Global climate change is now the leading cause of coral-reef degradation, but how intermediate-scale (‘mesoscale’) oceanography overprints climatic forcing is poorly understood. Reef ecosystems in Pacific Panamá collapsed from ~4100 to 1600 years ago. The primary cause of that 2500-year hiatus was increased variability of the El Niño–Southern Oscillation (ENSO). The goal of this study was to determine the influence of mesoscale oceanography on the condition of coral reefs in the eastern tropical Pacific (ETP). Reefs in Pacific Panamá exist along a natural oceanographic gradient, from the strongly upwelling Gulf of Panamá (GoP) to the weakly upwelling Gulf of Chiriquí (GoC). They provide a natural laboratory for studying how mesoscale oceanography modulates the effects of climate change.

Over millennial timescales, coral reefs at three sites in the GoC recovered more quickly from climatic perturbations driven by ENSO than three reef-sites in the GoP. In recent decades, corals in the GoC have correspondingly had higher growth rates than in the GoP. The situation appears to be changing, however. Although both gulfs have warmed significantly over the last 150 years, thermal extremes in the GoC are increasing faster. In contrast to historical trends, the cover of living corals, coral survival, and coral growth-rates were all higher in the GoP. Corals bleached extensively in the GoC following the 2015–2016 El Niño event, whereas upwelling in the GoP allowed the corals there largely to escape high-temperature stress. As the climate continues to warm, upwelling zones in the ETP may offer a temporary, localized refuge for coral populations, while reef growth in the rest of the ETP continues to decline.

The shifting balance between species that calcify and erode the calcium-carbonate framework of a coral reef determines whether it will grow vertically toward the surface and at what rate, while sea level is rising with climate change. In order to measure the scope for upward growth in the ETP, standardized blocks of coral of the genus Porites were placed at the three reefs in the GoP and the three in the GoC. Oceanographic instruments  characterized the thermal and chemical conditions of each gulf. Satellite data were used to examine differences in planktonic productivity, and divers surveyed the abundance of fish and sea-urchins that grazed the bottom. After two years, the coral blocks were collected and scanned using high-resolution computed tomography (CT) to quantify the extent to which they were altered at the surface and within the blocks. The GoP, which has seasonally higher productivity, cooler temperatures, and more acidic conditions, had higher rates of boring into the blocks, but also higher rates of calcium-carbonate deposition. The unexpectedly higher rates of calcification resulted from high colonization rates of filter-feeding species, particularly barnacles and snails. Higher nutrient levels in the GoP as a result of seasonal upwelling resulted in more phytoplankton, which meant higher colonization rates of filter-feeding calcifiers, but also filter-feeding, internal eroders. Surface erosion by fish and sea urchins was the dominant influence on coral blocks across both gulfs, causing greater erosion in the GoC. These results underscore the important role that fish and sea urchins play in not just removing algae from reefs, but also in eroding reef habitat.

Carbonate budgets—budgets of reef-framework growth and erosion—were calculated for the six reefs, taking into account the growth of coral populations, as well as the eroders and calcifiers other than corals in the coral-block study. The carbonate budgets showed that reefs in the GoP should be able to keep up with future sea-level rise if greenhouse gases are moderately controlled (RCP 4.5 of the IPCC). Coral reefs in the GoC, however, are producing calcium-carbonate production at low rates and remain highly vulnerable to future high-temperature events. This means that reefs in the GoC are severely limited in their potential to keep up with sea-level rise. Under the IPCC’s business-as-usual scenario, RCP 8.5, reefs in neither gulf would be able to keep up with rising sea level. The Gulf of Panamá, therefore, could provide a temporary refuge for coral reefs if action is taken to curb carbon emissions.

This research explored the effects of climate change on coral reefs, a subject of deep concern to the scientific community and the public. The study provided opportunities for undergraduate, graduate, and postdoctoral training. It provided research opportunities for students from underrepresented groups and promoted women in scientific careers. Two science lessons with accompanying laboratory exercises for high-school students were created and beta-tested. Investigators and students working on the project presented talks and led activities for the general public and K–12 students. K–12 activities included outreach to underrepresented students and a film festival on marine-science subjects for students. The project is providing important information to managers and policymakers on how mesoscale oceanography modulates climatic impacts on coral reefs.

Last Modified: 10/06/2021
Modified by: Richard B Aronson