For our RAPID project, our overarching hypothesis was that many coral reefs in the Eastern Tropical Pacific (ETP) are becoming more resilient in the face of multiple major ENSO disturbances as a result of adaptive processes.  If we are correct that reef systems can develop more tolerance and recover faster when subjected to repeated thermal disturbances, this will drastically change predictions regarding the fate of global coral reef ecosystems over the next 100 years of climate change. However, we also predicted there will be a limit to resilience beyond which reefs can no longer adapt and ecosystem functions break down. While many reefs of the ETP appear to be recovering rapidly, there has been no recovery at sites chronically exposed to high-CO₂/low pH conditions (e.g., the southern Galápagos), suggesting a possible acidification threshold or tipping point for reef resilience to repeated thermal stress. Thus, this ENSO provides a unique opportunity to transform our understanding of coral reef resilience, and especially its limits, in the face of climate-related disturbance.

We developed hypotheses that we tested by targeted sampling and experiments in the critical stages before and during the ENSO on reefs in Panamá (Uva and Saboga) and Galápagos (Darwin and Floreana). We will continue these efforts after this ENSO with funds from National Geographic and NOAA. Our study reefs span a documented gradient in acidification that provides a real-world model system for conditions expected throughout the tropics in a high-CO₂ world.  

In each site with a track record of recovery, methods included: (1) in situ measurement of physical parameters (temperature, conductivity, pH, DO, PAR, chlorophyll, turbidity, inorganic nutrients); (2) in situ measurement of carbonate chemistry and net ecosystem metabolism (calcification, production); (3) In situ measurements of coral and reef community responses including coral bleaching and mortality and the population responses of corallivores, bioeroders, herbivores, and benthic algal cover; (4) characterization of symbiont communities in major coral taxa before, during and after the bleaching event to compare with archived samples from the 1997-98 event;  

Key Outcomes

1) Bleaching was overall greatest on Uva Island reef compared to the Gulf of Panama or the Galapagos, especially the shallow zones.  Deeper reef zones bleached significantly less, providing a refuge from stress during this time.

2) Temperature data from Uva indicate that, for near surface corals, temperature conditions and stress accumulation were lower in the second year (2015-16) than in the first year of the event (2014-15).  These data are critical to help us understand the 2015-16 event in the context of past episodes and the potential adaptation of reefs to repeated warming.

3) At each site, we deployed paired SeaFET pH and SAMI- or Pro-Oceanus CO₂ sensors, providing a long time-series of ?_arag before, during, and after bleaching. Quality data were obtained for > 6 months, showing that reefs in Panama had a higher diurnal pH range than that from 68 different reefs across 10 regions spanning the Pacific and Indian Oceans (Manzello, unpub. data). In 2014 and 2015, the carbon metabolism of bleached reefs was investigated using the Benthic Ecosystem and Acidification Measurement System (BEAMS), which uses boundary layer flux methods. During bleaching, there was a decline in calcification and productivity, an increase in [H⁺] flux from the reef (i.e., localized acidification), and increased nighttime dissolution. To our knowledge this is the first time carbonate chemistry and reef metabolism were measured during a coral bleaching event. The documented localized acidification and increased dissolution during bleaching has serious implications for reef structural persistence with warming and acidification.

4) Top-down control by relatively intact herbivore communities strengthened significantly during ENSO in Panama, limiting algal proliferation during the time when coral was most vulnerable to competition. In contrast, the extremely strong baseline force of herbivory (pre-ENSO) in the Galapagos and did not change during ENSO.

5) Bottom up control of algae via nutrient-limitation also intensified during ENSO in Panama, limiting algal competitive abilities. However, while algae suffered widespread mortality during ENSO in the Galapagos, this was not related to nutrient limitation.

6) Changes in algal symbiont communities in favor of more thermally tolerant Symbiodinium glynni (D1). Although these shifts were not as severe as those during 1997-98, they support an emerging model in which D1 offers thermal tolerance, and colonies that survive bleaching increase their symbionts in favor of D1. Reversions in favor of the original symbionts occur over a timespan of years-decades, but this process may be slowing as temperatures continue to warm, and bleaching is repeated.  

Last Modified: 11/29/2017
Modified by: Andrew C Baker