Coral Resilience to Stressors
Despite having waxed and waned for millions of years, coral reefs today are exhibiting a precipitous decline unlike any before. The degradation we see can be attributed to multiple stressors that are unambiguously anthropogenic: dredging, pollution, sedimentation, overfishing, and most of all, ocean warming. Yet not all corals are equally affected by this onslaught. Different regions, species, and even individuals within a species, can all exhibit variation in how they respond to these impacts. In any given population, genetic variation and phenotypic plasticity give rise to this capacity for differential responses. When certain responses make a coral more likely to survive stressful conditions, the potential exists for the individual to reproduce and pass along the underlying genes to the next generation—the basis for evolution. It is through this process that coral resilience to stressors can increase at the population level. Our lab seeks to study current coral resilience to stressors and how it may be changing as a result of the evolution-global change interaction across spatial and temporal gradients.
Hurricane Irma, the most powerful Atlantic hurricane to hit the United States since Hurricane Katrina in 2005, made landfall in the Florida Keys in September of 2017. Experts estimate that Hurricane Irma totaled over 60 billion dollars in damage, but marine managers have barely begun to assess the ecological costs. Hurricanes have a multitude of harmful effects on coral reefs: waves cause physical damage, corals become buried and choked under sediment, rainfall significantly changes the salinity and seawater chemistry, and storm water runoff leads to nutrient loading and reductions in water quality and visibility. Approximately 80% of the coral cover has already been lost in the Caribbean over the past 50 years, so understanding the effects of this most recent natural disaster is vital to conservation and management of this ecologically and economically important region.
The Davies Lab received funding from the National Science Foundation to conduct coral reef damage assessment along the Florida Keys following Hurricane Irma in October of 2017. The worst damage appeared along the southernmost reefs near Key West, also where the most damage occurred on land. Reefs near the middle Keys (adjacent to Islamorada and Key Largo) had accumulated a lot of sediment but the corals appeared mostly undamaged.
Along with surveying the length of the Keys, we also compared damage between reefs that were closer to the coastline (inshore) and closer to open ocean (offshore). The results were unequivocal. While the inshore corals seemed healthy overall, we saw the widespread demise of coral colonies that were hundreds of years old at each offshore site. However, due to the amount of algae that had grown over these corals, they had clearly been dead for months already. This coral death must have occurred prior to Hurricane Irma, most likely due to disease or warming temperatures.
At every site, we took photographs and collected samples of multiple coral species to gather information on these corals’ growth, survival, and microbial communities. We will compare these results to data collected pre-Hurricane Irma in the springs of 2015 and 2016 and we will return in the summer of 2018 to follow up on the status of these corals’ recovery. Corals of the Florida Keys face multiple stressors: climate change, disease, and super-storms like Hurricane Irma. Continued research to monitor the effects of these stressors and to assess the corals’ capacity for resilience is of the utmost importance.
Hurricane Harvey brought record-breaking levels of rainfall and flooding to Texas in June of 2017, causing unprecedented freshwater runoff to travel through the Gulf of Mexico. Corals of the Flower Garden Banks National Marine Sanctuary (FGBNMS) are subject to relatively stable salinities, so the sudden influx of freshwater was cause for concern. As a result, The National Science Foundation promptly funded a team of researchers, including the Davies lab at Boston University, to investigate the results of Hurricane Harvey runoff in October of 2017.
The freshwater runoff finally reached the sanctuary, 100 miles from land, months after the storm. An automated buoy at the site showed a steep drop in salinity on September 28th and again on the first day of the researchers’ expedition on October 20th. Until October 25th, the team surveyed three sites at the FGBNMS extensively. Although they found the corals showed some signs of stress, overall the corals had made it through this storm event alive, though latent effects may be on the horizon. The team collected coral and seawater samples for genetic and microbial processing, so we will learn more about the effects of Harvey soon. In April, the team will return again to collect more samples to evaluate reef recovery.
Die off in the FGBNMS
In July of 2016, sports divers visiting the beautiful East Bank of the Flower Garden Banks National Marine Sanctuary (FGBNMS) discovered cloudy water, white muck covering the reef, and scores of dead animals. Scientists immediately surveyed the area and found a massive die-off of invertebrates, including corals, sponges, sea urchins, starfish, clams, and more. In three separate dive sites, Dr. Sarah Davies and other researchers sampled healthy and sick patches of the affected corals and other organisms, the mysterious white slime, and the ambient water to try to understand the event. Possible causes of the die-off include anoxic water or other water quality problems, oil spills, or disease pathogens. However, it is unclear how these issues would affect these isolated areas or how so many different organisms were affected. The composition of the white material and the cause of the mass mortality remains in question as researchers continue to analyze samples from the field.