Seawater microorganisms within overlying reef seawater change over six years

Reefs on the southern shore of St. John, United States Virgin Islands (USVI) were targeted for 11 opportunistic sampling events over six years from June 2016 to June 2022. The reefs included (from west to east) Dittlif, Cocoloba, Joels Shoal, Europa, Yawzi, Tektite, Booby Rock, and Ram Head, all of which are withinin the bounds of Virgin Islands National Park, except for Dittlif. Reef collections included surveys for benthic composition, small volumes of seawater for inorganic and organic nutrients and microbial abundances, larger volumes for microbial biomass and chlorophyll, and CTD casts for temperature and salinity. All collections occurred during daylight hours. Dates of seawater sampling events included June 10-12, 2016, October 28-29, 2016, March 25-28, 2017, July 26-30, 2017, November 27-30, 2017, April 11-13, 2018, November 5-9, 2018, August 6-10, 2020, January 17-24, 2021, October 20-25, 2021, and June 24-29, 2022. These sampling points surround two major stressors to St. John, USVI reefs: two category 5 hurricanes, Irma and Maria, which affected the reefs in September, 2017, and stony coral tissue loss disease, a multi-species disease outbreak that began emerging around St. John, USVI between January-June, 2020. As of August 2020, the disease just began affecting all reefs in the study area, except Europa and Cocoloba. All reefs were impacted by the next sampling in January 2021. An additional stressor, a coral bleaching event from October to November, 2019, impacted the reefs, but a gap in the dataset makes us unable to assess the impact of that event, and instead the study focuses on the impact of the hurricanes and stony coral tissue loss disease. To better understand how seawater microorganisms reflect changes in the underlying reef habitat, we sought to 1) identify the changes to the benthic reef habitat over a six year time period with the hypothesis that reefs would become more algal-dominant, 2) examine the changes in the overlying seawater nutrients, cell counts, and temperature, and 3) investigate the concurrent changes in specific microbial groups during that time period.