Coral reefs provide critical ecosystem services including food, income through fishing and tourism, and protection of coastal communities from storms. Despite their importance, coral reefs are threatened by a variety of global (climate change and ocean acidification) and local (overfishing, pollution) stressors. Human-driven climate change is increasing the severity and precipitation associated with hurricanes, which can physically damage reef structures. In addition, storm runoff from land can reduce reef health through changes in seawater quality. A severe mortality event occurred in 2016 on a portion of the Flower Garden Banks National Marine Sanctuary (FGBNMS, northwest Gulf of Mexico) East Bank coral reef that resulted in mortality of up to 82% of the coral colonies in the area along with other benthic invertebrates. This event was associated with freshwater runoff from flooding in Texas reaching the FGBNMS, and upwelling of deep water in the area. 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 coast. This runoff had the potential to impact nearby coral reefs in the FGBNMS if floodwaters were transported offshore.

Our experiments were among the first to test for the impacts of storm-derived floodwaters on offshore (>100 km) coral reef ecosystem water column chemistry and microbial communities, with implications for coral reef health. We sampled seawater chemistry and water column microbial communities before, immediately after, and a year after Harvey in order to track the response of the reef ecosystems over time. This work allows us to determine how: (1) floodwaters change coastal and coral reef water chemistry and (2) changes in water chemistry induce pelagic microbial shifts.

The large freshwater plume produced by Hurricane Harvey floodwater was primarily transported to the southwest along the Texas coast and not offshore to the FGBNMS (Figure 1). Consequently, our results show that surface water chemistry at the FGBNMS was normal for this time of year. However, low carbon dioxide levels along the West Texas coast where the freshwater plume was transported may suggest that nutrients in Harvey floodwaters enhanced phytoplankton growth along the Texas coast (Figure 2). Our results also show that Hurricane Harvey may have upwelled deep waters towards the surface in the area of the FGBNMS. We measured lower temperature and pH between 50-75 m depth immediately following Harvey compared to other time periods sampled (Figure 3). Water chemistry conditions were generally supportive for coral reef calcification and growth at the depth of the reefs (~20 m), but this data reveals that strong storms and eddies have the potential to bring stressful deep water conditions up onto the reefs. We are investigating the frequency of upwelling events in the FGBNMS and their impact on water chemistry and coral reef health using data collected from 2015 through 2019, including the samples collected as part of this project.

Depth explained 18% of the differences seen in water column prokaryotic community diversity (Figures 4, 5), but deeper analysis revealed that communities from the mixed layer are most similar to other samples from the same month, while samples from below the mixed layer do not exhibit this pattern (Figure 6). This indicates that there is more temporal variability in microbial diversity in the surface waters than deeper water and it reveals that changes in surface water column microbes may be important for the reefs and are likely indicators of environmental conditions potentially detrimental to the reef ecosystem. We also found that pathogens detected on diseased sponges were not present in the water column at the same time, indicating that those taxa were concentrated by the sponges as part of their pathology or that they are present in the water column in undetectably low abundances.

Our work provides fundamental insight into the potential impacts of high intensity and precipitation storms on coral reef environments. Our results clearly indicate that the effects of storms on both surface and deep water must be accounted for to fully assess impacts on coral reef ecosystems. In addition, establishment of microbial and water quality time series for nearshore and offshore reefs using standardized protocols will generate baseline data under normal conditions, providing critical context in which to detect and mitigate storm-derived stress on reefs.

A total of 1 postdoctoral scholar, 4 graduate students, and 8 undergraduate students from Texas A&M University were integrated into the work done for this grant. Half of these students are from groups underrepresented in the Geosciences, including first generation college, underrepresented ethnicity, and LGBTQ+ students. We also engaged communities in Texas to share our research, the importance of coral reef ecosystems, and ocean stewardship through field trips, summer camps, and middle and high school teacher trainings. Our project is listed on the BCO-DMO website to facilitate sharing our data at https://www.bco-dmo.org/project/746814.

Last Modified: 01/30/2021
Modified by: Kathryn Shamberger