Award: OCE-1807077

On September 10, 2017, Hurricane Irma passed over the Florida Keys with up to 130 mph winds causing extreme damage to both aquatic environments and populated land areas. Along parts of the Middle Keys, the hurricane force winds caused water to recede over 100 meters northward from the bayside shoreline. Exposure of the seafloor resulted in catastrophic damage to nearshore benthic communities including sponges, whose total biomass decreased by approximately 95 ± 5% on one shallow middle Keys embayment according to scientists at the Florida Fish and Wildlife Conservation Commission (FWC) located in Marathon, Vaca Key. In collaboration with FWC scientists we established five sampling stations (see colored dots in Figure 1) for repetitive time-series measurements out in the west central area (MB) and at four nearshore stations along the Middle Keys. Our post-Irma time-series measurements made from October 2017 through July 2018, focused on determination of the impacts of Irma and other episodic storm events on temporal variability in dissolved inorganic nitrogen (DIN), generally the nutrient element limiting productivity in western Florida Bay, and on potentially related phytoplankton bloom occurrences as indicated by enhanced chlorophyll-a (Chl-a) concentrations. We also investigated the relationship between changes in DIN and dissolved organic carbon (DOC) concentrations. Monthly whole water samples were collected by UNC and FWC scientists from October 2-17 through July 2018. The FWC laboratory provided essential support in the forms of scientific personnel, laboratory space, SCUBA equipment support and boating facilities. Each water sample was filtered[MC1] [MC2] [MC3] for chlorophyll-a (Chl-a) measurements, then analyzed for nitrate/nitrite (NOx-), ammonium (NH4+), and DOC. Drastic, systematic changes in all water quality parameters probably occurred within days of Irma’s passage and were still present and observed in water column samples collected for our first measurements in October (Figure 2). The initial changes included spiked DIN concentrations, that we hypothesized to result from release of NH4+-enriched porewaters during muddy sediment resuspension. Those high concentrations rapidly declined during subsequent phytoplankton blooms as estimated from Chl-a concentrations (Figure 2). Additional spikes in DIN and Chl-a concentrations occurred following a second but lower intensity storm in December 2017, during which the water column turned milky in color as a result of sediment resuspension. However, four months post-Irma in January 2018, DIN and Chl-a concentrations dropped to spring and early summer values typically observed in quarterly sampling and measurements from 1989 through 2017, conducted by scientists from Florida International University (FIU) as part of the SERC program. We conclude that changes from long-term baseline concentrations caused by strong storms, including Hurricane Irma, have immediate impacts on water quality that dissipate within months while causing rapid changes in phytoplankton communities. As a result of Hurricane Irma, interior locations of Florida Bay, including our MB station, experienced large-scale phytoplankton blooms and low DIN concentrations. Interestingly and in contrast, increased DIN concentrations occurred at nearshore stations without creating quick resultant blooms. Throughout Florida Bay, hurricanes and other significant storms likely cause immediate spikes in water column DIN concentrations though sediment resuspension which, in turn, releases large quantities of dissolved ammonium stored in the porewaters in the upper 5 to 10 cm of underlying muddy sediments. Dramatic sponge biomass losses occurred as Hurricane Irma impacted stations in central Florida Bay as well as in nearshore embayments. The importance of sponge nitrogen recycling to the overlying water column depends upon sponge speciation and which nutrient element is limiting to primary production in a given area of the Bay. The decimation of sponge biomass, a major source of recycled DIN, plus contributions of porewater DIN from storm-perturbed sediments and bloom DIN sequestration require new modeling strategies for understanding dissolved nitrogen seasonal cycles and budgets in Florida Bay. Last Modified: 06/03/2022 Submitted by: Christopher S Martens