Phytoplankton Responses to Mass Coral Spawning in the Flower Garden Banks, Gulf of Mexico

Mass coral spawning represents a nutrient input to coral reef systems that for Pacific reefs has been shown to stimulate pelagic and benthic processes. If phytoplankton in the water column over the reef are able to utilize this annual nutrient input, this could potentially alter phytoplankton biomass and community composition, in what is normally a very oligotrophic system. Sampling was performed at East Flower Garden Bank (EFGB), Gulf of Mexico during May, July, and August 2009. The annual coral spawning event occurred there August 11-14, 2009. Samples were collected morning and evening at three depths and analyzed for nutrients, chlorophyll a, accessory pigments, phytoplankton species composition, and carbon, hydrogen, and nitrogen (CHN). During spawning, only small changes in nutrient concentrations were detected. Dissolved inorganic nitrogen (DIN) peaked on the second day of spawning and N:P ratio was highest on 5/28, likely due to particularly phosphate concentration. Chl a biomass was significantly different between sample dates and the biomass increased steadily throughout the spawning period. The contribution of different phytoplankton classes to total chlorophyll a was determined using known pigment algorithms. Prokaryotes were the dominant class across the entire sampling period with 60-80 percent abundance. Trichodesmium spp. was the dominant genus throughout the study and genus specific changes per sample date were seen. On 8/11 and 8/13 two genera contributed the majority of chl a (Trichodesmium spp. and Ceratium spp.; Cylindrotheca spp. and Trichodesmium spp., respectively). Abundance showed variability during spawning with a peak at 11 cells/ml on 8/12. The high abundance of Trichodesmium spp. could indicate N limitation is alleviated at the Flower Garden Banks (FGB). Current literature on coral spawning is limited to studies performed in the Great Barrier Reef, with assessment areas close to a major shoreline. Genera found at EFGB were similar to those found in other reef systems. It cannot be determined if nutrient input increased diversity, as diversity was high prior to spawning as well. Greater increase in available forms of nitrogen would have likely been found several days post major spawning. The FGB were a unique system to study, as they are coral reefs, but are located 200 km offshore. This study provided a snapshot into phytoplankton dynamics as a result of spawning. Changes across the short time scale were seen in biomass and community composition.