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Abstract The impacts of pulsed nutrient injections or extreme runoff events on marine ecosystems are far less studied than those associated with long‐term eutrophication, particularly in regard to mechanisms regulating the response of plankton community structure. Over 800 million liters of nutrient‐rich water from a fertilizer mine were discharged over a 2‐week period into Tampa Bay, Florida, in 2021, providing a unique opportunity to document the plankton response. A 3D‐coupled hydrodynamic biogeochemical model was developed to investigate this response and to understand the observed succession of a large, short diatom bloom followed by a secondaryKarenia brevisbloom that lasted through the summer. The model reproduced the observed changes in nutrient concentration, total chlorophylla, and diatom andK. brevisbiomass in Tampa Bay. With a faster growth rate and spring temperature close to the optimal window of growth, diatoms had an initial competitive advantage, with 2/3 of the nutrient uptake due to ammonium and 1/3 due to nitrate. However, exhaustion of external nutrients led to the rapid decline of the diatom bloom, and the associated particular organic nitrogen sank onto the bay sediment. Enhanced sediment release of ammonium during the weeks following, and summer remineralization of dissolved organic nitrogen provided sufficient regenerated nitrogen to support slow‐growingK. brevisthat could capitalize on low nutrient conditions. Modeling analysis largely confirmed Margalef's conceptual model ofrtoK‐selected species succession and provided additional insights into nutrient cycling supporting the initial diatom bloom and the subsequent bloom of a slow‐growing harmful algal species. 

Abstract The availability of dissolved inorganic and organic nutrients and their transformations along the fresh to marine continuum are being modified by various natural and anthropogenic activities and climate-related changes. Subtropical central and eastern Florida Bay, located at the southern end of the Florida peninsula, is classically considered to have inorganic nutrient conditions that are in higher-than-Redfield ratio proportions, and high levels of organic and chemically-reduced forms of nitrogen. However, salinity, pH and nutrients, both organic and inorganic, change with changes in freshwater flows to the bay. Here, using a time series of water quality and physico-chemical conditions from 2009 to 2019, the impacts of distinct changes in managed flow, drought, El Niño-related increases in precipitation, and intensive storms and hurricanes are explored with respect to changes in water quality and resulting ecosystem effects, with a focus on understanding why picocyanobacterial blooms formed when they did. Drought produced hyper-salinity conditions that were associated with a seagrass die-off. Years later, increases in precipitation resulting from intensive storms and a hurricane were associated with high loads of organic nutrients, and declines in pH, likely due to high organic acid input and decaying organic matter, collectively leading to physiologically favorable conditions for growth of the picocyanobacterium, Synechococcus spp. These conditions, including very high concentrations of NH 4 + , were likely inhibiting for seagrass recovery and for growth of competing phytoplankton or their grazers. Given projected future climate conditions, and anticipated cycles of drought and intensive storms, the likelihood of future seagrass die-offs and picocyanobacterial blooms is high.