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Blooms of the toxigenic dinoflagellateKarenia brevisare an almost annual occurrence in the eastern Gulf of Mexico, typically initiating in late summer and early fall months and terminating in late spring or earlier. The question of whether blooms have been expanding in frequency or duration has long been debated. Recently, a Bloom Severity Index (BSI) was developed that captures changes in bloom magnitude based on cell concentrations normalized to maximum observed values. Here, changes in the BSI (severity and bloom duration) were examined for the period from 1970-2019, a period of rapid climate change and increased anthropogenic pressures. This time period encompassed several changes in the Oceanic Niño Index (the El Niño-Southern Oscillation), including a shift from a highly positive to a negative North Atlantic Oscillation in the mid 1990s, bringing with it increased precipitation and more intensive storms. Annual BSI and bloom duration have increased with increasing temperatures, and blooms have also become longer in duration in relation to increased temperatures and river flows since the mid 1990s. As increased precipitation is related to increased nutrient runoff, regional fertilizer use and the anthropogenic nitrogen (N) footprint based on population census data as proxies of nitrogen loads were examined. The duration of severe blooms was highly correlated with the increasing anthropogenic N footprint, especially when BSI values were averaged across multiple years. These relationships highlight the importance of climate changes and of increasing population since the 1980s and help to explain why earlier analyses of nutrient loads and bloom severity were inconclusive. To reduce bloom severity or duration in the future, reductions in N loads and releases from the Caloosahatchee River are needed more than ever to counteract the increasing pressures from climate change. 

Various recent reports, based on different approaches, data sets and time periods, have yielded different conclusions with regard to whether blooms of the Florida red tide dino􀀙agellate, Karenia brevis, have increased over time. Without question, however, there have been a number of recent blooms that have been long lasting, continuing through the summer months normally taken to be outside the ideal temperature niche for K. brevis. Here, using a recently developed bloom severity index, the time series of blooms from 1970 to 2019 is examined, focusing on how monthly patterns have changed over time. More severe blooms have been found since the mid 1990s, now lasting 4- to 5-months longer than in previous decades, a trend related to the Oceanic Ni˜no Index (El Ni˜no -Southern Oscillation). Since the mid-1990s, water temperature anomalies have been related to bloom severity with lags of 3 to 6 months. The most signi􀀼cant temperature increases have occurred in the latter months of the year when K. brevis growth typically is highest. Increased 􀀙ow from the Caloosahatchee River, and its total nitrogen load, are also predictors of recent bloom severity with lags of several months. Cells that survive the nowwarmer winter dry season have adequate nutrients and may experience more favorable nitrogen forms as the summer wet season develops, and as nutrients are recycled, may “over summer”. The stresses of increased warming and increased nutrient pollution on K. brevis blooms will continue to make managing these blooms a challenge for management as climate change trajectories continue. 

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 secondary Karenia brevis bloom that lasted through the summer. The model reproduced the observed changes in nutrient concentration, total chlorophyll a, and diatom and K. brevis biomass 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-growing K. brevis that could capitalize on low nutrient conditions. Modeling analysis largely confirmed Margalef's conceptual model of r to K-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.