Documenting decadal disturbance dynamics reveals archipelago-specific recovery and compositional change on Polynesian reefs

Differential effects of nutrients and consumer pressure on sympatric cryptic coral species (Pocillopora spp.)

Cryptic species (evolutionarily distinct lineages that do not align with morphologically defined species) are being increasingly discovered but are poorly integrated into ecological theory. In particular, we still lack a useful understanding of if and how cryptic species differ in ways that affect community recovery from disturbances and responses to anthropogenic stressors, such as the removal of consumers and pollution from nutrients. On coral reefs, nutrient pollution increases the growth of macroalgae that displace corals. Reductions in herbivorous fishes reduce the suppression of macroalgae, while reductions in coralivorous fishes reduce predation on corals. An unresolved question is if and how cryptic coral species respond differently to these impacts, thereby differing in their ability to influence coral community dynamics and maintain coral dominance. Therefore, we assessed how the response of cryptic Pocillopora species over a period of three years following a simulated disturbance from a cyclone depended on the experimental reduction of fish consumer pressure and nutrient addition. After three years, five morphologically cryptic, but genetically distinct, Pocillopora species recruited to the reef. However, recruitment was dominated by two species: Pocillopora tuahiniensis (46%) and Pocillopora meandrina (43%). Under ambient conditions, recruitment of P. tuahiniensis and P. meandrina was similar, but experimentally reducing consumer pressure increased recruitment of P. tuahiniensis by up to 73% and reduced recruitment of P. meandrina by up to 49%. In both species, nutrient enrichment increased recruitment and colony growth rates equally, but colonies of P. tuahiniensis grew faster and were up to 25% larger after three years than those of P. meandrina, and growth was unaffected by reduced consumer pressure. Predation by excavating corallivorous fish was higher for P. meandrina than for P. tuahiniensis, especially under nutrient enrichment. In contrast, polyp extension (an indicator of elevated heterotrophic feeding as well as susceptibility and attractiveness to corallivores) was lower for P. meandrina than for P. tuahiniensis, especially under low to medium consumer pressure. Overall, we uncovered ecological differences in the response of morphologically cryptic foundation species to two pervasive stressors on coral reefs. Our results demonstrate how cryptic species respond differently to key anthropogenic stressors, which may contribute to response diversity that can support ecological resilience or increase extinction risk.

A seascape dichotomy in the role of small consumers for coral reef energy fluxes

Biogeochemical fluxes through ecological communities underpin the functioning of ecosystems worldwide. These fluxes are often heavily influenced by small-bodied consumers, such as insects, worms, mollusks, or small vertebrates, which transfer energy and nutrients from autotrophic sources to larger animals. Although coral reefs are one of the most productive ecosystems in the world, we know relatively little about how small consumers make energy available to larger predators and how their roles may vary across reefs. Here, we use community-scale collections of small, bottom-dwelling ("cryptobenthic") reef fishes along with size spectrum analyses, stable isotopes, and demographic modeling to examine their role in harnessing and transferring carbon in two distinct coral reef habitats. Using a comprehensive dataset from Mo'orea (French Polynesia), we demonstrate that, despite only being separated by a narrow reef crest, forereef and backreef habitats harbor distinct communities of cryptobenthic fishes that play vastly divergent roles in carbon transfer. Forereef communities in Mo'orea are depauperate, largely consisting of predatory and planktivorous species that have comparatively high standing biomass (both individually and collectively). In these communities, the combination of size spectra and isotope values suggests important contributions of pelagic subsidies, but the rate of biomass production and turnover (i.e., the rate at which biomass is replenished) is relatively low. In contrast, cryptobenthic fish communities in the backreef are characterized by high abundances of the smallest bodied species, forming a traditional bottom-heavy trophic pyramid that is fueled by benthic autotrophs. In these communities, benthic productivity fuels rapid production and turnover of fish biomass, while pelagic energy channels are notably less productive. Our integrative approach demonstrates the utility of combining multiple methods (e.g., isotopically informed demographic models) to trace energy fluxes through small consumer communities in complex ecosystems. Furthermore, our results highlight that coral reef productivity dynamics are highly habitat-dependent and the role of the smallest coral reef consumers may be most pronounced in shallow systems with limited connectivity to the open ocean.

Niche breadth and divergence in sympatric cryptic coral species (Pocillopora spp.) across habitats within reefs and among algal symbionts

While the presence of morphologically cryptic species is increasingly recognized, we still lack a useful understanding of what causes and maintains co-occurring cryptic species and its consequences for the ecology, evolution, and conservation of communities. We sampled 724 Pocillopora corals from five habitat zones (the fringing reef, back reef, and fore reef at 5, 10, and 20 m) at four sites around the island of Moorea, French Polynesia. Using validated genetic markers, we identified six sympatric species of Pocillopora, most of which cannot be reliably identified based on morphology: P. meandrina (42.9%), P. tuahiniensis (25.1%), P. verrucosa (12.2%), P. acuta (10.4%), P. grandis (7.73%), and P. cf. effusa (2.76%). For 423 colonies (58% of the genetically identified hosts), we also used psbA ncr or ITS2 markers to identify symbiont species (Symbiodiniaceae). The relative abundance of Pocillopora species differed across habitats within the reef. Sister taxa P. verrucosa and P. tuahiniensis had similar niche breadths and hosted the same specialist symbiont species (mostly Cladocopium pacificum) but the former was more common in the back reef and the latter more common deeper on the fore reef. In contrast, sister taxa P. meandrina and P. grandis had the highest niche breadths and overlaps and tended to host the same specialist symbiont species (mostly C. latusorum). Pocillopora acuta had the narrowest niche breadth and hosted the generalist, and more thermally tolerant, Durusdinium gynnii. Overall, there was a positive correlation between reef habitat niche breadth and symbiont niche breadth-Pocillopora species with a broader habitat niche also had a broader symbiont niche. Our results show how fine-scale variation within reefs plays an important role in the generation and coexistence of cryptic species. The results also have important implications for how niche differences affect community resilience, and for the success of coral restoration practices, in ways not previously appreciated.

Asynchrony in coral community structure contributes to reef-scale community stability

Many aspects of global ecosystem degradation are well known, but the ecological implications of variation in these effects over scales of kilometers and years have not been widely considered. On tropical coral reefs, kilometer-scale variation in environmental conditions promotes a spatial mosaic of coral communities in which spatial insurance effects could enhance community stability. To evaluate whether these effects are important on coral reefs, we explored variation over 2006-2019 in coral community structure and environmental conditions in Moorea, French Polynesia. We studied coral community structure at a single site with fringing, back reef, and fore reef habitats, and used this system to explore associations among community asynchrony, asynchrony of environmental conditions, and community stability. Coral community structure varied asynchronously among habitats, and variation among habitats in the daily range in seawater temperature suggested it could be a factor contributing to the variation in coral community structure. Wave forced seawater flow connected the habitats and facilitated larval exchange among them, but this effect differed in strength among years, and accentuated periodic connectivity among habitats at 1-7 year intervals. At this site, connected habitats harboring taxonomically similar coral assemblages and exhibiting asynchronous population dynamics can provide insurance against extirpation, and may promote community stability. If these effects apply at larger spatial scale, then among-habitat community asynchrony is likely to play an important role in determining reef-wide coral community resilience.

Reef environments shape microbial partners in a highly connected coral population

Evidence is mounting that composition of microorganisms within a host can play an essential role in total holobiont health. In corals, for instance, studies have identified algal and bacterial taxa that can significantly influence coral host function and these communities depend on environmental context. However, few studies have linked host genetics to algal and microbial partners across environments within a single coral population. Here, using 2b-RAD sequencing of corals and metabarcoding of their associated algal (ITS2) and bacterial (16S) communities, we show evidence that reef zones (locales that differ in proximity to shore and other environmental characteristics) structure algal and bacterial communities at different scales in a highly connected coral population (Acropora hyacinthus) in French Polynesia. Fore reef (FR) algal communities in Mo'orea were more diverse than back reef (BR) communities, suggesting that these BR conditions constrain diversity. Interestingly, in FR corals, host genetic diversity correlated with bacterial diversity, which could imply genotype by genotype interactions between these holobiont members. Our results illuminate that local reef conditions play an important role in shaping unique host-microbial partner combinations, which may have fitness consequences for dispersive coral populations arriving in novel environments.

Mesophotic coral communities escape thermal coral bleaching in French Polynesia

Climate change and consequent coral bleaching are causing the disappearance of reef-building corals worldwide. While bleaching episodes significantly impact shallow waters, little is known about their impact on mesophotic coral communities. We studied the prevalence of coral bleaching two to three months after a heat stress event, along an extreme depth range from 6 to 90 m in French Polynesia. Bayesian modelling showed a decreasing probability of bleaching of all coral genera over depth, with little to no bleaching observed at lower mesophotic depths (greater than or equal to 60 m). We found that depth-generalist corals benefit more from increasing depth than depth-specialists (corals with a narrow depth range). Our data suggest that the reduced prevalence of bleaching with depth, especially from shallow to upper mesophotic depths (40 m), had a stronger relation with the light-irradiance attenuation than temperature. While acknowledging the geographical and temporal variability of the role of mesophotic reefs as spatial refuges during thermal stress, we ought to understand why coral bleaching reduces with depth. Future studies should consider repeated monitoring and detailed ecophysiological and environmental data. Our study demonstrated how increasing depth may offer a level of protection and that lower mesophotic communities could escape the impacts of a thermal bleaching event.