Carbonate production of Micronesian reefs suppressed by thermal anomalies and Acanthaster as sea-level rises

Summary

Coral reefs are essential to millions of island inhabitants. Yet, coral reefs are threatened by thermal anomalies associated with climate change and by local disturbances that include land-use change, pollution, and the coral-eating sea star Acanthaster solaris. In combination, these disturbances cause coral mortality that reduce the capacity of reefs to produce enough carbonate to keep up with sea-level rise. This study compared the reef-building capacity of shallow-water inner, patch, and outer reefs in the two islands of Pohnpei and Kosrae, Federated States of Micronesia. We identified which reefs were likely to keep up with sea-level rise under different climate-change scenarios, and estimated whether there were differences across habitats in the threshold of percentage coral cover at which net carbonate production becomes negative. We also quantified the influence of A. solaris on carbonate production. Whereas the northwestern outer reefs of Pohnpei and Kosrae had the highest net rates of carbonate production (18.5 and 16.4 kg CaCO₃ m^-2 yr^-1, respectively), the southeastern outer reefs had the lowest rates of carbonate production (1.2–1.3 and 0.7 kg CaCO₃ m^-2 yr^-1, respectively). The patch reefs of Pohnpei had on average higher net carbonate production rates (9.5 kg CaCO₃ m^-2 yr^-1) than the inner reefs of both Pohnpei and Kosrae (7.0 and 7.8 kg CaCO₃ m^-2 yr^-1, respectively). A. solaris were common on Kosrae and caused an average reduction in carbonate production of 0.6 kg CaCO₃ m^-2 yr^-1 on Kosraean reefs. Northern outer reefs are the most likely habitats to keep up with sea-level rise in both Pohnpei and Kosrae. Overall, the inner reefs of Pohnpei and Kosrae need ~ 5.5% more coral cover to generate the same amount of carbonate as outer reefs. Therefore, inner reefs need special protection from land-use change and local pollution to keep pace with sea-level rise under all climate-change scenarios.

Keywords:

Islands – Corals – Coral reefs – Sediment – Carbonates – Sedimentation – Starfish – Federated States of Micronesia

Introduction

Coral reefs are an integral component of global marine ecosystems and are essential to millions of people that benefit from the goods and services that coral reefs provide. For example, coral reefs reduce storm-wave energy by up to 97% [1], reducing the threat of coastal inundation during severe storms [2]. However, contemporary thermal-stress events, associated with global-climate change, cause coral bleaching and mortality, which can lead to shifts in species dominance [3–6]. These changes have reduced the capacity of coral reefs to accrete carbonate in some localities and keep up with sea-level rise [7]. Here we examine whether the reefs of Pohnpei and Kosrae, Federated States of Micronesia (FSM; Figure A in S1 File), are producing enough carbonate to keep up with sea-level rise, while experiencing thermal stress and local disturbances.

Rates of carbonate production have been studied using geological coring [8–10], hydrochemistry [11], modeling [12], and in situ estimates [13–15]. All approaches show considerable variation across ocean basins, with erosional forces only exceeding rates of carbonate production when gross calcification rates are low. In a global synthesis, Vecsei (2004) [16] showed that carbonate production decreased with depth and was lower on reef flats than in other habitats. These results agree with van Woesik and Cacciapaglia (2018) [17] who showed major differences in carbonate production among reef habitats in the Republic of Palau and the island of Yap, western FSM, with outer reefs averaging greater carbonate production (i.e., 10 kg CaCO₃ m^-2 yr^-1) than inner reefs (i.e., averaging 7 kg CaCO₃ m^-2 yr^-1). Yet, in less favorable environments, Perry et al. (2013) [7] estimated that Caribbean reefs have modern net carbonate production rates averaging only 1.5 kg CaCO₃ m^-2 yr^-1, with some reefs showing net negative carbonate budgets. On the shallow fore-reef slopes in the Maldives in the Indian Ocean, Perry and Morgan (2017) [18] showed that after a thermal-stress event net accretion rates were negative, at -3 kg CaCO₃ m^-2 yr^-1. Although, through the same thermal-stress event, Ryan et al. (2019) [15] showed evidence that the upper reef crest and reef flats of the same Maldivian reef maintained positive accretion rates. In a study in the Seychelles, Januchowski-Hartley et al. (2017) [19] showed that carbonate production was dependent on thermal stress, depth, macroalgal presence, wave energy, and the abundance of excavating parrotfish.

Carbonate production rates can be influenced also by other chronic local disturbances. For example, the coral predator Acanthaster solaris has long been known to reduce coral populations when the sea stars are in high densities [20]. While high densities of Acanthaster larvae have been associated with elevated nutrient concentrations through river discharge [21, 22], other aspects of their biology and ecology remain unresolved [23]. Here we examine the influence of A. solaris populations on the capacity of Pohnpeian and Kosraean shallow-water reefs to produce carbonate, and determine the density of A. solaris, relative to the available percentage of live coral cover, beyond which carbonate production is reduced to zero.

Quantifying carbonate production is critical when predicting how coral reef systems will respond to sea-level rise, especially as the rate of sea-level rise is predicted to accelerate rapidly into the future [24–26]. These estimates in carbonate production should also influence conservation targets, especially if inner reefs require more coral cover to produce the same amount of carbonate as outer reefs [17]. Here we examine the coral reefs on two islands, Pohnpei and Kosrae, FSM, and quantify the in situ rates of carbonate production to identify which reefs are likely to keep up with sea-level rise [27, 28] under different climate-change scenarios. We also estimate whether there are differences in the threshold of percentage coral cover, at which net carbonate production becomes negative, across habitats.

Methods

Study design and field methods

Twenty-four study sites were randomly selected in each of Pohnpei (6.2°N, 158.2°E) and Kosrae (5.3°N, 162.9°E) FSM using a randomly stratified sampling approach with the package sp [29] in R [30]. In Pohnpei, reefs were stratified as inner reefs, patch reefs, and outer reefs. In Kosrae, we only stratified the reefs as either inner reefs or outer reefs (because of the lack of patch reefs). Sample size of each strata was determined by calculating the geographic area of each reef type, using the area function from the R package raster [31], and allocating the number of sites in accordance with the area estimates. Reef surveys focused on the 2–5 meters depth contour to estimate shallow-water carbonate production.

Six, 10 m transects, using a modified line-intercept technique that followed the reef substrate, were used to measure the benthic composition for every centimeter, at each site of the 48 sites [32, 17]. A few meters gap was allocated between the ends of the transects to ensure no overlap of substrate between transects. Corals were recorded to species level, except massive Porites and encrusting Montipora, which were recorded in the field as growth forms. All other organisms along each transect were identified to the highest possible taxonomic resolution. Rugosity was recorded using the planar length of a second transect that spanned across the reef horizontally. Echinoids were recorded within 30 cm on either side of the 10 m tape. The urchins were recorded as Echinometra, Diadema, and ‘Other’, and the diameter of each echinoid test was measured to the nearest 0.5 cm. The abundance of Acanthaster solaris (crown-of-thorns sea star) were recorded within 5 m along each of the six 10 m transects. Herbivorous parrotfishes were videoed and identified to species and their estimated length was recorded to the nearest cm along six transects, each of which was 30 m long by 4 m wide. Care was taken to record the fish-transect videos ahead of the other transects to avoid any disturbance to the fishes.

Carbonate production

Net carbonate production (kg CaCO₃ m^-2 yr^-1) was estimated using the following equation:

Sedimentation

The accretion of reefs can be supplemented by calcareous sedimentation [9,10], or compromised by excessive amounts of terrestrial sedimentation (when > 0.05 kg m^-2 d^-1) [34], which causes coral smothering and reduces the rate of carbonate production [36]. The sedimentation rate that was used in Pohnpei and Kosrae was 0.4 kg CaCO₃ m^-2 yr^-1 following estimates from Montaggioni (2005) [10] and Hubbard (1997) [36]. We witnessed some terrestrial runoff and a high deposition of fine sediment in the southern bay of Kosrae (Utwe Bay), and we therefore introduced a negative sedimentation component to Eq 1, using -0.4 kg CaCO₃ m^-2 yr^-1 [37], at sites that were downstream of river runoff at that location.

Erosion

Reef erosion was comprised of three biological components, echinoids or sea urchins, herbivorous fishes, and macroboring organisms. Gross erosional rates were calculated as:

Erosion by sea urchins (kg CaCO₃ m^-2) was estimated after [14] using the following equation:

Acanthaster

While Glynn (1973) [41] estimated the densities of Acanthaster that would overwhelm the ability for corals to persist, there have been no studies aimed at quantifying the influence of Acanthaster on carbonate production rates. We used field estimates to evaluate the effect of Acanthaster solaris on carbonate production as follows:

To convert net carbonate production, from Eq 1, to vertical reef growth (in mm) we used:

Carbonate thresholds

We used an additive mixed effects model in a Bayesian framework [44] to estimate the value of coral cover, for the different habitats, at which net carbonate production became negative, using the following:

We would also like to thank Eugene Joseph the Director of the Conservation Society of Pohnpei and Andy George the Director of the Kosrae Conservation Society for granting us permission to conduct research on Pohnpei and Kosrae respectively.

Results

Gross carbonate production on Pohnpei averaged 8.2 kg CaCO₃ yr^-1, and was on average higher on patch reefs (9.1 kg CaCO₃ m^-2 yr^-1) than on outer reefs (7.7 kg CaCO₃ m^-2 yr^-1) and on inner reefs (6.8 kg CaCO₃ m^-2 yr^-1) (Table 1). Net carbonate production rates closely followed rates of gross production (Table 1), although within-habitat differences were considerable (Figs 1 and 2). For example, the outer northwestern reefs of Pohnpei supported the highest rates of net carbonate production (18.5 kg CaCO₃ m^-2 yr^-1), and the lowest rates were recorded on the southeastern outer reefs (1.2–1.3 kg CaCO₃ m^-2 yr^-1), (Table 1 and Fig 2).

Gross carbonate production on Kosrae averaged 7.4 kg CaCO₃ m^-2 yr^-1, although on average the outer and inner reefs did not vary greatly (7.6 and 7.0 kg CaCO₃ m^-2 yr^-1, respectively). Similar to Pohnpei, carbonate production on the outer reefs on Kosrae was variable and was highest on the northern outer reefs (16.4 kg CaCO₃ m^-2 yr^-1), and lowest on southeastern outer reefs (0.7 kg CaCO₃ m^-2 yr^-1) (Table 1 and Fig 3). Net production differed from gross production at sites where sedimentation and erosion were much higher than background rates. This occurred in Utwe Bay in southern Kosrae, where terrestrially derived sediment was much higher than elsewhere (personal observations). Terrestrially derived sediment smothers coral colonies and thereby reduces carbonate production.

The reefs of Pohnpei and Kosrae supported similar coral assemblages, although there were some differences in species dominance. The reefs of Pohnpei, particularly the patch and inner reefs, were dominated by Porites rus, Porites cylindrica, and Porites lobata. The outer reefs were dominated by encrusting Montipora and Acropora hyacinthus (Fig 4). These five species contributed 82% of the gross carbonate production on Pohnpei. The reefs of Kosrae were dominated by encrusting Montipora, Porites rus, Goniastrea retiformis, Porites lobata, and Porites lichen (Fig 4). These five species contributed 78% of the total gross carbonate production on Kosrae. Importantly, the inner reefs of both Pohnpei and Kosrae, and the patch reefs of Pohnpei had a higher live-coral-cover threshold than the outer reefs of both islands, although there was considerable uncertainty in the thresholds for the inner reefs (i.e., high 95% credible intervals) (Fig 5).

Acanthaster solaris were observed on reefs of both islands although in 2018 populations indicative of an outbreak (>30 hectare^-1) were only observed on some of the shallow reefs of Kosrae. Since outer reefs tended to have the highest densities of A. solaris, carbonate production on these outer reefs were most affected (Table 1 and Figs 6 and 7). We re-ran the carbonate production model for both islands to incorporate A. solaris and found that carbonate production was reduced on Kosrae by an average 0.6 kg CaCO₃ m^-2 yr^-1 and on Pohnpei by 0.04 kg CaCO₃ m^-2 yr^-1, across all habitats (Table 1 and Figs 6 and 7). A. solaris densities did not reduce carbonate production to negative values, although at two northwestern sites on Kosrae gross carbonate production was reduced by 80% and 62%, where A. solaris densities were 17 and 14 (per 300 m²) and where coral cover was low. For mitigation purposes, and to sustain a productive reef, we found that A. solaris densities should be kept below a density threshold that is proportional to 7.3% of the relative coral densities (Figure B in S1 File). For example, if a 100 m² site supports 30% live coral cover, any more than two Acanthaster in that site for one year will likely reduce gross carbonate production to zero.

Other echinoids also reduce carbonate production, particularly Echinometra mathaii, and related species in high densities [38]. We noticed that large populations of E. mathaii were common on southeastern outer reefs and caused considerable erosion (Figures C and D in S1 File) and are less common on inner and patch reefs. Carbonate erosion by parrotfishes was also high on the southeastern reefs and along western reefs, and was lower elsewhere (Figures E and F in S1 File).

Discussion

This study aimed to identify the spatial variation of shallow-water carbonate production in Pohnpei and Kosrae, Federated States of Micronesia, to assess which reefs are likely to keep up with sea-level rise, and to determine what role Acanthaster solaris plays in carbonate production. While the leeward, northern and northwestern facing reefs had the highest rates of net carbonate production (16.5–18.5 kg CaCO₃ m^-2 yr^-1), the windward, southeastern facing reefs showed the lowest rates of net carbonate production (0.7–1.3 kg CaCO₃ m^-2 yr^-1). Such high variation in carbonate production along the outer shallow-water reefs is important, especially since habitats with low rates may not have the capacity to keep up with predicted sea-level rise. Based on different greenhouse-gas-emission scenarios, most frequently conveyed as Representative Carbon Pathways (RCPs) 2.6. 4.5, 6.0, and 8.5 Wm^-2, the predicted rates of sea-level rise by the year 2100 have been conservatively estimated at 5, 6.5, 6.7, and 9 mm yr^-1, respectively [25]. Converting the field estimated rates of carbonate production to vertical rates of reef accretion (following Eq 18) the northwestern shallow-water outer reefs of Pohnpei and the northern shallow-water outer reefs of Kosrae are estimated to vertically accrete at 11.8 mm yr^-1 and 11.2 mm yr^-1, respectively. These rates of vertical accretion are relatively high for contemporary reefs and exceed the rates of sea-level rise under RCP 8.5. Therefore, if effectively managed, the northern shallow-water outer reefs of Pohnpei and Kosrae will likely have the capacity to keep up with sea-level rise and maintain their essential ecosystem functions.

By contrast, the southeastern shallow-water reefs of Pohnpei and Kosrae, have estimated vertical accretion rates of only 1.3 mm yr^-1. Although we did not directly measure sedimentation rates nor did we measure micro-bioerosion rates, our data suggest that the southeastern reefs are not likely to keep up with sea-level rise by the year 2100. Indeed, the projections of our model show that without considering spatial variation, on average, Pohnpei’s and Kosrae’s shallow-water outer reefs fall short of the moderate rates of sea-level rise projected under RCP 4.5 (i.e., 6.2 mm yr^-1 accretion and 6.5 mm yr^-1 of sea-level rise under RCP 4.5, Figure G in S1 File). Most concerning is that the shallow-water inner reefs of both islands have estimated rates of vertical accretion averaging 5.9 mm yr^-1, which is lower than most predicted rates of sea-level rise, even the conservative rates associated with RCP 4.5 by the year 2100 (Figure G in S1 File). Although there is some uncertainty in the live-coral-cover thresholds for the inner reefs of Pohnpei and Kosrae, these inner reefs on average require around 5.5% higher live coral cover than outer reefs to produce the same amount of carbonate (Fig 4). These results provide a strong conservation message that in order for nearshore reefs to have a chance to keep up with sea-level rise, it is critical to mitigate land-use discharge and pollution to nearshore shallow-water reefs. Without conserving these relatively sensitive, nearshore reefs our projections suggest that they would likely drown in the near future. Additionally, mitigating terrestrial runoff may also prevent large, persistent outbreaks of Acanthaster [21, 22].

Increases in the survival of Acanthaster brachiolaria-stage larvae have been associated with river discharge and elevated nutrient concentrations [21, 22]. Therefore, implementing management strategies on small Pacific islands that mitigate terrestrial discharge will not only reduce sedimentation stress on corals but will also effectively suppress chronically dense Acanthaster populations that reduce carbonate production. In Kosrae, A. solaris reduced carbonate production by on average 0.6 kg CaCO₃ m^-2 yr^-1, with a maximum reduction of 5.1 kg CaCO₃ m^-2 yr^-1 when A. solaris were at high densities (> 15 individuals per 300 m²). Although our estimates of the impact of A. solaris on carbonate production are novel, they still comprise a degree of uncertainty because of the assumptions underlying Eqs 14–17. Therefore, future improvements in these estimates can be made by examining these assumptions, which could include an adjustment for coral composition.

The coral species that were the most important contributors to carbonate production on both islands were Porites rus and Porites lobata, particularly on the shallow-water inner reefs. The most important contributors to carbonate production on the shallow-water outer reefs were encrusting Montipora, merulinids, and acroporids. There was a lack of patch reefs and large lagoons in Kosrae, therefore Porites cylindrica was less common on Kosrae than on Pohnpei. Yet, if the shallow-water outer reefs are unable to keep up with sea-level rise and are breached by offshore waves, the patch reefs of Pohnpei and the inner reefs of both islands will likely become more similar in coral composition to that of the outer reefs [47]. Still, whether these reefs, even altered in composition, will be able to produce enough carbonate to keep up with sea-level rise is an open question.

The shallow-water coral-reef carbonate production rates measured on both Pohnpei and Kosrae are lower than the field estimates recorded farther west on Palau and Yap (~2.2 kg CaCO₃ m^-2 yr^-1 less, when averaged among habitat types) [17]. The lower carbonate production rates are most likely a result of reduced Acropora cover caused by the recent thermal-stress events on both Kosrae and Pohnpei in 2016 and 2017 (Peter Houk, pers. comm.). At the same time, similar thermal-stress events were not recorded in Palau and Yap. Thermal-stress events are known to significantly reduce a reefs’ capacity to produce calcium carbonate [19], and under extreme events can temporarily reduce net accretion to negative values [18]. Although there are some studies on the net ecosystem calcification of coral reefs and the influence of coral bleaching on that process [47–51], more field studies are needed that examine (i) thermal-stress events and the dynamics of carbonate production through those events, and (ii) the rates of recovery of carbonate production from thermal-stress events.

The capacity of coral reefs to keep up with rising sea level is important for coastal residents and is particularly relevant to residents of low-lying islands who cannot move to higher elevations. Historically, healthy coral reefs have kept up with dynamic shifts in sea level through glacial-interglacial periods [52], yet disturbances to modern reefs are suppressing the capacity of coral reefs to produce enough carbonate [53] and protect island residents from storm-wave damage. In addition, drowned reefs will not be able to provide goods and services or support fisheries. If the coral species contributing to reef complexity and carbonate production are unable to persist under the stress of climate change then the coral reefs will not keep up with sea-level rise and drown.

Supporting information

S1 File [a]
Seven supporting figures.

S1 Data [rar]
Data and R code.