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Three lines of evidence to link outbreaks of the crown-of-thorns seastar Acanthaster planci to the release of larval food limitation

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Abstract and Figures

Population outbreaks of the coral-eating crown-of-thorns seastar, Acanthaster planci, continue to kill more coral on Indo-Pacific coral reefs than other disturbances, but the causes of these outbreaks have not been resolved. In this study, we combine (1) results from laboratory experiments where larvae were reared on natural phytoplankton, (2) large-scale and long-term field data of river floods, chlorophyll concentrations and A. planci outbreaks on the Great Barrier Reef (GBR), and (3) results from A. planci—coral population model simulations that investigated the relationship between the frequency of outbreaks and larval food availability. The experiments show that the odds of A. planci larvae completing development increases ~8-fold with every doubling of chlorophyll concentrations up to 3μgl−1. Field data and the population model show that river floods and regional differences in phytoplankton availability are strongly related to spatial and temporal patterns in A. planci outbreaks on the GBR. The model also shows that, given plausible historic increases in river nutrient loads over the last 200years, the frequency of A. planci outbreaks on the GBR has likely increased from one in 50–80years to one every 15years, and that current coral cover of reefs in the central GBR may be 30–40% of its potential value. This study adds new and strong empirical support to the hypothesis that primary A. planci outbreaks are predominantly controlled by phytoplankton availability. KeywordsCrown-of-thorns starfish-Seastar-Trophic limitation-Great Barrier Reef- Acanthaster planci -Eutrophication-Phytoplankton-Chlorophyll
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Three lines of evidence to link outbreaks of the crown-of-
thorns seastar Acanthaster planci to the release of larval food
K. E. Fabricius
*, K. Okaji
, G. De’ath
1 Australian Institute of Marine Science, PMB No. 3, Townsville MC, Qld 4810, Australia.
2 Coralquest Inc., Asahicho 1-34-10, Atsugi, Kanagawa 243-0014, Japan
* Corresponding author: , phone: +61-747534412, fax +61-747725852.
Running title: A. planci outbreaks and phytoplankton
Key words: Crown-of-thorns starfish, seastar, trophic limitation, Great Barrier Reef, Acanthaster planci,
eutrophication, phytoplankton, chlorophyll.
Population outbreaks of the coral-eating crown-of-thorns seastar, Acanthaster planci, continue to kill
more coral on Indo-Pacific coral reefs than other disturbances, but the causes of these outbreaks have
not been resolved. In this study we combine (1) results from laboratory experiments where larvae were
reared on natural phytoplankton, (2) large-scale and long-term field data of river floods, chlorophyll
concentrations and A. planci outbreaks on the Great Barrier Reef (GBR), and (3) results from A. planci
coral population model simulations that investigated the relationship between the frequency of
outbreaks and larval food availability. The experiments show that the odds of A. planci larvae
completing development increases ~8-fold with every doubling of chlorophyll concentrations up to 3
µg L
. Field data and the population model show that river floods and regional differences in
phytoplankton availability are strongly related to spatial and temporal patterns in A. planci outbreaks
on the GBR. The model also shows that, given plausible historic increases in river nutrient loads over
the last 200 years, the frequency of A. planci outbreaks on the GBR has likely increased from one in
50 80 years to one every 15 years, and that current coral cover of reefs in the central GBR may be 30
40% of its potential value. This study adds new and strong empirical support to the hypothesis that
primary A. planci outbreaks are predominantly controlled by phytoplankton availability.
On most Indo-Pacific coral reefs, including the Great Barrier Reef (GBR), coral cover has
been declining at rates of 0.2 1.5% per year since the 1960s (Bruno and Selig 2007). To
date, predation of coral by crown-of-thorns seastar (Acanthaster planci) accounts for a large
proportion of the observed decline in coral cover on the GBR. Between 1985 and 1997,
population outbreak of A. planci were observed on ~32% of monitored reefs on the GBR,
with their coral cover averaging 9% one year after the outbreak, compared with a mean of
28% coral cover on reefs that had not experienced an outbreak in the same period (Lourey et
al. 2000). These figures suggest a GBR-wide reduction in coral cover of 0.5% yr
due to A.
planci alone in this 12 years period. Population outbreaks of A. planci, i.e., the sudden
emergence of a large population after a period of relative rarity (Moran 1986), were first
recorded throughout the Indo-Pacific in the 1960s. The abrupt population increase by orders
of magnitude from a small parent population is called a ‘primary outbreak’ (Birkeland and
Lucas 1990), and questions concerning the cause(s) of such primary outbreaks, and whether
or not human activities have changed their frequency, have to date remained unresolved. In
contrast, secondary outbreaks are simply the consequence of the large numbers of gametes
Coral Reefs (2010): Volume 29, pp 593-605: A. planci outbreaks and phytoplankton
produced upstream by a primary outbreak or another secondary outbreak population. Such
secondary outbreaks have been reconstructed using hydrodynamic models (Moran 1986;
Dight et al. 1990).
There are many hypotheses that relate to the control of A. planci populations (reviewed in
Birkeland and Lucas 1990; Brodie et al. 2005). Echinoderms that release large numbers of
planktotrophic larvae such as A. planci have a propensity to population fluctuations (Uthicke
et al. 2009), and some primary A. planci outbreaks have been recorded even on coral reefs
that remained relatively unaltered by human activities (Birkeland 1982, Birkeland and Lucas
1990). However, the observed widespread decline in coral cover since the 1960s suggest that
the recently observed frequency of primary outbreaks of once in ~15-years is unsustainable
(Seymour and Bradbury 1999). Even before mass bleaching started to inflict additional
mortality, most reefs were estimated to take 10 25 years for full recovery of coral cover,
with 25% of reefs showing no signs of recovery from A. planci coral mortality (Lourey et al.
2000). Two hypotheses that specifically address the apparent increase in the frequency of
primary outbreaks have been widely debated. They are: (1) the ‘terrestrial runoff hypothesis’
aka ‘larval starvation hypothesis’ that argues that nutrient limited survival of the pelagic
planktotrophic larvae of A. planci controls population outbreaks (Birkeland 1982; Lucas
1982; Brodie et al. 2005), and (2) the ‘predator removal hypothesis’, which postulates that
more juveniles survive to maturity due to the removal of fish predators through human
exploitation (reviewed in Birkeland and Lucas 1990). Both hypotheses are based more on
circumstantial than empirical support.
The terrestrial runoff hypothesis has strong correlative evidence (reviewed in Brodie et al.
2005). More outbreaks occur on reefs near high Pacific islands or continental coasts from
which terrestrial runoff occurs, compared to low atoll islands without terrestrial runoff, and
most outbreaks follow large or drought-breaking floods that carry high nutrient and sediment
loads (Birkeland 1982). The apparent increase in A. planci outbreak frequencies is attributed
to increased nutrient levels resulting from the terrestrial runoff of fertilizers, sewage and
eroding soils, as recorded throughout the Indo-Pacific in modern times (Brodie et al. 2005).
The planktotrophic larvae of A. planci feed on nano- and microphytoplankton (>3 µm cells)
(Okaji et al. 1997) that multiply at high nutrient levels. Earlier feeding experiments with
cultured microalgae showed that larval development was optimal at 2 6.5 µg L
chlorophyll, while few larvae completed their development at <0.6 µg L
chlorophyll (Lucas
1982). However, the types of cultured microalgae also determined the developmental success
of A. planci larvae (Lucas 1982), limiting inferences from these experiments about larval
development in natural phytoplankton communities.
The predator removal hypothesis states that seastar populations are largely controlled by
predation, and that increased human exploitation of fish predators has resulted in increased
numbers of seastars surviving to maturity (McCallum 1987). Dulvy et al. (2004) have
suggested a relationship between A. planci outbreaks and human population density (but not
fish predator densities) on 7 of 13 investigated reefs in Fiji, and Sweatman (2008) has shown
a relationship between reef protection status and A. planci outbreaks on pairs of reefs open
and closed to fishing in the GBR. However, the more commonly fished large predatory fish
don’t usually prey upon A. planci (Sweatman 1995) and to date, no fish predator has been
identified that can effectively regulate A. planci populations, although predation on juveniles
undoubtedly occurs (Keesing et al. 1996). It is argued (but difficult to show empirically) that
the removal of large predators may (1) suppress prey switching behavior, i.e., the fewer large
predatory fish stop eating the less preferred A. planci, and/or (2) have caused some complex
Coral Reefs (2010): Volume 29, pp 593-605: A. planci outbreaks and phytoplankton
trophic cascades, eventually resulting in fewer bottom-dwelling invertebrates that eat juvenile
seastar (Keesing et al. 1996; Sweatman 2008).
In this study we re-examine the contribution of terrestrial runoff to limiting the frequency of
primary outbreaks, by combining new empirical evidence from A. planci larval feeding
experiments, large-scale and long-term river flood and chlorophyll monitoring data for the
GBR, and model-based simulations of the population dynamics of A. planci and its main
drivers. Extensive long-term and large-scale coral reef, water quality and river monitoring
data are now available for the GBR (Brodie et al. 2007; Sweatman et al. 2008), thereby
providing a new opportunity to resolve the A. planci issue that affects the health of many
Indo-Pacific coral reefs.
Laboratory Experiments
Eight separate laboratory experiments were used to quantify rates of development and
survival of larvae reared at different concentrations of natural phytoplankton as detailed in
Okaji (1996). Seawater was collected from the ocean and filtered through a 25 µm mesh to
remove large zooplankton and detrital matter. This coarsely filtered seawater was used to
create fresh batches daily of the following treatments:
(a) 0.45-FSW: filtration through a 0.45 µm GF/B filter;
(b) 2-FSW: filtration through a 2 µm polycarbonate membrane filter;
(c) 25-FSW: no further treatment of the coarsely filtered seawater;
(d) NES: nutrient-enriched seawater: 2 ml of Guillard’s f/2 nutrient solution (Guillard 1975)
were added to 20 L of freshly collected and coarsely filtered seawater. This nutrient-
enriched seawater was incubated in a 500 L water bath outdoors without shading for 2
days prior to use to develop the phytoplankton communities. Nutrient enrichment
increased the concentration of eukaryotic phytoplankton cells and the concentration of
chlorophyll on average 20-fold in NES compared with 25-FSW, while the concentration
of cyanobacteria cells increased to a lesser extent (2- to 3-fold, Table 1).
The treatment levels of the 8 experiments are listed in Table 1. Each treatment was done in
triplicates for Experiments 1 to 6, and in duplicates for Experiments 7 and 8. Experiments 1,
2, 5 and 6 were conducted at Lizard Island Research Station in the northern Great Barrier
Reef (GBR), which is surrounded by clear offshore waters. Experiments 3 and 4 were
conducted at the University of the Ryukyus (Okinawa, Japan), using water sampled from the
front of Chatan Reef, Okinawa, from often clear coastal waters. Experiments 7 and 8 were
conducted at the Australian Institute of Marine Science (central GBR), with the often turbid
coastal seawater collected off Cape Bowling Green once a week and stored outdoors in
aerated 500 L tanks. For Experiments 1 to 4, duplicate seawater samples were taken from
each container every day, and for Experiments 5 and 6 every second day. The chlorophyll a
concentration of these samples was determined using fluorometry, and the densities of
eukaryotic algal and cyanobacteria cells were counted with an epifluorescence microscope
(Table 1). In Experiments 7 and 8, chlorophyll a of the NES was determined fluorometrically
before use, and NES was diluted with 0.45-FSW to obtain the final chlorophyll
concentrations. The use of total chlorophyll a concentrations overestimates food availability
Coral Reefs (2010): Volume 29, pp 593-605: A. planci outbreaks and phytoplankton
Table 1: Mean percentage of surviving A. planci larvae that completed their development (late
brachiolarian stage or metamorphosed to juveniles) at age 22 days, in treatments with
contrasting concentrations of natural phytoplankton. Treatments: 0.45-FSW, 2-FSE, and 25
FSE = seawater filtered using 0.45, 2 and 25 µm filters, respectively; NES = nutrient enriched
Chl. a
g L
± SD)
Eukaryote density
± SD)
± SD)
(% of survivors ±
0.07 ± 0.03
(not detected)
25 ± 16
0 ± 0
0.17 ± 0.10
0.004 ± 0.006
55 ± 36
0 ± 0
0.40 ± 0.20
0.214 ± 0.112
65 ± 36
0 ± 0
0.08 ± 0.03
(not detected)
31 ± 16
0 ± 0
0.25 ± 0.11
0.004 ± 0.004
75 ± 34
0 ± 0
0.52 ± 0.21
0.234 ± 0.086
83 ± 30
0 ± 0
0.08 ± 0.03
0.163 ± 0.125
6.4 ± 5.9
0 ± 0
0.29 ± 0.10
0.437 ± 0.222
7.1 ± 6.1
18.7 ± 8.5
0.28 ± 0.08
0.385 ± 0.178
6.7 ± 5.1
0 ± 0
0.19 ± 0.10
0.004 ± 0.002
56 ± 40
0 ± 0
0.28 ± 0.10
0.207 ± 0.077
62 ± 41
88.3 ± 8.6
50% NES
2.91 ± 1.35
2.435 ± 0.564
142 ± 90
100 ± 0
100% NES
5.25 ± 2.32
4.441 ± 0.989
202 ± 157
100 ± 0
0.19 ± 0.10
0.004 ± 0.002
56 ± 40
0 ± 0
0.28 ± 0.10
0.207 ± 0.077
62 ± 41
97.2 ± 1.7
50% NES
2.91 ± 1.35
2.435 ± 0.564
142 ± 90
99 ± 0.7
5.25 ± 2.32
4.441 ± 0.989
202 ± 157
100 ± 0
0 ± 0
0 ± 0
0 ± 0
32.2 ± 0.5
50.2 ± 26.6
0 ± 0
0 ± 0
6.8 ± 0.9
38.6 ± 4.1
61.6 ± 4.4
for A. planci, because the contribution of nano- and microphytoplankton, the preferred food of
A. planci larvae, is typically <50% of chlorophyll a in Indo-Pacific waters (Charpy and
Blanchot 1999; Crosbie and Furnas 2001). The rest is picoplankton (<3 µm cells) that
constitutes <10% of the diet of A. planci larvae (Okaji et al. 1997). However, the use of
natural phytoplankton communities in our experiments, in which the relative contribution of
nano- and microphytoplankton to chlorophyll reflects that found in the field, enabled us to
relate the experimental results to the GBR long-term chlorophyll data.
Batches of A. planci larvae were reared in the laboratory (Okaji 1996). Actively swimming
and healthy early bipinnaria larvae with fully developed alimentary canal were collected 2
days after the in vitro fertilization of gametes, and 100 larvae (150 larvae in Experiments 7
and 8) were added to each treatment chamber (1 litre volume for Experiments 1 and 2; 2 litres
for all others), which were gently aerated and kept in the temperature range 26.5 29C.
Coral Reefs (2010): Volume 29, pp 593-605: A. planci outbreaks and phytoplankton
Larvae were sieved with a 60 µm mesh and transferred to a clean set of containers of freshly
prepared seawater every day. Every second to forth day, larvae were individually examined
under a dissecting microscope, and their developmental stages recorded following Lucas
(1982). For Experiments 7 and 8 the body lengths of 10 to 20 randomly selected larvae per
chamber were measured along their longest axes every fourth day with an ocular micrometer.
When the first larvae reached brachiolaria stage, aeration was reduced and a few small chips
of crustose coralline algae were introduced as settlement substratum. These chips were
checked daily in order to count metamorphosing larvae and settled juveniles.
The diameters of juvenile seastar after completion of metamorphosis were measured with an
ocular micrometer in Experiment 5. The rate of successful completion of development was
defined as the proportion of surviving larvae that were either in late brachiolaria or juvenile
stages after 22 days. These two stages were combined since late brachiolaria larvae are
competent to metamorphose to juveniles. Absolute survivorship was not analyzed because
abnormal or regressed larvae can remain alive for extended periods of time without any
prospect of further development (‘living ghosts’), and there were no rigorous criteria to
distinguish these from healthy larvae. Clearly regressed or abnormal larvae were scored as
For the statistical analyses, the results of all 8 experiments were combined (Table 1). The
relationship between rate of successful completion of development after 22 days and
chlorophyll concentration was analyzed using a generalized linear mixed model; a logistic
regression model where the response was the proportion of successful developmental
completion. Two severe outliers (88% and 97% survivorship in E5, 25-FSW and E6, 25-
FSW; Table 1) were excluded from the final model. Runs in Experiment 2 that were
terminated after 18 days due to a lack of development in all treatments were scored as ‘zero
completion’. All analyses were done using the statistical software package R (R Development
Core Team 2009).
GBR Flood History and Chlorophyll Data
Data of the cumulative discharge volumes of the Burdekin River since 1922 and the five
largest Wet Tropics rivers (Herbert, Tully, Johnstone, Russell and Barron Rivers, latitude
16.5 18.5S) were obtained from the Queensland Department of Environment and
Resource Management, and the values for the Burdekin River plotted for each ‘water year’
October 30
September). The Burdekin River is the largest river entering into the GBR
lagoon, and the most important factor determining inter-annual variability in flood plumes,
since the annual discharge from its dry subtropical catchment varies by two orders of
magnitude between its wettest and driest years (long-term annual mean discharge: 8.5 km
; CV = 106% of annual mean). In contrast, discharges from the many annually flooding
rivers in Wet Tropics catchments, which jointly supply ~40% of the total annual runoff to the
GBR (Furnas 2003), vary less between years (CV = 34 37% of annual means for Tully,
Johnstone and Russell River, and 75 76% for the Herbert and Barron Rivers). Continuous
monitoring of the Burdekin River started in 1922, and of the Wet Tropics Rivers between
1967 and 1983.
Data of summer chlorophyll a concentrations were extracted from the GBR long-term
Chlorophyll Monitoring Program Data Base, which has sampled chlorophyll concentrations
along fixed transects across the continental shelf monthly since 1993 (Brodie et al. 2007).
Most A. planci on the GBR spawn in December to January (Lucas 1973; Babcock and Mundy
1992). Under experimental conditions, the duration of the pelagic larval phase is 12 to 22 days
Coral Reefs (2010): Volume 29, pp 593-605: A. planci outbreaks and phytoplankton
for well-fed larvae (Birkeland and Lucas 1990), and >50 days for larvae reared in filtered
seawater (Lucas 1982). Thus chlorophyll records from November to March were used to
quantify long-term average summer values for the far northern (FN, 12.0 15.0S) and
central/northern regions (CN, 15.1 19.2S). The data from each region were further split
into the inner <25 km of the shelf (containing inshore and midshelf reefs in CN and FN) and
outer locations, as most small to medium-sized river flood plumes remain within the inner 25
km of the shelf and travel along the coast towards the north due to the prevailing
hydrodynamic patterns and Coriolis forcing (King et al. 2001; Devlin and Brodie 2005).
Differences in the probabilities of larvae completing their development were then calculated
for FN and CN based on the estimated differences in larval survival rates for given
chlorophyll levels using the response curve from the laboratory experiments and long-term
average chlorophyll concentrations in the GBR.
The A. planci Coral Simulation Model
The model was used to simulate spatial-temporal distributions of A. planci on the GBR. By
varying the drivers and parameters of the model, running various scenarios and conducting
sensitivity analyses, we investigated the temporal dynamics and the spatial patterns of the
outbreaks. The simulation model comprised four linked sub-models:
(1) The A. planci model: This is an age-structured meta-population model with two juvenile
and six adult stages, each of one year duration. Life history parameters included age-
dependent size, rates of survival across age cohorts, age-dependent fertility and feeding rates
on corals (Table 2; Moran 1986; Birkeland and Lucas 1990; Scandol 1993).
(2) The coral model: The parameters included the rates of growth of coral as the prey of
juvenile and adult A. planci (Scandol 1993; Sweatman et al. 2001; Wolanski and De’ath
(3) The chlorophyll model: The spatial-temporal variation in chlorophyll was based on data
from the GBR Long-Term Chlorophyll Monitoring Program (Brodie et al. 2007). Compared
to long-term change, temporal variation was large on seasonal and short time scales. Both
observed chlorophyll data (i.e., field observations) as well as simulated data including
gradients and spikes originating from floods were used in the model.
(4) The connectivity model: The connectivity of a reef to other reefs determines its capacity
to provide larvae to itself (i.e., to self-seed), to other reefs (i.e., be a source), and to receive
larvae from other reefs (i.e., be a sink). Hydrodynamic models provided estimates of the self-
seeding, source and sink levels for 321 reefs in the central and northern GBR (James et al.
A justifiable criticism of population modeling is that, given enough parameters, one can
reproduce most observed data. We have safeguarded against this problem by (1) focusing on
relative not absolute effects (determining the ratios and 90% confidence ranges of values for
A. planci and corals for FN and CN), and (2) using sensitivity analyses: The robustness of the
model assumptions were tested by adding various levels of stochastic noise to any of the A.
planci and coral life history variables, and by varying many of the model parameters within
reasonable ranges (e.g., variations in the rates of coral replenishment and consumption, and
juvenile and adult survival and fecundity). The models and all analyses of outputs were
programmed in the statistical software package R (R Development Core Team 2009).
Coral Reefs (2010): Volume 29, pp 593-605: A. planci outbreaks and phytoplankton
a) Laboratory experiments
The laboratory experiments, in which freshly hatched A. planci larvae were reared in seawater
at 0.01 to 5.25 µg L
chlorophyll a, showed that the proportion of larvae completing their
development increased rapidly with increasing natural phytoplankton concentration (Fig. 1).
At 0.01 0.25 µg L
chlorophyll, few larvae developed from the bipinnaria to early
brachiolaria stage, none developed beyond the early brachiolaria stage, and most regressed at
days 10 to 14. The odds of completion of development increased by a factor 8.3 (95% CI =
4.7, 17.7) for each doubling of concentrations of chlorophyll (Fig. 2a, Table 1). At low to
moderate chlorophyll concentrations (<0.5 µg L
), this was equivalent to increasing the
probability of completing development by a factor 7 8 for each doubling of chlorophyll. At
higher concentrations, the rate of increase in the probability of completion slowed, and
plateaued at >3 µg L
where completion was certain.
Figure 1: Developmental success of A. planci larvae that were exposed to different phytoplankton concentrations
in 8 experiments (E1 E8). Larval developmental stages: white = bipinnaria, wide diagonal hatch= early
brachiolaria, narrow diagonal hatch = mid brachiolaria, grey shade = late brachiolaria, black = metamorphosed
juvenile seastar. The latter two stages together were scored as ‘completed development’. Abbreviations: 0.45-
FSW, 2-FSE and 25 FSE = seawater filtered using 0.45, 2 and 25 µm filters; NES = nutrient enriched seawater.
Growth rates, developmental speed and final body sizes of the larvae and early seastar also
depended on the availability of phytoplankton. Larvae reared at ≥0.8 µg L
reached the maximum observed size of 1.2 1.3 mm at 17 to 20 days of age, suggesting
growth was not food limited (Fig. 2b). At <0.5 µg L
chlorophyll, larvae initially grew from
0.8 to 1.0 mm but growth arrested after days 12 to 15 (Fig. 3). The time for 50% of surviving
larvae to complete development decreased from 41 to 14 days when chlorophyll increased
from 0.5 to >2 µg L
(not shown). Similarly, larvae that developed at 0.28 µg L
metamorphosed into seastars with significantly smaller mean diameter (0.44 mm, SE = 0.07
mm) than larvae reared at 2.9 or 5.2 µg L
chlorophyll (0.66 mm, SE = 0.05 mm; 0.64 mm,
SE = 0.09 mm).
Coral Reefs (2010): Volume 29, pp 593-605: A. planci outbreaks and phytoplankton
Combining the data on rates of developmental completion and growth suggests the following
chlorophyll thresholds. At <0.25 µg L
(<220 eukaryotic cells m L
; Table 1), a negligible
proportion of larvae complete development, suggesting starvation. At 0.25 0.8 µg L
670 eukaryotic cells mL
) this proportion is moderate, but development is slow and body
sizes of larvae and juveniles remain small, suggesting severe food limitation. Finally, at >2 µg
(>1700 eukaryotic cells mL
) larval developmental success is high, developmental speed
is fast, and both larvae and juveniles grow to their maximum observed size, suggesting release
from trophic limitation.
Figure 2: (a) Relationship between chlorophyll a concentration and the proportion of A. planci larvae completing
their development. (b) Body length of A. planci larvae at 17-20 days of age. Each point represents the mean
results of duplicate or triplicate deployments per treatment. Black lines are model fits, the thin black lines are 2
SE of the mean.
Figure 3: Patterns of growth of A. planci larvae at increasing concentrations of chlorophyll a (Experiments 7 and
8). Shrinkage observable around days 16 to 20 reflects contraction for metamorphosis.
b) Temporal and spatial correlations between chlorophyll availability and A. planci
primary outbreaks
The patterns in Burdekin River discharges showed strong temporal and spatial agreement with
the timing and location of primary outbreaks of A. planci in the GBR. On the GBR, primary
outbreaks were first observed at 16.75S in 1962 and 1979, and between 14.7 16.1S in
Coral Reefs (2010): Volume 29, pp 593-605: A. planci outbreaks and phytoplankton
1993/94 (Moran et al. 1992; Miller 2002; Sweatman 2008). The three largest recorded floods
of the Burdekin River yielded freshwater discharges of 28, 54 and 40 km
in 1958, 1974 and
1991 (Fig. 4). In 1974, all Wet Tropics rivers except the Herbert also produced >90
percentile floods, and in 1991, the Herbert and Barron produced >90
percentile floods and
the other three rivers were above median levels. Therefore, the 1979 and 1994 outbreaks
occurred three to five years after the two wettest years on record. The 1962 outbreak is
difficult to interpret since the 1958 flood occurred in February-March and hence may have
been too late in the season to feed A. planci larvae, and because few data exist from the Wet
Tropics rivers. A large flood also occurred very early in the 1950/51wet season, but no A.
planci data exist from that period (Fig. 4). The region north of Latitude 16.75S is the only
section of the whole GBR where the dense matrix of large mid- and outer-shelf reefs such as
Green Island frequently encounters river plumes (Fig. 5; Devlin and Brodie 2005; Brodie et
al. 2005).
Figure 4: Cumulative discharge volumes of the
Burdekin River into the GBR for each year since
1922. Red lines indicate the three large floods that
preceded the three recorded primary outbreaks of A.
planci in 1966, 1979 and 1994. The dark grey line
shows an early large flood in 1951, but no data exist
from that period. The blue lines show the large 2008
and 2009 Burdekin floods, potentially predicting the
onset of a fourth primary outbreak.
A satellite image from a moderate flood event on the central and northern GBR (Fig. 5)
illustrates: (1) the Burdekin plume extending >200 km to the north where it merges with the
plumes from the Herbert and many Wet Tropics rivers; (2) the plumes intersect mid-and
outer-shelf reefs around latitude 16 17S due to offshore diversion by the Cape Grafton
headland and a narrow continental shelf; and (3) the plume waters do not intersect with any
large reefs elsewhere as the remaining reef tract is too far offshore and all inshore reefs are
very small.
Strong regional differences in the long-term average summer chlorophyll concentrations are
also apparent on the GBR. Along the inner 25 km of the GBR, chlorophyll values were on
average twice as high in the central/northern GBR (CN) compared to the far northern GBR
(FN) (0.54 vs 0.26 µg L
). Assuming our experimental results were indicative of food
limitation in the field, this ~2-fold difference in chlorophyll concentrations between CN and
FN would translate into a ~8-fold higher rate of successful larval development in the former.
Additionally, levels of chlorophyll exceeding 0.5 µg L
occur for ~37% of summer values in
the inner CN, compared to 5.7 6.4% in the remaining sectors.
A re-analysis of AIMS Long-Term Monitoring Program data (Sweatman et al. 2008) of A.
planci outbreaks and coral cover shows that in the period 1985 2007, 12.9% ± 1.7% (SE) of
reefs in CN were in a state of ‘active or incipient outbreak’ at anyone time, and coral cover
averaged 16.5% ± 0.8%. In contrast, in FN only 5.5% ± 0.01% of reefs had A. planci
outbreaks, no outbreak waves have been observed, and coral cover averaged 28.0% ± 1.0%.
Coral Reefs (2010): Volume 29, pp 593-605: A. planci outbreaks and phytoplankton
c) A. planci coral spatial-temporal simulation model
We used the A. planci coral spatial-temporal simulation model to investigate and quantify
the relationships between inshore chlorophyll, seastar populations and coral cover (Figs. 6 and
7, Table 2). The two principal drivers of the A. planci populations were both food-resource
related, and comprised:
(1) The concentration of chlorophyll in the water column which determines the
probability of A. planci larvae to survive until settlement and metamorphosis;
(2) The availability of hard coral for consumption by the juvenile and adult A. planci.
The former is governed by the empirical relationship between larval survival and
concentrations of chlorophyll in the water (Fig. 6a c). At low levels of chlorophyll the larval
survival was low and thus A. planci populations remained low and coral cover was high.
Conversely, at high levels of chlorophyll the abundant larvae led to large populations of A.
planci adults that can deplete the hard coral cover within a few years.
Table 2: Model parameters for the A. planci - coral simulation model. The values were based on Scandol (1993).
The temporal dynamics and patterns of the model results were relatively insensitive to variation of these
parameters. Abbreviations: J1, J2: juveniles aged 1 and 2 years, A1 A5: adults aged 3 to 7 years.
Life Stage
Coral consumed
Figure 5: Satellite image of the central GBR
(Modis, 10
February 2007), also showing the
locations of the mouths of the main rivers, and
towns (filled square). All inshore reefs, and the mid-
and outer-shelf reefs north of latitude 17S (the
presumed location of source reefs for primary A.
planci outbreaks on the GBR, red box) are
inundated by flood waters from the merged plumes
of several rivers, while the remaining mid- and
outer-shelf reefs are not intercepted by the flood
plumes during this moderate flood event.
Coral Reefs (2010): Volume 29, pp 593-605: A. planci outbreaks and phytoplankton
Figure 6: Relationship of Acanthaster planci population dynamics and chlorophyll in the Great Barrier Reef
(GBR) off the NE of Australia. (a) Map of the GBR. (b). Long-term average chlorophyll concentrations in the
GBR in the far northern (FN, blue) and central/northern (CN, red) region, monitored near-monthly since 1992.
Applying the results from the laboratory experiments (c) showed that the odds for survival of A. planci larvae
was ~8-fold higher at chlorophyll levels found in CN compared with FN. Simulations of A. planci and coral
population dynamics show that in FN (d), outbreaks occur at 50 80 year intervals and coral cover recovers
between outbreaks (Table 4). In CN (e), outbreaks occur at 15 year intervals and corals only recover to 30-40%
of potentially obtainable values. These data form the basis to model the transition (f) in chlorophyll, A. planci
and coral cover in CN from pre-European (blue) to contemporary levels (red).
The model was used to compare the CN and FN regions using observed water quality data to
drive the populations (Fig. 6 d - f). Averaged over many generations, with distributions of
chlorophyll concentrations reflecting those in the CN inshore region (Table 3, mean
chlorophyll = 0.54 µg L
), the model population formed outbreaks at 12 15 year intervals,
consistent with present-day outbreak frequencies and intensities in CN (Fig. 6e, Table 4).
Coral recovery between outbreaks remained incomplete, with coral cover averaging 20 28%
of typical maximum values. At the chlorophyll distributions recorded in the FN (mean
chlorophyll = 0.26 µg L
), adult and juvenile seastar densities were 0.04 - 0.25 and 0.25 -
0.63 of CN densities respectively, outbreaks occurred only once in 50 80 years, and coral
cover recovered to 75 90% of typical maximum values between outbreaks (Fig. 6d, Table
4). Taking the relatively pristine FN as reflective of conditions ~150 years ago, a potential
transition from pristine to contemporary outbreak conditions in CN becomes apparent,
demonstrating increasing outbreak frequencies and progressively declining coral cover (Fig.
Coral Reefs (2010): Volume 29, pp 593-605: A. planci outbreaks and phytoplankton
Table 3: Summer chlorophyll a concentrations on the inner (<25 km off the coast) and outer section of the
continental shelf in the far northern (FN, latitude 12 15.0S) and central/northern (CN, latitude 15.1 - 19.2S)
regions of the Great Barrier Reef (Brodie et al. 2007). N = number of samples. Shown are means, medians, and
the percentage of water samples with chlorophyll concentrations below 0.25 µg L
and exceeding 0.5 and 0.8 µg
(µg L
± SE)
(µg L
µg L
µg L
µg L
Inner 25 km
0.26 ± 0.01
0.54 ± 0.02
0.27 ± 0.01
0.24 ± 0.01
The model was also used to investigate the characteristics of A. planci, coral cover and the
outbreak frequencies and intensities (Fig. 7a-f). In terms of the model, outbreaks were defined
as events that reduced the coral cover to <2% and lead to mass mortality of seastars. The age
structure of the A. planci populations changed with increasing chlorophyll levels (Fig. 7a-c).
As chlorophyll increased, population sizes increased, and the age structure shifted to
relatively more young seastars. Outbreaks only occurred at chlorophyll levels above ~0.25 µg
and seastar populations were <10% of their maximum levels (Fig 7d). At chlorophyll
levels >0.5 µg L
coral cover declined by 75% of the initial cover and was dominated by
young corals with slower growth rates, the rate of growth in the coral population slowed, and
less coral was consumed (Fig. 7e - f). The maximum frequency of the outbreak waves was
one cycle per 10 15 years, and was controlled by the rate of coral recovery, with slower
rates resulting in lower outbreak frequencies.
Table 4. Results of simulation runs of the single-reef A. planci coral simulation model (Fig. 6). The far
northern region (FN, latitude 12.0 15.0S) has little agriculture and a low human population density, whereas
the central/northern GBR (CN, latitude 15.1 19.2S) experiences elevated nutrient loads from rivers. Ratios
between the two contrasting regions rather than absolute values and sensitivity analyses were used to overcome
the effects of model assumptions.
(Figs. 6b, e)
(Figs. 6b, d)
Adult A. planci (mean
relative density)
40 63
16 25
1.8 3.2
Juvenile A. planci
(mean relative density)
4,000 10,000
400 1,000
6 15
Coral Cover (mean %
of typical maximum)
20 28
75 90
0.25 0.35
Interval between
outbreaks (yrs)
12 15
50 80
0.13 0.22
Coral Reefs (2010): Volume 29, pp 593-605: A. planci outbreaks and phytoplankton
Figure 7. Simulation results from the population model shows the averaged effects of varying levels of mean
chlorophyll on relative population sizes of (a) larvae, (b) juveniles and (c) adults, and on (d) the frequency of
seastar outbreaks, (e) the percent of coral eaten and (f) coral cover. Results are averages over model runs
spanning 200 years and excluding a 20-years ‘burn-in’ period.
Finally the data on reef connectivity and their source, sink and self-seeding levels were related
to seastar population and outbreak intensities. We found that:
(1) Patterns of inter-reef connectivity had far less effect on the large-scale wave-like patterns
of secondary outbreaks than differences in chlorophyll concentrations. Provided there is at
least a low level of connectivity amongst reefs, further increases in the strength of
connectivity had little effect on the outbreak patterns.
(2) At the scale of individual reefs, the risk of a reef having a severe A. planci outbreak
increased with its capacity to retain larvae through self-seeding and acting as a sink, and
decreased with its capacity to reduce larvae by acting as a source. Thus, reefs that were
the origin of outbreaks needed not outbreak themselves, and hence reefs where primary
outbreaks are first observed may not be the source of the outbreak. Furthermore, reefs that
were predominantly sources of larvae (i.e., had low larval retention) were four times less
likely to outbreak than reefs that retained more of their larvae.
Identifying the causes of ecological patterns and distinguishing anthropogenic changes from
natural dynamics is exceedingly complex, but synthesis of information from various sources
such as experiments, field surveys, long term monitoring and simulation models can form a
basis for attribution of causality (Fabricius and De'ath 2004). Using this approach, our study
adds new and strong support to the hypothesis that food availability controls primary
outbreaks of A. planci by enhancing the survival of larvae. It is important to differentiate
between the different processes that govern population dynamics on the three reef types
(source reefs, primary outbreak reefs, and secondary outbreak reefs). Primary outbreaks can
arise from small populations living on source reefs that encounter highly productive waters
during spawning times. Hydrodynamics dictate that source reefs may be located either
upstream of the primary outbreak reefs, or become primary outbreak themselves reefs through
self-seeding. However both source and primary outbreak reefs are likely to experience
Coral Reefs (2010): Volume 29, pp 593-605: A. planci outbreaks and phytoplankton
seasonal phytoplankton blooms as the larvae together with the phytoplankton move with the
currents. Following these primary outbreaks, secondary outbreaks are then observed on
individual reefs in downstream progressing waves (Moran 1986; Moran et al. 1992,
Sweatman et al. 2008) that can be reconstructed using hydrodynamic models (Dight et al.
1990, our A. planci model). Primary outbreaks therefore develop from a small source
population of starfish, with each mature starfish producing an extremely large number of
offspring due to the release of larval food limitation. In contrast, secondary outbreaks
comprise a large population of mature starfish and hence can sustain outbreaks in conditions
where larval survival is relatively low.
Our larval feeding experiments showed a strong and non-linear (logistic) dose-response
relationship between the availability of natural eukaryotic phytoplankton and larval
development success. By using unfiltered natural seawater, the larvae in our experiments
grew in conditions where both phytoplankton food and potential planktonic predator
communities underwent natural successions in response to nutrient availability. We
demonstrated larval food availability to be a strong driver, with low chlorophyll leading to a
low rate of developmental completion, a prolonged pelagic phase of the larvae and small sizes
of the post-metamorphosis juveniles, likely leading to higher pre- and post-settlement
mortality (Allison 1994). The role of larval food limitation to echinoderm population
dynamics in the field is also corroborated by the observation that only echinoderm groups
with planktotrophic larvae have the propensity to exhibit boom and bust population dynamics,
while echinoderms with non-feeding (lecitotrophic) larvae have more stable populations
(Uthicke et al. 2009).
Identifying the origin of A. planci primary outbreaks is key to successful management of the
GBR. The first two outbreaks were first detected at Green Island off Cairns (Moran et al.
1992), a major tourist destination that was more frequently visited than many other reefs near-
by. The third outbreak was first reported from Lizard Island in October 1993 and 10-11
months later from 7 other reefs in the Cairns and Lizard Island regions (16013C, Evening
Reef, Swinger, Startle, Mackay, North Direction and Macgillivray Reefs), with a number of
additional reefs having A. planci densities only slightly below outbreak threshold levels in
1994 (LTMP data, not shown). The multiple outbreak locations, and the multiple A. planci
size classes (including juveniles) observed at Lizard Island in 1996 (Pratchett 2005) suggested
a gradual population build-up in the whole region through several successful spawning events
in prior years, indicating that not only the large 1991 flood but also conditions in following
years provided conditions suitable for high recruitment success. Floods have reached or
crossed this part of the shelf in 1991, 1994, 1995 and 1996 (Devlin and Brodie 2005),
retention times of flood materials on the continental shelf may be long (Luick et al. 2007), and
chlorophyll levels frequently exceed 0.5 and even 0.8 µg L
(Table 3). The weak and
bidirectional currents may further increase the vulnerability of reefs in this region to develop
outbreaks due to their relatively high rates of larval retention and self-seeding (James et al.
Long-term average chlorophyll values on the inner 25 km were twice as high in CN compared
to FN. As offshore chlorophyll values were similar in both regions, it is unlikely that the high
CN chlorophyll values were attributable to latitude or upwelling (Brodie et al. 2007). In CN,
present river loads of nutrients and sediments are estimated to be 2 10 fold higher than
before western colonization in ~1860, while river loads in the sparsely inhabited FN are
considered largely unaltered (McKergow et al. 2005). As rivers are the main source of new
nutrients to GBR inshore waters, regional differences in chlorophyll have been attributed to
Coral Reefs (2010): Volume 29, pp 593-605: A. planci outbreaks and phytoplankton
differences in river nutrient loads, reflecting past and present terrestrial runoff (Devlin and
Brodie 2005; Brodie et al. 2007), although an explicit link between increasing river loads and
changes to inshore water quality on the GBR has not been established.
The fact that primary A. planci outbreaks occurred on locations where floods intercepted large
reefs on the GBR three to five years earlier, shows that not only long-term chlorophyll
concentrations but also large floods are a strong driver for A. planci primary outbreaks, in
agreement with previous findings (Birkeland 1982, Brodie et al. 2005). Flood plume models
(King et al. 2001) showed that the 1974 and 1991 floods reduced salinity for >60 days during
the time when A. planci larvae are pelagic. Due to the high incidence of cloud cover during
and after rain events, few satellite images are available to track the spread of flood plumes.
However, Devlin and Brodie (2005) used aerial surveys to show a spreading of plumes into
the main reef matrix, similar to the patterns shown in Fig. 5, after cyclones in 1996 and 1999,
and to an even greater extent in 1994. These river floods are the largest source of new
nutrients for the inshore GBR, and trigger phytoplankton blooms that average 2 µg L
chlorophyll and at times exceed 4 µg L
(Devlin and Brodie 2005). The application of
experimental findings to field settings necessitates caution. Bearing this in mind, if we take
the experimentally observed rates of change as indicative of the relative differences in
developmental rates in the field, the odds of successful development of A. planci larvae could
be up to ~60-fold higher during floods with 2.0 µg L
chlorophyll compared to the long-term
average of 0.54 µg L
for the central/northern GBR.
In combination, the three components of this study, together with previous evidence
(Birkeland 1982; Brodie et al. 2005), strongly support the assertion that removal of larval
food limitation causes primary population outbreaks of A. planci on the GBR. In contrast to
the parsimonious explanation of food limited control of A. planci populations, explanations of
population control by predators rely on complex arguments (Birkeland and Lucas 1990). To
date, there is no empirical evidence to support these arguments. Unlike water quality,
predation pressure does not fluctuate widely on short time scales, hence it remains unclear
how a chronic release from predation would occasionally lead to sudden increases in
population densities. However the reported correlation between reef protection status and A.
planci outbreaks on a subset of GBR reefs (Sweatman 2008) suggests that both hypotheses
may not necessarily be exclusive, and that predation may play some additive role in
determining the propensity of individual reefs to be afflicted by A. planci outbreaks.
Coastal GBR water quality is considered amenable to benefit from improved land
management (Haynes et al. 2007) due to the dominant role of the rivers in providing new
nutrients to the inshore GBR, and the potentially long residency times of these newly
imported materials (Luick et al. 2007). Large and early river floods from the Burdekin and
Wet Tropics rivers occurred again in January 2008 and 2009 (Fig. 4), potentially providing
conditions to trigger a new A. planci primary outbreak wave that may again kill a significant
proportion of GBR corals. Our study suggests that reductions in phytoplankton biomass to
summer values of <0.5 µg L
(De'ath and Fabricius 2010) through better land management
could reduce the frequency of primary A. planci outbreaks. Legislation and incentives have
now been put in place to reduce river discharges of nutrients, sediments and pesticides from
agricultural areas. However until a reduction in nutrient levels is achieved, two additional
precautionary management measures should aim to maintain very low A. planci densities in
the high-risk area (the midshelf reefs and hard bottom inter-reefal areas that are directly
intercepted by floods). These are: (a) large permanent fishing closures in the high-risk area,
allowing fish populations to reach carrying capacity to safeguard against cascading changes in
Coral Reefs (2010): Volume 29, pp 593-605: A. planci outbreaks and phytoplankton
food webs, and (b) potentially some targeted efforts by divers, especially in the years
following large floods, to remove some of the A. planci before they start to aggregate and
spawn, This three-pronged approach constitutes the best presently available strategy to slow
or reverse the loss of coral cover throughout the whole central and southern GBR, at a time
where rising seawater temperatures exert increasing pressure on coral reefs, and increasing
climatic instability may increase the frequency of extreme floods.
Many thanks to the GBR Long-Term Chlorophyll Monitoring Program for the chlorophyll data, to the AIMS
Long-Term Monitoring Program for coral cover and A. planci field distribution data, to J. Scandol for
compilation of the A. planci life history data, and to M. Slivkoff for the processing of the Modis satellite image.
KO conducted the laboratory experiments, and GD developed the coral A. planci simulation model. We
gratefully acknowledge support for the experimental study by T. Ayukai and J. Lucas. We thank J. Caley, B.
Schaffelke, S. Uthicke and H. Sweatman for constructive comments on earlier versions of the manuscript, and J.
Brodie, K. Day and E. Wolanski for sharing ideas. The study was funded by the Marine and Tropical Sciences
Research Facility (MTSRF) and the Australian Institute of Marine Science, with the experimental study being
funded by the Great Barrier Reef Marine Park Authority and James Cook University.
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... Many of these hypotheses are not mutually exclusive, and it is unlikely that any single hypothesis can explain the initiation of all outbreaks (Babcock et al., 2016a;Pratchett et al., 2014). The role of nutrient enrichment, specifically the hypothesis that high nutrient availability increases phytoplankton biomass and enhances CoTS larval growth and survival leading to mass recruitment events and outbreaks, has been put forward several times Brodie, 1992;Fabricius et al., 2010;Lucas, 1973;Pearson and Endean, 1969;Wolfe et al., 2017). Indeed, the 'nutrient enrichment' hypothesis, and specifically the role of nutrient enrichment from land-based run-off (i.e. the 'terrestrial run-off' hypothesis) (Birkeland, 1982), is one of the main hypotheses proposed to account for an increased frequency of CoTS population outbreaks Fabricius et al., 2010;Wolfe et al., 2017). ...
... The role of nutrient enrichment, specifically the hypothesis that high nutrient availability increases phytoplankton biomass and enhances CoTS larval growth and survival leading to mass recruitment events and outbreaks, has been put forward several times Brodie, 1992;Fabricius et al., 2010;Lucas, 1973;Pearson and Endean, 1969;Wolfe et al., 2017). Indeed, the 'nutrient enrichment' hypothesis, and specifically the role of nutrient enrichment from land-based run-off (i.e. the 'terrestrial run-off' hypothesis) (Birkeland, 1982), is one of the main hypotheses proposed to account for an increased frequency of CoTS population outbreaks Fabricius et al., 2010;Wolfe et al., 2017). ...
... During the summer-wet season, mean chlorophyl-a (Chl-a) concentrations in GBR surface waters are approximately 50 % greater than in the winter-dry season (May to November) (Brodie et al., 2007). Further, mean surface Chl-a concentrations from November to March are higher in the inner <25 km of the shelf of the central/northern GBR (15.1 • -19.2 • S), encompassing the CoTS initiation zone, compared to that in the far northern GBR (12.0 • -15.0 • S) (Fabricius et al., 2010). The composition of phytoplankton communities also changes after flood events, with a subsequent increase in zooplankton abundance in the nearshore region (Richardson et al., 2021). ...
Full-text available
Crown-of-Thorns Starfish (CoTS) population outbreaks contribute to coral cover decline on Indo-Pacific reefs. On the Great Barrier Reef (GBR), enhanced catchment nutrient loads are hypothesised to increase phytoplankton food for CoTS larvae in the outbreak initiation zone. This study examines whether catchment load reductions will improve water quality in this zone during the larval period. We defined the i) initiation zone's spatial extent; ii) larval stage's temporal extent; and iii) water quality thresholds related to larval food, from published information. We applied these to model simulations, developed to quantify the effect of catchment load reductions on GBR water quality (Baird et al., 2021), and found a consistently weak response of chlorophyll-a, total organic nitrogen and large zooplankton concentrations in the initiation zone. Model results indicate marine and atmospheric forcing are more likely to control the planktonic biomass in this zone, even during major flooding events purported to precede CoTS outbreaks.
... Obviously here an impact of combination of both catalysts, which are believed to cause invasions of Acanthaster, namely, the enrichment of the water area with nutrients and the removal of natural enemies of this starfish from the ecosystem at all stages of its life cycle as a result of overfishing (Fabricius et al., 2010;Pratchett et al., 2014Pratchett et al., , 2017Brodie et al., 2017). The annual increase in the production of phytoplankton, on which the planktonic larvae of Acanthaster feed, directly correlates with the concentration of chlorophyll-a (Chl-a) in water (Hieu et al., 2021), while the level of 0.6-1 mg/L Chl-a is the optimal concentration for the development of larvae of this starfish (Wolfe et al., 2015;Brodie et al., 2017). ...
... The annual increase in the production of phytoplankton, on which the planktonic larvae of Acanthaster feed, directly correlates with the concentration of chlorophyll-a (Chl-a) in water (Hieu et al., 2021), while the level of 0.6-1 mg/L Chl-a is the optimal concentration for the development of larvae of this starfish (Wolfe et al., 2015;Brodie et al., 2017). Increasing the concentration of Chl-a to 3 mg/L respectively increases the survival rate of COTS larvae by 8 times (Fabricius et al., 2010). According to our data (Tkachenko et al., 2020b), autumn peak concentrations of Chl-a in the bay even exceed 3 mg/L which can contribute to the development and survival of COTS larvae. ...
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Until recently, the city of Nha Trang, which stretches along Nha Trang Bay of southern Vietnam, was known as the "Riviera of the South China Sea" with clean and white beaches, untouched islands, and rich coral reefs with high biodiversity. Nevertheless, complex and long-term anthropogenic impacts caused by enlargement of tourist resorts on the coast of the bay and on its islands, dredging, boom of mariculture development, and overfishing led to degradation of more than a half of the coral reefs in the bay already by the beginning of the 2010s. By that time, only a third of the remaining reefs in the seaward part of the bay were characterized by rather high coral cover and diversity. Finally, in just three years from 2017 to 2019, more than 90% of these remaining rather healthy reefs have died off as a result of an outbreak of the main coral predator: crown-of-thorns starfish Acanthaster sp. By April 2019, the abundance of this starfish reached 4.2 individuals per 100 m 2. Such abundance is 8-fold higher than the maximum at which the coral community may exist without decline. An abrupt increase in starfish abundance in the bay was determined by an increase in phytoplankton production (food source of starfish larvae) due to eutrophication of the bay and withdrawal of all natural enemies of the starfish from the coral reef ecosystem because of overfishing. In June 2019, the subsequent strongest sea surface temperature anomaly caused bleaching and mortality of surviving coral colonies. The cascade degradation of coral reefs in Nha Trang Bay and significant degradation of coral reefs in the neighboring provinces of Vietnam do not make it possible to give an optimistic prediction on recovery of coral reefs in this area in the near future.
... This increase in nutrients can make reefs more vulnerable to CoTS, as it may cause blooms of phytoplankton. Phytoplankton are the primary food for CoTS larvae , with relationships having been observed between CoTS larval survival and phytoplankton concentration (Fabricius et al., 2010) and therefore nutrient input (Brodie et al., 2005), leading to the conclusion that increased nutrient availability is itself a potential cause of outbreaks (Babcock et al., 2016). ...
... For Cebu City, we also simulated cases in which an MPA was established on parts of Mactan and Cordova Islands and urban development there was limited, to examine the efficacy of this conservation strategy. We controlled for the effects of nutrient availability on CoTS larval survival (Fabricius et al., 2010;Wolfe et al., 2017) by taking = 30 (the baseline value) and 3. Additional details on the model fitting and parametrization for these scenarios are available in the Appendix, sections C-G. ...
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Crown-of-thorns starfish (CoTS) outbreaks rank among the greatest threats to coral throughout the Indo-Pacific. In the future, reefs already stressed by CoTS will be further burdened by overfishing and nutrient loading. How much these two factors will exacerbate CoTS outbreak severity is still uncertain. Furthermore, the CoTS management literature has focused on the Great Barrier Reef, whereas outbreak damage is rising across the Indo-Pacific. Here, we use a metacommunity model to simulate CoTS outbreaks in areas with high and growing levels of fishing pressure and offshore nutrient input. We model outbreaks on reefs adjacent to two cities within the range of CoTS that have less prior literature coverage: Cebu City, Philippines, and Jeddah, Saudi Arabia. We observe that the combination of population increases and urbanization of previously rural areas can drive complex patterns of multi-stressor interaction. We find that CoTS removal on intermediate spatial scales significantly improves regional-scale coral health, and provide guidelines under which each of four CoTS management strategies is optimal for conservation. We find that coral decline due to overfishing can be sharper on reefs with CoTS, and that nutrification can induce a shift from discrete outbreak waves to continuous CoTS presence. Our work shows the importance of long-term planning for reef management, and highlights how reef stressors can interact in potentially unforeseen ways.
... CoTS densities can increase up to six orders of magnitude in two years, depriving reefs of up to 90% of living coral tissue (Yamaguchi, 1986) and, by selectively preying on the coral taxa most susceptible to climate change such as acroporid corals, CoTS can exacerbate coral reef degradation after bleaching events (Keesing et al., 2019). A combination of environmental conditions such as elevated phytoplankton concentrations, low current velocities, and lack of predators are thought to contribute to the initiation of CoTS outbreaks (Birkeland & Lucas, 1990;Fabricius et al., 2010;Hock et al., 2014;Kroon et al., 2021;Sweatman, 2008;Wooldridge & Brodie, 2015). Yet, the relative contribution of each driver to the dynamics of CoTS is unclear Pratchett et al., 2017). ...
... Australia's UNESCO World Heritage-listed Great Barrier Reef (GBR) has undergone four CoTS outbreak waves since the 1960s . The initiation of all four CoTS outbreak waves has been detected on mid-shelf reefs between 14 S and 17 S, a region just north of Cairns identified as the "initiation box" (Fabricius et al., 2010;Pratchett et al., 2014) from which each outbreak has propagated southward with prevailing hydrodynamic conditions (Reichelt et al., 1990). Monitoring of CoTS outbreaks on the GBR started after the second major outbreak wave initiated, with data showing that up to 42% of coral cover loss (from an average 28% coral cover) was attributed to CoTS between 1985and 2012(De'ath et al., 2012. ...
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Abstract Crown‐of‐thorns starfish (CoTS) naturally occur on coral reefs throughout the Indo‐Pacific region. On Australia's Great Barrier Reef (GBR), outbreaks of CoTS populations are responsible for ecologically significant losses of corals, and while they have been documented for decades, they now undermine coral recovery from multiple stressors, especially anthropogenic warming. Culling interventions are currently the best approach to control CoTS outbreaks on the GBR, but assessing control effectiveness under multiple stressors is complicated. Using an ensemble of two reef community models simulating the temporal and spatial dynamics of CoTS and corals under future climate scenarios, we evaluate the present‐day and future effectiveness of the current implementation of the GBR CoTS Control Program. Specifically, we determine the culling effort needed (i.e., number of vessels) to achieve the maximum ecological benefits as predicted by the models under possible warming futures. Benefits were measured by comparing projections of coral cover and CoTS densities under scenarios of increasing control effort and baseline scenarios where no control was simulated. Projections of present‐day control efforts (five vessels) show that the number of individual reefs subject to CoTS outbreaks is reduced by 50%–65% annually, yielding a benefit of 5%–7% of healthy GBR coral area per decade, equivalent to gaining 104–150 km2 of live corals by 2035. A threefold increase in current control efforts is sufficient to reach more than 80% of the maximum coral benefits predicted by each model, but the future amount of effort required to control CoTS effectively depends on the intensity of warming and the early detection of CoTS outbreaks. While culling CoTS across the entire GBR is unfeasible, we provide a framework for maximizing ecosystem‐wide benefits of CoTS control and guide management decisions on the required culling effort needed to reduce CoTS outbreaks to levels that may ensure coral persistence in the face of future climate change impacts.
... River nutrients can influence CoTS outbreak dynamics (Schaffelke et al. 2017) as wet season nutrient inputs from the central GBR rivers, typically discharge when phytoplankton-feeding CoTS larvae are present in the water column (November to March) (Devlin et al. 2012(Devlin et al. , 2013. The increase in nutrients provides food for the phytoplankton blooms which allows a greater number of CoTS larvae to survive to a stage where they are able to settle out on a coral reef (Brodie et al. 2005;Fabricius et al. 2010;Brodie et al. 2017). ...
... Studies highlight that the number of outbreaks have increased through the period where the GBR inshore waters have experienced increases in nutrient loads from agriculture. This has resulted in the frequency of CoTS waves on the GBR moving from low frequencies of about every 50-80 years to about every 15 years (Brodie 1992;Fabricius et al. 2010;Brodie et al. 2017;Pratchett et al. 2017). Increased Coral Bleaching Susceptibility DIN availability plays an important part in the coralalgae symbiosis, with elevated DIN concentrations disrupting the ability of the coral host to maintain an optimal population of algal symbionts . ...
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Excess nutrientsfrom fertiliser application, pollution discharge and water regulations outflow through rivers from lands to oceans, seriously impact coastal ecosystems. Terrestrial runoff of waters polluted with nutrients (primarily nitrogen [N] and phosphorus [P] compounds) from point source/s, such as sewage treatment plant (STP) discharges, and diffuse sourcesvia river discharges, such as fertiliser losses, are having devastating adverse effects in coastal and marine ecosystems globally (Carpenter et al., Ecol Appl 8:559–568, 1998; Halpern et al., Science 319:948–952, 2008; Crain et al., Ecol Lett 11:1304–1315, 2008; Smith and Schindler, Trends Ecol Evol 24:201–207, 2009). The nutrients can be dissolved such as dissolved nitrate and Phosphate typically discharged from STPs or agricultural runoff or in a particulate form, often associated with soil erosion.
... When outbreaks occur, Acanthaster planci are capable of killing up to 80% of corals across large areas of reefs (Baird et al. 2013). There have been a number of factors that suggest how COTs outbreaks are initiated and these include concentration of nutrients in near-shore waters (Fabricius et al. 2010) as well as overfishing of fish species that feed on COTs (Dulvy et al. 2004;Sweatman 2008). Colgan (1987) provided evidence of reef degradation as a result of the Crown-of-Thorns starfish in Guam. ...
In 2000, Fiji suffered an extensive coral bleaching disturbance caused by elevated sea temperatures that resulted in mass mortalities of corals around the country. From 2000 to 2013, temporal changes in reef community, sea temperatures and relevant parameters from different sites in Fiji were analyzed to document changes. Coral composition and recovery varied among sites and years suggesting that one site may not be representative of all sites across Fiji’s exclusive economic zone of 10, 550, 000 km2. Localized disturbances such as tropical cyclones, heavy rainfall events, ENSO and human causes, influence the reef systems. Geographically, sites located near each other were similar having likely recruited corals from the same area. The dominance of a benthic category such as Acropora in earlier years for several sites shifted in later years to other coral benthic forms including submassive corals due to disturbances that halted survival and growth of Acropora.
... COTS outbreaks are a major cause of coral mortality in the Indo-Pacific region with impacts flowing on to reef fish and benthic communities (Fabricius, Okaji, & De'Ath, 2010;Great Barrier Reef Marine Park, 2019;Hughes, Hughes, & Smith, 2014;Kayal et al., 2012). Although a link between outbreaks and terrestrial runoff has been postulated, the cause is not fully understood (Hughes et al., 2014). ...
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This research presents an investigation into the major factors impacting the long-term sustainability of recreational scuba diving tourism (RSDT) in the Cairns section of the Great Barrier Reef Marine Park. This section of the GBRMP hosts one of the most iconic, globally renowned and largest agglomerations of RSDT in the world providing significant societal, financial and employment contributions across local and international entities through various linkages ). Cairns is often described by travel writers and the Cairns Regional Council as the Gateway to the Great Barrier Reef, and in scuba diving publications as one of the world’s most desirable RSDT destinations.
... However, landuse changes since European settlement in coastal catchments draining into the GBR have increased riverine nutrient and sediment loads three-fourfold, reducing GBR water quality (McCloskey et al., 2021a,b). Increased nutrient loads have been linked to increased phytoplankton growth (Bell et al., 2014) and macroalgal growth (Chen et al., 2019), changes in the coral community composition (Thompson et al., 2014), increased coral disease (Willis et al., 2004) and the enhanced growth of crown-of-thorns starfish (COTS) larvae whose adults prey on corals (Fabricius et al., 2010;Babcock et al., 2016). ...
The "larval starvation hypothesis" proposed that the growing frequency of Crown-of-Thorns Starfish (CoTS) outbreaks could be attributed to increased availability of phytoplankton, which is a nutritional supply for starfish larvae. However, comprehensive field investigations on the living environment of CoTS larvae and the food availability of phytoplankton are still lacking. A cruise was conducted in June 2022 in Xisha Islands, South China Sea, to study the interaction between environmental conditions and phytoplankton communities during CoTS outbreak period. The average concentrations of dissolved inorganic phosphorus (0.05 ± 0.01 μmol L-1), dissolved inorganic nitrogen (0.66 ± 0.8 μmol L-1) and chlorophyll a (0.05 ± 0.05 μg L-1) suggested that foods may be limited for CoTS larvae in Xisha Islands. Microscopic observation and high-throughput sequencing were used to study the composition and structure of the phytoplankton communities. Bacillariophyta predominated in phytoplankton community with the highest abundance and species richness. 29 dominant species, including 4 species with size-range preferred by CoTS larvae, were identified in Xisha Islands. The diversity index of all stations indicated a species-rich and structure-stable phytoplankton community in Xisha Islands during the period of CoTS outbreak, which may potentially contribute to CoTS outbreak. These findings revealed the structure of phytoplankton community structure and environmental factors in the investigated area during CoTS outbreak, providing the groundwork for future research into the causes and processes of CoTS outbreak.
The radially symmetric body of starfish has major implications on their nervous system including eyes and vision. All the up to 50 arms are structurally identical, and most examined species have a small compound eye basally on the terminal tube foot of each arm. The 20–300 ommatidia of the compound eyes are lens-less but hold approximately 100 photoreceptors with outer segments made of a combination of microvilli and a modified cilium. The eyes support image forming vision but of low spatial resolution and extremely low temporal resolution with flicker fusion frequencies ≤1 Hz. Starfish are color-blind, and vision seems to be based on a single rhabdomeric opsin although many other types of opsins are expressed in their eyes. Starfish also possess extraocular photoreceptors, but little is known about their identity and function. Not many visually guided behaviors are known from starfish so far, but habitat recognition is well documented in a couple of tropical species. More behavioral data are urgently needed, but interestingly, recent data suggest that at least in some situations vision is integrated with olfaction and rheotaxis forming a sensory hierarchy, where olfaction is dominating. Such processing and integration putatively take place in the central nervous system. The eyes are direct extensions of the radial nerve, which constitute the major part of the CNS of starfish and other echinoderms. In general, the echinoderm CNS is enigmatic and the functionality is at best speculative. Here we present new data showing differentiations of the radial nerve along the length of the arms and differences in radial nerve structure between eye-possessing and eyeless species.KeywordsCompound eyesOmmatidiaLow resolution visionRadial nerveSea starRadial symmetryEchinodermTemporal resolutionHabitat recognition
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The successful management of ecosystems depends on early detection of change and identification of factors causing such change. Determination of change and causality in ecosystems is difficult, both philosophically and practically, and these difficulties increase with the scale and complexity of ecosystems. Management also depends on the communication of scientific results to the broader public, and this can fail if the evidence of change and causality is not synthesized in a transparent manner. We developed a framework to address these problems when assessing the effects of agricultural runoff on coral reefs of the Australian Great Barrier Reef (GBR). The framework is based on improved methods of statistical estimation (rejecting the use of statistical tests to detect change), and the use of epidemiological causal criteria that are both scientifically rigorous and understood by nonspecialists. Many inshore reefs of the GBR are exposed to terrestrial runoff from agriculture. However, detecting change and attributing it to the increasing loads of nutrients, sediments, and pesticides is complicated by the large spatial scale, presence of additional disturbances, and lack of historical data. Three groups of ecological attributes, namely, benthos cover, octocoral richness, and community structure, were used to discriminate between potential causes of change. Ecological surveys were conducted along water quality gradients in two regions: one that receives river flood plumes from agricultural areas and one exposed to runoff from catchments with little or no agriculture. The surveys showed increasing macroalgal cover and decreasing octocoral biodiversity along the gradients within each of the regions, and low hard coral and octocoral cover in the region exposed to terrestrial runoff. Effects were strong and ecologically relevant, occurred independently in different populations, agreed with known biological facts of organism responses to pollution, and were consistent with pollution effects found in other parts of the world. The framework enabled us to maximize the information derived from observational data and other sources, weigh the evidence of changes across potential causes, make decisions in a coherent and transparent-manner, and communicate information and conclusions to the broader public. The framework is applicable to a wide range of ecological assessments.
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Numerical hydrodynamic models of the northeastern Queensland shelf, forced by regional winds and modelled boundary currents in the northern Coral Sea, are used to provide improved estimates of general flow trajectories and water residence times within the Great Barrier Reef (GBR) shelf system. Model performance was checked against a limited set of current metre records obtained at Lark Reef (16°S) and the Ribbon Reefs (15.5°S). Estimates of water parcel trajectories are derived from a series of numerical tracer experiments, with daily releases of neutrally buoyant, un-reactive particles at 320 sites along the coast between Cape York (10.7°S) and Hervey Bay (25°S). Flow trajectories and residence times for tracer particles introduced to the GBR lagoon in the southern—ca. 22°S, central—19°S, and northern reef—14°S are emphasised. For purposes of the analysis, the year was divided into two seasons based on mean alongshore current direction. Most coastal sourced tracers entering the central GBR lagoon between 16° and 20°S during the northward-current season (January–August) primarily encounter the outer-shelf reef matrix after exiting the lagoon at its northern “head” (nominally 16°S), after 50–150 days. Up to 70% of tracer particles entering in the southward-current season (August–December) eventually crossed the lagoon to the outer-shelf reef matrix, with median crossing times between 20 and 330 days. During favourable wind conditions, tracers introduced at the coast may move rapidly across the lagoon into the reef matrix. The tracer experiments indicate that most coastal-sourced tracers entering the GBR lagoon remain near the coast for extended periods of time, moving north and south in a coastal band. Residence times for conservative tracer particles (and implied residence times for water-borne materials) within the GBR shelf system ranged from ca. 1 month to 1 year—time frames that are very long relative to development times of planktonic larvae and cycling times for nutrient materials in the water column, implying they are transformed long before reaching the outer reef matrix.
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Manta tow surveys of the perimeters of reefs throughout the Great Barrier Reef (GBR) assessed broad-scale changes in hard coral cover on reefs impacted by, recovering from and unaffected by Acanthaster planci outbreaks. Mean coral cover was 16 to 40% on reefs with no history of A. planci outbreaks, depending on location on the GBR. Coral cover increased at approximately 2% yr(-1) on southern reefs, while there was no significant increase on such reefs in other regions. Hard coral cover on reefs with A. planci outbreaks declined at a mean annual rate of 6% to an average level of 9%. Coral cover on southern reefs that were recovering from sustained A. planci outbreaks increased at about 4% yr(-1) while such reefs showed an annual increase of 0.8% in the remaining regions. A total of 78% of recovering reefs showed a positive growth rate, assuming linear growth, the time for coral cover to increase by 30%, was estimated at between 5 yr and well over 1000 yr. In addition to providing regional estimates of the decline and recovery of reefs due to A. planci outbreaks, this study highlights the variability in rate of recovery between reefs and raises the possibility that not all reefs will recover from sustained outbreaks.
CotSim is a size-structured metapopulation model of the crown-of-thorns (Acanthaster planci) on the central Great Barrier Reef (GBR). The populations of starfish and the coral cover on 269 individual reefs are modelled for up to 200 years. Starfish are represented as larvae, two age classes of juveniles and three size classes of adults. Coral can either be modelled as a single type or as two types each with a characteristic growth rate, equilibrium cover and susceptibility to starfish predation. Reefs are connected using simulated dispersal data for A. planci on the central GBR. These data were generated using a particle tracking program where simulated currents displaced particles representing dispersing larvae after an A. planci spawning episode. The dispersal data represented patterns expected from the 1976/77 to 1989/90 spawning season. The starfish growth model is a density-dependent matrix model. When coral cover is low, survival within classes is law and the transitions into larger classes is impeded. In contrast, at high coral cover the reverse patterns occur. Both the starfish and coral data are filtered through an interpretation model to generate observed patterns. The starfish interpretation model represents the important difficulty in detecting smaller adults. Results from the model using the default parameters correspond with published patterns of starfish/coral dynamics and the overall patterns of starfish outbreaks on the GBR. The model is an interactive event-driven 32-bit Windows application requiring Windows 95 or Windows NT 3.51/4.0. Most parameters are able to be altered by the user with three tabbed dialogue boxes (for the simulation, starfish and coral parameters). Biologically justifiable default parameters are provided for all parameters. Parameters and initial starfish populations are stored in simple coded ASCII files. Simulations are controlled using 'Run', 'Pause/Continue' and 'Stop' operations. Maps of the GBR illustrate the spatial and temporal structure of the metapopulation dynamics including the patterns of dispersal. Once paused, populations on individual reefs can be examined using two types of plots (time series and single time bar charts). Overall patterns can be displayed using latitude versus time plots of observed reef slate. Starfish populations and coral cover can be edited, which enables users of the model to become associated with some of the key issues regarding large-scale starfish control programs. Results from the model can be written to ASCII files for additional analysis. The speed of a simulation is able to be controlled and colours for important graphical elements can be altered. CotSim includes indexed online context-sensitive help and a graphical install routine. The program adheres to published guidelines for Windows applications.
Synechococcus was more abundant and had a greater biomass than Prochlorococcus at most inshore and mid-shelf sites in the central (17°S) Great Barrier Reef (GBR), and at all shelf sites in the southern (20°S) GBR. Significant Prochlorococcus populations were confined to mid-and outer-shelf sites with mixed or partially stratified water columns of greater oceanic character in the central GBR, where depth-weighted average Synechococcus and Prochlorococcus abundances were better correlated with salinity, shelf depth and chlorophyll a concentration, than with concentrations of NH4+, NOx- (i.e. NO2- + NO3-), or PO43-. Vertical gradients of normalized mean cellular red and orange fluorescence of Synechococcus and Prochlorococcus populations imply that vertical mixing rates were sufficiently low to allow these populations to photoacclimate at depth at shelf locations in the central GBR, but too greater substantial photoacclimation to be observed at sites in the southern GBR. The presence of Prochlorococcus populations at inshore sites in the central GBR in the absence of extensive intrusion events suggests that Prochlorococcus populations were actively growing.
Acanthaster planet (L.) and Nardoa novaecaladoniae (Perrier, 1875) are two coral reef asteroids having planktotrophic and lecithotrophic larval development, respectively. Comparative sizes at metamorphosis are 0.5 to 0.7 mm for A. planci and 1.2 to 1.6 mm for N. novaecaladoniae. Mortality rates of small juveniles (one month old) of each species were measured experimentally in the field on the Great Barrier Reef, Australia. Mortality rates of N. novaecaladoniae were low (1.5 %.d-1) compared to 7.8 %.d-1 for A. planci. Survival of the two species was similar between habitats. However, mortality rates of A. planci were highly variable both within- sites and between-sites within-habitats (fore reef 15 m depth, reef flat 2 m and back reef lagoon 12 m). There was no apparent effect of density of A. planci on mortality rates. Mortality is thought to be principally due to predation by infauna which are abundant in the coral reef rubble. A study of survival rates of newly metamorphosed Nardoa sp. (1.0 to 1.2 mm) in Okinawa, Japan, found very low mortality rates of just 0.2 %.d-1. The abundance of potential predators among the rubble infauna was very low on the Okinawan reef compared to the Great Barrier Reef. These studies provide evidence of the importance of predation as a determinant of survival rates of small starfish and that a reproductive strategy providing for a large size at settlement facilitates greater survivorship.