Century-scale Records of Coral Growth and Water Quality from the Mesoamerican Reef Reveal Increasing Anthropogenic Stress and Decreasing Coral Resilience
Coral reefs provide extensive ecosystem goods and services to the communities that depend upon them including food, shoreline protection, and tourism income. Unfortunately, reefs worldwide are being devastated by a range of factors including overexploitation, pollution, and ocean warming and acidification. This study was undertaken with a conservation-minded focus: I wanted to investigate why reefs in Mesoamerica were dying, in order to inform management decisions regarding resource allocation for reef protection. I suspected that runoff was a major impact in the region that was not being taken into account. While the establishment of marine protected areas is important, these boundaries do not prevent polluted runoff from reaching the reefs.
In order to investigate whether runoff was negatively impacting the reef, I collected numerous core samples from large Montastraea faveolata coral heads. I measured coral growth rates, metal content, and stable carbon and oxygen isotopes in order to reconstruct changes in coral health and water quality, respectively. I found that the site with the highest levels of runoff had a decreasing trend in extension rates beginning in the 1970’s, indicating that as runoff has increased, coral growth has begun a slow decline. There was no long-term decline at other sites, but all sites were severely impacted by bleaching in 1998. Bleaching (the loss of the coral’s symbiotic algae) on this large of a scale is caused by thermal stress. I investigated long-term records of heat stress, and found that 1998 was not an exceptional year: 1958 was even warmer in this region, yet no bleaching had occurred, as indicated by a lack of skeletal growth anomalies. This might be explained by a reduction in the coral’s thermal tolerance threshold in recent years, and I hypothesize that local anthropogenic stress is the culprit. I also found that coral growth rates did not recover, even 8 years after the bleaching event, at sites that experience higher local stress, quantified here using a combination of local human population, fish abundance, sedimentation, and nutrient runoff. These findings indicate that coral resistance to bleaching and resilience after bleaching (as quantified by growth rates) are both reduced with local stress is high.