Coral reefs are ecologically and economically important ecosystems, and are threatened by a variety of global (climate change) and local (overfishing, pollution) stressors. Human-driven climate change is increasing the frequency and severity of storms, which can physically damage reef structures and reduce reef health through changes in seawater quality. In August of 2017, Hurricane Harvey caused widespread flooding in southeast Texas when it released more than 50 trillion liters of rain, which then accumulated in the ocean along the Texas Shelf. This runoff had the potential to impact nearby coral reefs in the Flower Garden Banks National Marine Sanctuary (FGBNMS, northwest Gulf of Mexico) if eddies and jets transported coastal flood-influenced waters offshore.

Our experiments were among the first to test for the impacts of storm-derived floodwaters on offshore (>100 km) bottom-dwelling organisms, like corals and sponges. We sampled seawater chemistry, microbial communities (water column and reef bottom), coral colony gene expression, and coral and sponge-associated microbial communities before, immediately after, and 12 months after Harvey runoff entered FGBNMS in order to track the response of coral and sponge health, microbial community diversity, and the trajectory of reef communities over time. This work ultimately allows us to determine if: (1) changes in water chemistry induce pelagic microbial shifts; (2) if microbial communities typically associated with corals and sponges are altered; and (3) whether feedbacks occur between these potential drivers of benthic invertebrate mortality.

The flood-derived offshore water mass following Harvey did not move directly over FGBNMS, and therefore, our work documents that significant mortality of reef organisms did not occur. Our results show, however, that offshore corals and sponges did experience sub-lethal stress effects following Hurricane Harvey. This was evident based on genes that were differentially expressed in corals immediately after the hurricane, which indicated increases in cellular oxidative stress responses that are consistent with a stress response. In addition, shifted microbiomes and the presence of human microbial pathogens in offshore sponges were observed after extreme storm events in 2016 (Tax Day Flood) and 2017 (Hurricane Harvey). Such effects were not detectable in samples collected during the same season of a non-flooding baseline year (2018). Analyses of potential microbiome shifts associated with offshore corals and coral gene expression and microbiome responses to low dissolved oxygen and hyposalinity stressors during tank-based experiments are ongoing. Overall, our findings to date suggest that offshore systems, at least in the Gulf of Mexico, are not necessarily buffered from the effects of floodwaters. Thus, local actions such as improved wastewater management and reductions in terrestrial runoff could benefit offshore coral reefs in the Gulf of Mexico. Our findings clearly demonstrate the urgent need for: 1) continued mitigation of stormwater runoff and climate change impacts; and 2) establishment of microbial and water quality time series for near- and offshore reef using standardized protocols. This latter program will generate baseline data on the physiology and microbiomes of key reef organisms under normal conditions, providing critical context in which to detect and mitigate floodwater-derived stress on reefs. Taken together, our findings suggest that offshore systems, such as coral reefs, should be considered in future floodwater management decisions.

Our work provides fundamental insight into the potential impacts of floodwaters on coral reef environments. The Flower Garden Banks contain reefs with among the highest remaining coral cover in the wider Caribbean, yet they have recently experienced increases in macroalgae, a coral competitor. Long-term monitoring of reef bottom cover at FGBNMS has documented an increase in macroalgae, without a corresponding loss in corals, over the last two decades and suggests nutrient input may have been affecting these offshore reefs since 1999. Growth of macroalgae on coral reefs is generally detrimental as macroalgae outcompete corals for light and space and promote coral disease. If unchecked, continued ‘pulses’ of nutrients from storm-derived flooding events may underlie or exacerbate increased coral-algal interactions and trigger the decline of these ‘reef of hope’, which currently boast coral cover greater than 50%.

A total of 3 postdoctoral scholars, 19 graduate students, 25 undergraduate students, and 7 high school students were integrated into the projects of this grant. The majority (78%) of these trainees were female. These students have received a broad introduction to our science that expands their horizons as to what marine biology actually is. We also made significant efforts to engage local communities in southeast Texas and Boston, Massachusetts, and to increase public knowledge of the threats to coral reefs and how our research addresses those threats.

Our project is listed on the BCO-DMO website to facilitate sharing our water column chemistry and microbial 16S data, our coral gene expression and microbial 16S data, and our sponge microbial 16S community data. The URL for the BCO-DMO project is https://www.bco-dmo.org/project/746814.

Last Modified: 05/18/2020
Modified by: Sarah W Davies