Eastern Tropical Pacific Reefs Research Articles

Increased dominance of heat-tolerant symbionts creates resilient coral reefs in near-term ocean warming

Climate change is radically altering coral reef ecosystems, mainly through increasingly frequent and severe bleaching events. Yet, some reefs have exhibited higher thermal tolerance after bleaching severely the first time. To understand changes in thermal tolerance in the eastern tropical Pacific (ETP), we compiled four decades of temperature, coral cover, coral bleaching, and mortality data, including three mass bleaching events during the 1982 to 1983, 1997 to 1998 and 2015 to 2016 El Niño heatwaves. Higher heat resistance in later bleaching events was detected in the dominant framework-building genus, Pocillopora, while other coral taxa exhibited similar susceptibility across events. Genetic analyses of Pocillopora spp. colonies and their algal symbionts (2014 to 2016) revealed that one of two Pocillopora lineages present in the region (Pocillopora "type 1") increased its association with thermotolerant algal symbionts (Durusdinium glynnii) during the 2015 to 2016 heat stress event. This lineage experienced lower bleaching and mortality compared with Pocillopora "type 3", which did not acquire D. glynnii. Under projected thermal stress, ETP reefs may be able to preserve high coral cover through the 2060s or later, mainly composed of Pocillopora colonies that associate with D. glynnii. However, although the low-diversity, high-cover reefs of the ETP could illustrate a potential functional state for some future reefs, this state may only be temporary unless global greenhouse gas emissions and resultant global warming are curtailed.

Functional potential of coral assemblages along a typical Eastern Tropical Pacific reef tract

Coral reefs are considered to be valuable ecosystems due to the goods and services that they provide. However, these ecosystem services are highly dependent on the physical structure of the reef, which in turn depends on the species composition of hermatypic corals. In this study, we explored the functional potential of reefs with a new index that takes into account coral cover, the calcification rate, rugosity, and the height of each species. In June 2019, we surveyed 11 reefs from the southern Mexican Pacific (15°40′N, 96°29′W−15°45′N, 96°5′W) to estimate coral cover and collect quantitative morphometric data of ~15 haphazardly selected coral colonies per site (N = 102). We estimated the calcification rates and structural complexity of the species present and then calculated the Reef Functional Index (RFI), which estimates the species-specific functional contribution of the corals in reef systems by integrating their capacity to create complex three-dimensional structures through calcium carbonate precipitation with morphological complexity. We modelled the effects of shifting species compositions through a permutation approach with 6435 possible combinations. We found a mean coral cover of 48.5 ± 18.09% (±SD) dominated by Pocillopora damicornis, Pocillopora verrucosa, and Pocillopora capitata and low abundance of Porites panamensis and Pavona gigantea. The mean RFI was 0.71 ± 0.1. The permutation model showed that the RFI was very close to its maximum potential (96%) when the relative abundance of pocilloporids was ≥60% and decreased as the relative abundance of massive corals increased. At maximum evenness, the RFI value was at 92% of its maximum potential. Our data highlight the importance of pocilloporids to the functioning of reefs in the Eastern Tropical Pacific (ETP) and confirm that P. damicornis is the primary reef builder in the region. Nevertheless, our findings also highlight the vulnerability of these reefs given that branching corals are more susceptible to environmental disturbance than slow-growing massive corals. In a rapidly changing environment, the future of ETP reefs depends not only on the survival of pocilloporids but also on the ability of coral reefs to withstand highly variable oceanographic conditions as complete ecosystems.

Coral reef resilience to thermal stress in the Eastern Tropical Pacific.

Coral reefs worldwide are threatened by thermal stress caused by climate change. Especially devastating periods of coral loss frequently occur during El Niño-Southern Oscillation (ENSO) events originating in the Eastern Tropical Pacific (ETP). El Niño-induced thermal stress is considered the primary threat to ETP coral reefs. An increase in the frequency and intensity of ENSO events predicted in the coming decades threatens a pan-tropical collapse of coral reefs. During the 1982-1983 El Niño, most reefs in the Galapagos Islands collapsed, and many more in the region were decimated by massive coral bleaching and mortality. However, after repeated thermal stress disturbances, such as those caused by the 1997-1998 El Niño, ETP corals reefs have demonstrated regional persistence and resiliency. Using a 44year dataset (1970-2014) of live coral cover from the ETP, we assess whether ETP reefs exhibit the same decline as seen globally for other reefs. Also, we compare the ETP live coral cover rate of change with data from the maximum Degree Heating Weeks experienced by these reefs to assess the role of thermal stress on coral reef survival. We find that during the period 1970-2014, ETP coral cover exhibited temporary reductions following major ENSO events, but no overall decline. Further, we find that ETP reef recovery patterns allow coral to persist under these El Niño-stressed conditions, often recovering from these events in 10-15years. Accumulative heat stress explains 31% of the overall annual rate of change of living coral cover in the ETP. This suggests that ETP coral reefs have adapted to thermal extremes to date, and may have the ability to adapt to near-term future climate-change thermal anomalies. These findings for ETP reef resilience may provide general insights for the future of coral reef survival and recovery elsewhere under intensifying El Niño scenarios.

Sponge diversity in Eastern Tropical Pacific coral reefs: an interoceanic comparison

Sponges are an important component of coral reef communities. The present study is the first devoted exclusively to coral reef sponges from Eastern Tropical Pacific (ETP). Eighty-seven species were found, with assemblages dominated by very small cryptic patches and boring sponges such as Cliona vermifera; the most common species in ETP reefs. We compared the sponge patterns from ETP reefs, Caribbean reefs (CR) and West Pacific reefs (WPR), and all have in common that very few species dominate the sponge assemblages. However, they are massive or large sun exposed sponges in CR and WPR, and small encrusting and boring cryptic species in ETP. At a similar depth, CR and WPR had seven times more individuals per m2, and between four (CR) and five times (WPR) more species per m2 than ETP. Perturbation, at local and large scale, rather than biological factors, seems to explain the low prevalence and characteristics of sponge assemblages in ETP reefs, which are very frequently located in shallow water where excessive turbulence, abrasion and high levels of damaging light occur. Other factors such as the recurrence of large-scale phenomena (mainly El Niño events), age of the reef (younger in ETP), isolation (higher in ETP), difficulty to gain recruits from distant areas (higher in ETP), are responsible for shaping ETP sponge communities. Such great differences in sponge fauna between the three basins might have consequences for coral reef structure and dynamics.

Calcification of coral assemblages in the eastern Pacific: Reshuffling calcification scenarios under climate change

The rearrangement of coral assemblages may produce significant changes in coral community calcification, yet it is not understood how the modification of community structure in depauperate areas under climate change scenarios may affect reef functionality. Observed coral community calcification (OCC) was calculated using coral cover data from 126 sites across the eastern tropical Pacific (ETP). To assess the effect that species assemblages exert on potential coral community calcification (PCC) of ETP reefs, we implemented a novel permutation approach for this purpose. We contrasted OCC across the ETP against the PCC of hypothetical monospecific and maximum ecological evenness (ME) ETP reefs and monogeneric Indo-Pacific (IP) and Caribbean (C) reefs. Average coral cover (21 ± 23%; mean ± SD) and OCC (8.23 ± 11.32 kg m−2 yr−1; mean ± SD) in the ETP were not related to species richness but to Pocillopora abundance and calcification. For any level of coral abundance or species richness, the permutation model indicates that PCC depends on community structure: the PCC of Pocillopora monospecific reefs reaches its maximum; PCC drops to half of its potential when ME is attained; and PCC reaches its minimum when slow-growing species turn dominant. A dynamic model with changing community structure based on the differential species tolerance to climate change showed a similar pattern as the permutation model. ETP Pocillopora reefs have lower PCC than IP Acropora, IP Pocillopora and C Acropora. Massive taxa are more tolerant to climate change and could replace branching taxa if environmental pressure trends in the ETP continue, meaning an ∼85% drop in PCC.

Rapid recovery of a coral dominated Eastern Tropical Pacific reef after experimentally produced anthropogenic disturbance

Local anthropogenic stressors such as overfishing, nutrient enrichment and increased sediment loading have been shown to push coral reefs toward greater dominance by algae. In a few cases this shift has been temporary, with the ability to recover to a healthy coral-dominated community after disturbance, suggesting some systems have considerable resilience. However, an understanding of the circumstances under which reefs may recover is only beginning to emerge. We monitored recovery of a coral-dominated reef in the Eastern Tropical Pacific (ETP) after cessation of a ∼6 month multiple stressor experiment (with herbivore exclosure, nutrient addition, and sediment addition). We observed substantial recovery from small-scale disturbances, though there were differences in both the extent and temporal dynamics of recovery between treatments. Plots that had been caged showed the largest recovery in absolute terms and recovery was quite rapid, while nutrient and sediment addition plots were slower to recover. We also observed different recovery patterns depending on the type of algae that replaced coral during or after disturbances. Macroalgae that established during manipulation were almost completely removed within 2 weeks, revealing that a significant proportion had covered still-living coral. Turf algae persisted longer, but were almost completely replaced by regenerating coral within 18 months. Very little crustose coralline algae were apparent during manipulations, but coverage did increase during recovery. This rapid recovery of corals after simulated anthropogenic disturbance to ETP reefs underscores the value of management of local stressors for short-term recovery and perhaps as a buffer for longer-term global stressors.

Functional diversity of fish and invertebrates in coral and rocky reefs of the Eastern Tropical Pacific

AbstractTo define the functional groups of fish and macroinvertebrates in the Eastern Tropical Pacific (ETP), visual censuses were performed in 18 areas of four biogeographic provinces: Cortés, Mexican, Panamic and Oceanic Islands. A total of 257 fish and macroinvertebrate species were recorded, and from them, 27 morpho‐functional groups (MFGs) were constructed on the basis of trophic level, maximum size, taxonomy and morphology. Biomass, richness, diversity and evenness of MFGs were calculated for each province and compared statistically; a regression analysis between taxonomic and functional diversity was conducted to observe the relationship between these two indicators. There were significant differences in all ecological indices (p < .002), highlighting the high biomass and richness of MFGs in the Cortés and Oceanic Islands provinces (>400 g/m2), associated with the influence of cold currents in the northernmost region and less fishing pressure in protected areas. A decreasing pattern of MFG richness towards the tropics was observed, which demonstrates that in the ETP, the relationship between habitat heterogeneity and species diversity has been translated into functional complexity. The Mexican province was the most functionally diverse (biomass well distributed in the MFG; H′ = 0.46 ± 0.009). Related to this, it is predicted that biomass is biased towards certain functional groups (i.e., large carnivores), which shows that the H′ index of the MFG is not a good indicator of the conservation status of ETP reefs. Finally, regression analysis suggests that functional diversity increases at low species diversity but eventually reaches an asymptote (almost all possible functions are represented).

Mechanisms of resilience: empirically quantified positive feedbacks produce alternate stable states dynamics in a model of a tropical reef

Summary Alternate stable states (ASS) theory is a dominant conceptual framework for understanding processes that support resilience of ecological communities in the face of multiple anthropogenic disturbances. For decades, coral reefs have been cited as a classic example of ASS, yet this position remains highly controversial, largely because convincing empirical evaluations have been elusive. Using a combination of empirical measurements of positive feedback processes and simulation modelling, we assessed ASS in coral reefs of the Eastern Tropical Pacific (ETP) by identifying the potential for multiple basins of attraction from a mechanistic perspective. Using bioassays of algal growth and consumption, we quantified two spatially localized positive feedback mechanisms on coral reefs: increasing herbivore activity associated with higher coral abundances and amelioration of nutrient limitation leading to higher algal growth in areas with higher algal abundance. Analysis of a model of benthic community dynamics incorporating these feedbacks showed they were central to producing hysteresis and bistability in the model, two hallmarks of ASS. Further, reefs simulated with conditions matching those measured at Isla Contadora, Panamá, displayed ASS dynamics while producing spatial patterns matching field observations. This provides model‐derived support, based on empirically measured conditions of ETP reefs, for the presence of ASS. Synthesis. The combination of experimental bioassays to measure feedback strengths and simulation models to evaluate the influence of those feedbacks provides a novel, non‐destructive approach to evaluating ASS dynamics that can be applied in threatened ecosystems where classic approaches are not viable.

Poorly cemented coral reefs of the eastern tropical Pacific: Possible insights into reef development in a high-CO 2 world

Ocean acidification describes the progressive, global reduction in seawater pH that is currently underway because of the accelerating oceanic uptake of atmospheric CO(2). Acidification is expected to reduce coral reef calcification and increase reef dissolution. Inorganic cementation in reefs describes the precipitation of CaCO(3) that acts to bind framework components and occlude porosity. Little is known about the effects of ocean acidification on reef cementation and whether changes in cementation rates will affect reef resistance to erosion. Coral reefs of the eastern tropical Pacific (ETP) are poorly developed and subject to rapid bioerosion. Upwelling processes mix cool, subthermocline waters with elevated pCO(2) (the partial pressure of CO(2)) and nutrients into the surface layers throughout the ETP. Concerns about ocean acidification have led to the suggestion that this region of naturally low pH waters may serve as a model of coral reef development in a high-CO(2) world. We analyzed seawater chemistry and reef framework samples from multiple reef sites in the ETP and found that a low carbonate saturation state (Omega) and trace abundances of cement are characteristic of these reefs. These low cement abundances may be a factor in the high bioerosion rates previously reported for ETP reefs, although elevated nutrients in upwelled waters may also be limiting cementation and/or stimulating bioerosion. ETP reefs represent a real-world example of coral reef growth in low-Omega waters that provide insights into how the biological-geological interface of coral reef ecosystems will change in a high-CO(2) world.