and demography (Rijnsdorp et al. 2009, Potts et al. 2015,
Poloczanska et al. 2016). Behaviour modifications are often a first response to ocean temperature changes as
fish may seek to avoid unfavourable temperatures (Rijnsdorp et al. 2009). Behaviours such as swimming speed
or foraging behaviour are temperature sensitive (Brownscombe et al. 2014, Johansen et al. 2014) and fish may
seek temperatures that maximise these behavioural processes or adjust behaviours when temperatures are
sub-optimal (Freitas et al. 2015). Temperature-induced behavioural modifications can affect fish population
phenology (the timing of cyclical events), as temperature is often a cue for seasonal migrations (Sousa et al.
2016), diel migrations (Keyser et al. 2016) and spawning migrations (Sims et al. 2004). A global analysis of sea
surface temperature trends by Lima and Wethey (2012) identifies the coast off South Africa as having some of
the highest advances in seasonal warming. When temperature stressors are chronic over time, these short-term
behavioural and phenology responses can manifest into negative impacts on the abundance and productivity of
fish populations (Crozier et al. 2008).
Over longer timescales, changing ocean temperatures can drive distributional shifts in fish populations across
latitudes or depths (Pecl et al. 2017). For example, Perry et al. (2005) show that nearly two-thirds of North Sea
fishes responded to warming temperatures by shifting their distribution either deeper or to higher latitudes. In
the warm-temperate region of South Africa, James et al. (2008) reported an increased occurrence of certain
tropical fish species in temperate estuaries. Similarly, in a 19-year study of a sub-tropical reef fish community in
South Africa, Llyod et al. (2012) found a general increase in the abundance of tropical species as well as a
change in the proportion of tropical versus temperate species. These findings were attributed to warming of the
Agulhas Current. Temperature also affects demographic processes like growth, mortality and recruitment, such
that even small changes in temperature can have big impacts on population biomass and abundance (Brander
2010, Poloczanska et al. 2016). For example, temperatures that exceed the narrow thermal tolerance of fertilised
eggs may lead to faster larval development, resulting in increased mortality and reduced recruitment into
fisheries (Pankhurst & Munday 2011). Warming temperatures can also result in faster juvenile and adult growth
rates, which alter fish production (Audzijonyte et al. 2016). Ultimately these behavioural, phenological,
demographic, and distributional changes affect the abundance, productivity, and distribution of fish populations
that the human population relies on for goods and services (Brander 2007).
Future Research Areas
Much of the leading work on this subject has come from South Africa, partly because of the sharp gradients in
temperature facilitating detection of changes in distributions over short times and distances, as well as a longer
history of studies of fish species and assemblages. Research on climate impacts on fish will require complex
laboratory-based experimental studies, as well as in-situ ecological, biological and environmental data collection.

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In order to generate baseline data, long-term monitoring programmes need to be initiated, particularly in data-
poor counties and in areas situated at the boundary of species distributional ranges, to better assess the impacts
of climate change on fish assemblages. These programmes should monitor the relative abundance of fishes as
well as the stability and resilience of fish assemblages.
With a myriad of potential changes likely, an assessment of key species climate resilience, defined as the ability
to resist or recover from a climate-related stressor (Côté & Darling 2010, Hodgson et al. 2015), is also a research
priority. Determining fish population resilience to climate change has generally been studied through correlations
between environmental variables and historical population responses (Horodysky et al. 2015). Correlative
studies mostly take a “black box” approach and do not consider the underlying mechanisms that govern
population responses (Helmuth 2009), which can make forecasting responses unreliable (Horodysky et al.
2015). Furthermore, assigning biological responses of populations to climate change is challenging, given the
complexity of interacting mechanisms, variable species responses, and the potential of cascading effects
(Poloczanska et al. 2016). While it is still useful to assess past responses to climate change, incorporating
experimentally derived information on the sensitivity of underlying processes can improve inference and
forecasting accuracy (Wernberg et al. 2011).
The incorporation of physiology into the assessment of vulnerability or resilience of fishes to climate change is
relatively new but holds great promise as a method to develop a suite of more appropriate management tools
(McKenzie et al. 2016). For example, in Papua New Guinea, Rummer et al. (2014) used metabolic physiology
to identify fish species vulnerable to climate change, while Fitzgibbon et al. (2014) used metabolic physiology to
identify the most vulnerable life history stage of spiny lobster (Sagmariasus verreauxi). Another recent global
fisheries example of the incorporation of physiological research into adaptive management plans is the Fraser
River sockeye salmon (Oncorhynchus nerka) (Patterson et al. 2016). Physiological research involved identifying
the impacts of high water temperature and high river discharge in terms of thermal tolerance, energy metabolism
and respiratory capacity, which among other factors, influence successful spawning migrations, and adjusting
catch limits when appropriate (Clark et al. 2010, Eliason et al. 2011, 2013). In the WIO, future research
quantifying the biological effect of climate change on fish populations must therefore take a species-specific
process-based, physiological approach.
The way forward
We have discussed three key marine organism groups and ecosystems that are instrumental in providing
important ecosystem goods and services for local communities and coastal economies in the Western Indian
Ocean. To date, for these systems, there have been varied efforts in studying the different climate impacts
across geographies as well as thematically in terms of the types of threats.
The current evidence suggests that temperature increases due to global warming will have the widest ranging
and most immediate impact across the marine environment and is therefore justifiably the most widely studied
to date, but significant investment still needs to be made on the other climate threats e.g. acidification, to
understand their full effects. This will help identify and develop more holistic strategies and actions to mitigate
against global climate change in the region.
Although a lot of progress has been made in the last decades, a common theme raised among all research
communities, regardless of study system, is that research conducted within the WIO needs greater collaboration
and partnerships amongst universities, laboratories, institutions, research groups, NGOs, and most recently the
use of citizen science. This should enhance learning across and within fields and facilitate the transfer of
knowledge, technology, experiences and data in order to expand the scope and number of studies. Additionally,
improved standardisation of sampling designs and methodologies, in particular for studies of climate change
where monitoring studies are an important part, would not only provide important baseline data, but also enable
detection of minor changes in ecological communities. Increased collaboration could also help fill a key research
gap on the associated or cross-effects of climate effects across ecosystems (cascading impacts).
To address the numerous research areas related to climate change and provide a comprehensive study of
biological and ecological responses of organisms, the physical environment and habitat condition to these
effects, a holistic and varied number of scientific techniques will be required. This includes genetic and molecular

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analyses, laboratory experiments and in-situ observations. For instance, information about genetic variation
within populations can provide valuable insights of species and populations uniqueness and their evolvability in
order to decide if certain populations should be designated as Evolutionary Significant Units (ESU) (Conner and
Hartl 2004). Currently, it would not be feasible to establish several advanced laboratories in the WIO due to the
high financial and human capacity requirements. Core facilities that have the expertise and equipment to conduct
advanced molecular analysis should be developed at a few key locations in WIO where the infrastructure is
already relatively well developed. Exchange programs from across the WIO to these centres of excellence will
enable students to learn specialist techniques e.g. molecular tools, with the aim of establishing new or
progressively improving existing laboratories and research station facilities in their home countries
Finally, how ecosystems will be affected is not only dependent on the direct effects of the perturbations on
individual species but also on the potential unequal pressure across trophic levels which could lead to
imbalances in the trophic structure, causing trophic cascades that may change the dynamics of the ecological
community (Ober 2016). If we are to fully understand the complex effects of a changing climate, this is a key
area to consider.
Conclusion
The WIO has an established group of researchers in government and private research institutions working
across different geographic areas and fields. There are also active regional monitoring networks e.g. GCRMN,
associations like WIOMSA which can coordinate regional collaboration, as well as regional inter-governmental
policy frameworks e.g. UN Nairobi Convention. The marine science sector in the WIO is therefore at a point
where there is potential to significantly grow and improve with the right investment. The UN Decade of Ocean
Science starting in 2020 provides a timely opportunity for this to happen. At the same time government interest
and intention to invest in the Blue Economy requires complementary investment in monitoring and research of
ecosystems and other resources, as climate impacts will underpin the benefits that are obtained from the ocean.
This review highlights that there will most certainly be both biological and human “winners” and “losers” in certain
areas of the WIO, it is our contention that understanding, and perhaps successfully predicting, ecological
responses to global climate change will be aided greatly by a deeper investment in a multitude of research areas
such as field ecology, genetics and social science, as well as investing in mechanisms to increase collaboration
and transfer expertise across countries, institutions and subject areas.

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