Predicting algal blooms in waterways receiving inflows from multiple sources is challenging since blooms and their drivers can originate from diverse sources. Models that mechanistically simulate the formation and transport of algal blooms are often computationally intensive, creating barriers to using them for daily decision- making. Given this challenge, we developed a statistical risk forecasting framework for the Caloosahatchee River and Estuary in southwest Florida, United States of America, which receives engineered water releases from the eutrophic Lake Okeechobee, as well as hydrologic inputs from the surrounding watershed. The forecasting approach considers two different hydrologic regimes (i.e., lake- versus runoff-dominated conditions) while maintaining structural simplicity such that water managers could readily apply the model for short-term decision-making. Using daily mean discharge at two United States Geological Survey stations and more than 14 years of discrete water quality sampling data with diverse temporal resolution from the South Florida Water Management District, two regression tree models were trained and tested for day-ahead bloom risk forecasting: one for conditions in which lake releases dominated, and one when watershed inputs dominated. For the model capturing lake-dominated conditions, the main bloom predictors were 30-day lagged total suspended solids at the lake and canal outlets averaged over transect residence time (R2 = 0.78 and RMSE = 6.10 mu g/L). For the model predicting watershed-dominated conditions, 30-day lagged dissolved phosphorus load, and chlorophyll-a averaged over transect residence time at the canal outlet were the most important predictors of next-day chlorophyll-a (R2 = 0.49 and RMSE = 14.50 mu g/L). Critically, algal blooms driven by Lake Okeechobee releases are potentially controllable, as they are related, at least in part, to the operation of water control structures. Thus, the strong performance of the model for lake-dominated conditions demonstrates its utility for informing release scheduling to mitigate downstream blooms.

This study explored the use of data-driven models to develop management-oriented prediction tools for algae blooms (ABs) represented by Chlorophyll-A (Chla) concentrations, using the Caloosahatchee and St. Lucie canals in Lake Okeechobee, Florida, as case studies. By comparing two modeling approaches, i.e., cascading modeling and time-lag modeling, the study aims to understand the differences in Chla dynamics between the two canals, identify the main drivers and predictors of Chla concentration in each, and develop suitable forecasting models for the canals' operation purposes. Throughout this study, both approaches demonstrated their value in improving the understanding of water quality dynamics in Lake Okeechobee canals. While some water quality parameters such as Dissolved Oxygen (DO) and Nitrate-Nitrite (NOx) were critical to ABs in the Caloosahatchee and St. Lucie canals, respectively, the effect of operation decisions on ABs was more significant on the St. Lucie than on the Caloosahatchee. From a modeling perspective, the time-lag modeling approach achieved higher predictive accuracy for Chla concentrations in both Caloosahatchee and St. Lucie canals. Particularly, at station S80 of St. Lucie canal, the XGBoost (XGB) algorithm achieved R2= 99% and RMSE = 0.001 μg/l in training, and R2= 60.1% and RMSE = 4.58 μg/l in testing. At station S79 of Caloosahatchee canal, Random Forest (RF) appeared to be the best model with R2= 85.7% and RMSE = 5.63 μg/l in training, and R2= 39% with RMSE = 10.06 μg/l in testing. In this study, the time-lag modeling approach was proven to offer decision-makers flexible tools for implementing better management strategies based solely on operation decisions and meteorological conditions.

Phytoplankton composition and biomass were investigated in the C-43 Canal in southwest Florida during a period of shifting discharges from water control structures. The canal receives regulated discharges from eutrophic Lake Okeechobee via the S77 structure. During periods of high S77 discharge in spring and early summer, cyanobacteria biomass dominated the phytoplankton community, including blooms of the harmful algal bloom (HAB) species Raphidiopsis raciborskii, Limnothrix redekei and Microcystis aeruginosa. During periods of low discharges from the lake, in mid-summer and autumn, water inputs to the canal came primarily from tributaries in the watershed surrounding the C-43. Phytoplankton biomass decreased, but the relative importance of dinoflagellates increased, including a bloom in July. The dinoflagellate community included Ceratium, Durinskia baltica, Glochidinium penardiforme, Gymnodinium fuscum, Parvodinium goslaviense, Parvodinium umbonatum/inconspicuum complex, Peridiniopsis quadridens, Woloszynskia reticulata, and an unidentified thecate and athecate species. D. baltica and P. goslaviense were recorded for the first time in Florida. Data was also obtained on water temperature, conductivity, fluorescent dissolved organic matter, chlorophyll a, total nitrogen, dissolved inorganic nitrogen, total phosphorus, PO4, discharge rates from water control structures, and water residence times. Results show that temporal shifts in the sources and rates of water inputs to the C-43 influence the character of environmental conditions that define phytoplankton composition and biomass in the canal. This suggests that management of discharges can play a role in mitigating HABs in the canal and downstream coastal environments receiving water from the canal.

The Caloosahatchee Estuary in southwest Florida, USA, is regularly subject to the introduction of toxic Microcystis aeruginosa blooms, often originating from the eutrophic Lake Okeechobee via the C-43 Canal. The focus of this study was to determine the responses of one of these introduced blooms to progressively elevated salinity levels as the bloom water mass moved through the estuary. In the upper estuary, salinities were freshwater, and surface blooms of large colonies of M. aeruginosa were observed, along with peak microcystin toxin concentrations up to 107 mu g L- 1, all in the particulate fraction. In the mid-estuary, salinity levels increased to 2-6, and surface blooms were again observed, with peak microcystin concentrations up to 259 mu g L- 1, however, significant levels of extracellular toxin were also observed (i.e., 17.8 mu g L- 1), suggesting a level of osmotic stress on M. aeruginosa. In the lower estuary, salinities ranged from 6 to 25 and very few viable M. aeruginosa colonies were observed, but significant levels of extracellular microcystin (i.e., 0.5 mu g L- 1) were present throughout the water column. It is noteworthy that average total microcystin concentrations in the water column (i.e., particulate + extracellular) remained constant throughout the movement of the bloom water mass during its transit through the estuary, revealing the negligible rate of microcystin degradation during the ten-day transit. The results also provide insights into the changes in the distribution of particulate and extracellular microcystin along the salinity gradient, which has implications for management of risks for ecosystem and human health, and how these risks may be affected by management of releases from three water control structures in the C-43 Canal. Discharge rates from the water control structures play major roles in the rate of movement of blooms through the C-43 CanalCaloosahatchee Estuary ecosystem. The potential implications of discharge regulation for the management of M. aeruginosa in the ecosystem are discussed from the perspectives of blooms of allochthonous and autochthonous origin.

Phosphorus (P) sustainability is a complex problem - it is a limited resource critical for agricultural productivity, but fertilizer production generates extensive phosphogypsum waste and can impair downstream water quality. Industrial, urban, and agricultural activities contribute to cultural eutrophication, thereby degrading both coastal and inland ecosystems and storing legacy nutrients in sediments. This study investigated the long-term effects of phosphogypsum wastewater discharges on legacy nutrient accumulation, an unintended impact of the fertilizer industry that is often overlooked and understudied. Sediment cores were collected to reconstruct the depositional history of two sites in Tampa Bay, Florida, USA that experienced past wastewater releases; the most recent in 2021. Sediments had high concentrations of stored or legacy nutrients (total P: 0.11 - 15.01 mg g-1; total nitrogen: 0.04 - 0.37 %) particularly during discharge timeframes, as assessed by short-lived radioisotopes, and were predominantly in bioavailable forms, as assessed by bulk pools and 31P nuclear magnetic resonance spectroscopy. These values are comparable to hypereutrophic lakes impacted by agriculture and urbanization. Sediment accumulation rates were elevated relative to other Florida estuaries (Bishop Harbor: 13,092 - 46,706 g m2 yr-1; Piney Point Creek: 3,064 - 23,990 g m2 yr-1), which can alter biogeochemical cycling and the fate of nutrient loading. Phosphorus accumulation rates and other proxies had downcore peaks corresponding to discharge events from 2001 to 2004, 2011, and 2021 with P accumulation rates ranging from 0.5 - 559 g m- 2 yr-1. These findings indicate that estuarine nutrient budgets need to incorporate stored sedimentary nutrient pools and internal benthic fluxes and highlight the need for a more sustainable P supply chain.

IntroductionSuccessful management and mitigation of harmful algal blooms (HABs) requires an in-depth understanding of the physiology and nutrient utilization of the organisms responsible. We explored the preference of various nitrogen (N) and phosphorus (P) substrates by two novel groups of HAB-forming phytoplankton originating from the Indian River Lagoon (IRL), Florida: 1) a consortium of picocyanobacteria (Crocosphaera sp. and ‘Synechococcus’ sp.) and 2) ananochlorophyte (Picochlorum sp.).MethodsShort-term kinetic uptake experiments tested algal use and affinity for inorganic and organic N substrates (ammonium (NH4+), nitrate (NO3-), urea, and an amino acid (AA) mixture) through 15N and 13C isotope tracing into biomass.ResultsPicocyanobacteria exhibited Michaelis-Menten type uptake for the AA mixture only, while nanochlorophytes reached saturation for NH4+, the AA mixture, and urea at or below 25 µM-N. Both picocyanobacteria and nanochlorophyte cultures had highest affinity (Vmax/Ks) for NH4+ followed by the AA mixture and urea. Neither culture showed significant uptake of isotopically-labeled nitrate. Disappearance of glucose-6-phosphate (G6P) added to culture medium suggesting use of organic P by both cultures was confirmed by detection of alkaline phosphatase activity and the tracing of 13C-G6P into biomass.DiscussionTogether, our results suggest that these HAB-forming phytoplankton groups are able to use a variety of N and P sources including organic forms, and prefer reduced forms of N. These traits are likely favorable under conditions found in the IRL during periods of significant competition for low concentrations of inorganic nutrients. Bloom-forming phytoplankton are therefore able to subsist on organic or recycled forms of N and P that typically dominate the IRL nutrient pools.

IntroductionCultural eutrophication threatens numerous ecological and economical resources of Florida’s coastal ecosystems, such as beaches, mangroves, and seagrasses. In April 2021, an infrastructure failure at the retired Piney Point phosphorus mining retention reservoir garnered national attention, as 814 million liters of nutrient rich water were released into Tampa Bay, Florida over 10 days. The release of nitrogen and phosphorus-rich water into Tampa Bay – a region that had been known as a restoration success story since the 1990s – has highlighted the potential for unexpected challenges for coastal nutrient management.MethodsFor a year after the release, we sampled bi-weekly at four sites to monitor changes in nutrients, stable isotopes, and phytoplankton communities, complemented with continuous monitoring by multiparameter sondes. Our data complement the synthesis efforts of regional partners, the Tampa Bay and Sarasota Bay Estuary Programs, to better understand the effects of anthropogenic nutrients on estuarine health.ResultsPhytoplankton community structure indicated an initial diatom bloom that dissipated by the end of April 2021. In the summer, the bay was dominated by Karenia brevis, with conditions improving into the fall. To determine if there was a unique carbon (C) and nitrogen (N) signature of the discharge water, stable isotope values of carbon (δ13C) and nitrogen (δ15N) were analyzed in suspended particulate material (SPM). The δ15N values of the discharge SPM were −17.88‰ ± 0.76, which is exceptionally low and was unique relative to other nutrient sources in the region. In May and early June of 2021, all sites exhibited a decline in the δ15N values of SPM, suggesting that discharged N was incorporated into SPM after the event. The occurrence of very low δ15N values at the reference site, on the Gulf Coast outside of the Bay, indicates that some of the discharge was transported outside of Tampa Bay.DiscussionThis work illustrates the need for comprehensive nutrient management strategies to assess and manage the full range of consequences associated with anthropogenic nutrient inputs into coastal ecosystems. Ongoing and anticipated impacts of climate change – such as increasing tropical storm intensity, temperatures, rainfall, and sea level rise – will exacerbate this need.

Seasonal patterns of phytoplankton biomass and composition in the inner continental shelf off Cape Canaveral on the east coast of Florida were examined for a 6-year period (2013–2019). In situ water samples were collected and analyzed for chlorophyll a , phytoplankton biomass and composition, along with water quality parameters. Regional satellite data on chlorophyll a, and temperature was also obtained from NASA. Average chlorophyll a values over the study period ranged from 0.63 ± 0.03 μg L −1 in the summer to 2.55 ± 0.10 μg L −1 in the fall. Phytoplankton community composition also showed seasonal differences, with persistent dominance by picoplanktonic cyanobacteria in the summer, but mixed dominance by picocyanobacteria and dinoflagellates in the fall. Seasonal differences were attributed to a shift in predominant seasonal wind directions, which drive water along the coast from the north in the fall and winter, but from the south in the spring and summer, including eddies and upwelling from the Gulf Stream. Water masses moving along the Florida coast from the north are influenced by nutrient and phytoplankton-enriched inputs from estuaries along the north coast of Florida, explaining the higher phytoplankton biomass levels on the Cape Canaveral shelf in the fall and winter. Seasonal patterns observed in this study demonstrate the importance of allochthonous influences on phytoplankton biomass and composition, and highlight the potential sensitivity of phytoplankton communities to continuing cultural eutrophication and future climate changes, including the frequency and intensity of tropical storms, and alterations in discharges from land.

Study region: Algal blooms (ABs), often exacerbated by excess nutrients from anthropogenic activities, can pose serious risks to public health, fisheries, and ecosystem structure and functions. Lake Okeechobee is located in southcentral Florida (USA), and with a surface area of 1730 km2, it is the largest subtropical lake in the United States. This lake is shallow, nutrient-rich, and subject to frequent and intense blooms of cyanobacteria, some of which are toxic. Study focus: In this study, a three-dimensional (3D) model was developed, coupling long-term monitoring data with complex physical, chemical, and ecological processes at fine spatial (15- 1000 m horizontal mesh size) and temporal (1 h) resolution. We used this model to understand the influence of environmental factors and nutrient management on ABs dynamics in Lake Okeechobee. New Hydrological Insights for the Region: The model showed that ABs mostly developed in shallow nearshore regions near canal outlets, and then spread over the lake as a result of prevailing winds and currents. Hypothetical scenario modeling showed that reducing both nitrogen and phosphorus inputs by 50%-75% would be more effective at reducing ABs in the lake than targeting a single nutrient. The model could be used as a tool to assess the effectiveness of different nutrient management strategies in Lake Okeechobee and its watershed, while the framework could be adopted to other large water bodies facing similar issues.

Legacy mining facilities pose significant risks to aquatic resources. From March 30th to April 9th, 2021, 814 million liters of phosphate mining wastewater and marine dredge water from the Piney Point facility were released into lower Tampa Bay (Florida, USA). This resulted in an estimated addition of 186 metric tons of total nitrogen, exceeding typical annual external nitrogen load estimates to lower Tampa Bay in a matter of days. An initial phytoplankton bloom (non-harmful diatoms) was first observed in April. Filamentous cyanobacteria blooms (Dapis spp.) peaked in June, followed by a bloom of the red tide organism Karenia brevis. Reported fish kills tracked K. brevis concentrations, prompting cleanup of over 1600 metric tons of dead fish. Seagrasses had minimal changes over the study period. By comparing these results to baseline environmental monitoring data, we demonstrate adverse water quality changes in response to abnormally high and rapidly delivered nitrogen loads.

From March 30th to April 9th, 2021, 814 million liters of legacy phosphate mining wastewater and marine dredge water from the Piney Point facility were released into lower Tampa Bay (Florida, USA). This resulted in an estimated addition of 186 metric tons of total nitrogen, exceeding typical annual external nitrogen load estimates to lower Tampa Bay in a matter of days. Elevated levels of phytoplankton (non-harmful diatoms) were first observed in April in the lower Bay. Filamentous cyanobacteria blooms (Dapis spp.) peaked in June, followed by a bloom of the red tide organism Karenia brevis. Reported fish kills tracked K. brevis concentrations, prompting cleanup of over 1600 metric tons of dead fish. Seagrasses had minimal changes over the study period. By comparing these results to baseline environmental monitoring data, we conclude that many of the biological responses observed after the release from Piney Point are abnormal relative to historic conditions.

Within the Indian River Lagoon (IRL), blooms of the marine microalga Aureoumbra lagunensis in excess of 1310 cells ml 1 have occurred on three occasions after first being locally identified in 2012. All blooms coincided with times of peak reproduction and growth of the eastern oyster Crassostrea virginica in Mosquito Lagoon (northern IRL). Over six years of field data on monthly recruitment of C. virginica were compared to bi-monthly cell counts of A. lagunensis. In addition to expected temperature and salinity-driven variations in oyster recruitment, cell counts of A. lagunensis in excess of 10,000 ml 1 reduced oyster recruitment. A second method of analysis similarly found a negative association with oyster recruitment when cell counts exceeded 9658 ml . Understanding the impact of blooms of A. lagunensis is essential for managing harvesting, aquaculture, and restoration of this foundation shellfish species.

Mesozooplankton, as abundant grazers of microalgae in coastal systems, have the potential to prevent or mitigate harmful algal blooms (HABs) and their effects. The Indian River Lagoon (IRL) is a subtropical estuary in eastern Florida (United States) where repeated blooms, dominated by the toxic dinoflagellate Pyrodinium bahamense , the brown tide species Aureoumbra lagunensis , pico/nano planktonic cyanobacteria and other nano-eukaryotes, have highlighted the need to better understand fluctuations in the grazing potential of mesozooplankton populations across bloom cycles. Mesozooplankton and abiotic environmental data were collected at five sites in the northern IRL system at 6-week intervals from November 2013 through June 2016. A total of 107 taxa from 14 phyla were detected. Communities varied across sites, dates and between bloom and non-bloom periods, with densities up to 338 individuals L –1 . Eight taxa comprising 85–94% of the total population at each site were identified as primary potential grazers, including barnacle nauplii, cladocerans, adult copepods, gastropod veligers, larvaceans, and polychaete metatrochophores. Although abundant, the estimated grazing potential of the primary taxa, calculated from their measured densities and previously published grazing rates, suggest that mesozooplankton lack the capacity to suppress phytoplankton once they reach bloom levels. These findings illustrate the utility of monitoring data and underscore the importance of systematically evaluating algal bloom controls with a consideration for the dynamic conditions of each unique ecosystem.

This paper examines the character of phytoplankton blooms in a restricted sub-tropical lagoon along the Atlantic coast of central Florida. The results of the 23-year study (1997–2020) provide evidence for multiple types of variability in bloom activity, including cyclical patterns, stochastic events, and most prominently a regime shift in composition and intensity. Cyclical patterns (e.g., El Niño/La Niña periods) and stochastic events (e.g., tropical storms) influenced rainfall levels, which in turn impacted nutrient concentrations in the water column and the timing and intensity of blooms. In 2011, a major change occurred in the character of blooms, with a dramatic increase in peak biomass levels of blooms and the appearance of new dominant taxa, including the brown tide species Aureoumbra lagunensis and other nanoplanktonic species. Results of quantitative analyses reveal system behavior indicative of a regime shift. The shift coincided with widespread losses of seagrass community and reduced drift algae biomass. A combination of exceptionally low water temperatures in the winters of 2009/2010 and 2010/2011, hypersaline conditions associated with drought conditions, and high light attenuation caused by blooms appear to have contributed to the widespread and protracted decline in seagrass and drift macroalgal communities in the lagoon, leading to shifts in distribution of internal and external nutrient sources toward phytoplankton.

A long-term exposure outdoor microcosm study was conducted to evaluate the effects of zinc (Zn) on zooplankton, phytoplankton, and periphyton in a freshwater system. Five Zn treatment concentrations (nominal: 8, 20, 40, 80, and 160 μg/L Zn) and an untreated control with 3 replicates each were used. Various physical and chemical characteristics of the microcosms and biological assessment endpoints (e.g., total abundance, group abundance, species richness, chlorophyll a, etc.) were measured to determine the effects of Zn over time. In general, physical and chemical characteristics (e.g., total dissolved solids, total suspended solids, dissolved oxygen, pH, dissolved organic carbon) of water fluctuated over time, but they were not significantly different within treatments and controls during the study. Zinc significantly affected the population dynamics and community structure of plankton. The effects occurred 7 d after initial treatment exposures began and continued to the end of the treatment phase, especially at the high treatment concentrations. Total and group abundance, species richness, the Shannon index, and chlorophyll a concentrations for high Zn treatment concentrations were significantly lower than the controls during the treatment phase. The no-observed-effect, lowest-observed-effect, and median effect concentrations were generally lower than the literature-reported results from single-species toxicity tests for fish and invertebrates, suggesting that plankton are more sensitive to Zn than planktivores. Although primary producers play an important role in the ecosystem, they have not been consistently incorporated into numerical environmental quality criteria for freshwater organisms, at least in the United States. The results of the present study are useful for development of environmental quality guidelines for freshwater ecosystems and ecological risk assessment. Environ Toxicol Chem 2021;40:2053-2072. © 2021 SETAC.

The linearity of the BZI (biomass, photic depth and irradiance) regression model for the estimate of depth-integrated primary productivity in the water column is evaluated. It is shown that the linear model can be derived from traditional production-irradiance (P-I) curves by assuming a linear production-irradiance relationship. A correction tem is obtained for water depth shallower than the photic depth. The analysis revealed that the slope of the linear BZI model is determined by the slope of the P-I curve. Following similar steps and assumptions, some of the well-known nonlinear light functions such as Steele's function, Smith's function and the Monod function were analytically integrated yielding nonlinear BZI models which go through the origin naturally allowing zero productivity at zero light. The nonlinear model integrated from Steele's function was successfully applied to three subtropical estuaries in Florida: the St. Lucie Estuary on the east coast, the Caloosahatchee River and Estuary and the Escambia Bay on the west coast. Despite being more faithful to theory (zero intercept), the non-linear version yielded only slightly better results than the linear model.

A microscopic study of microplankton in two coastal lagoons in the Florida Keys coincidently, and unexpectedly, revealed the widespread presence of high concentrations of polystyrene microplastic particles. The polystyrene particles were first observed in the second year of a 2-year study of phytoplankton communities, with peak densities in the spring/summer of 2019 at all ten sampling sites in the two lagoons. Polystyrene particle densities reached levels up to 76,000 L −1 . The particles ranged in size from 33 to 190 µm, similar to the size range of microplanktonic algae (20–200 µm). Over the period of peak polystyrene densities, average particle densities were similar to average densities of microplanktonic algae cells. The latter observation highlights the potential significance of the microplastic particles for the ecology of the pristine waters of the Florida Keys, if they persist.

The Indian River Lagoon (IRL), located on the east coast of Florida, is a complex estuarine ecosystem that is negatively affected by recurring harmful algal blooms (HABs) from distinct taxonomic/functional groups. Enhanced monitoring was established to facilitate rapid quantification of three recurrent bloom taxa, Aureoumbra lagunensis, Pyrodinium bahamense, and Pseudo-nitzschia spp., and included corroborating techniques to improve the identification of small-celled nanoplankton (<10 μm in diameter). Identification and enumeration of these target taxa were conducted during 2015–2020 using a combination of light microscopy and species-specific approaches, specifically immunofluorescence flow cytometry as well as a newly developed qPCR assay for A. lagunensis presented here for the first time. An annual bloom index (ABI) was established for each taxon based on occurrence and abundance data. Blooms of A. lagunensis (>2 × 108 cells L–1) were observed in all 6 years sampled and across multiple seasons. In contrast, abundance of P. bahamense, largely driven by the annual temperature cycle that moderates life cycle transitions and growth, displayed a strong seasonal pattern with blooms (105–107 cells L–1) generally developing in early summer and subsiding in autumn. However, P. bahamense bloom development was delayed and abundance was significantly lower in years and locations with sustained A. lagunensis blooms. Pseudo-nitzschia spp. were broadly distributed with sporadic bloom concentrations (reaching 107 cells L–1), but with minimal concentrations of the toxin domoic acid detected (<0.02 μg L–1). In summer 2020, multiple monitoring tools characterized a novel nano-cyanobacterium bloom (reaching 109 cells L–1) that coincided with a decline in A. lagunensis and persisted into autumn. Statistical and time-series analyses of this spatiotemporally intensive dataset highlight prominent patterns in variability for some taxa, but also identify challenges of characterizing mechanisms underlying more episodic yet persistent events. Nevertheless, the intersect of temperature and salinity as environmental proxies proved to be informative in delineating niche partitioning, not only in the case of taxa with long-standing data sets but also for seemingly unprecedented blooms of novel nanoplanktonic taxa.

Future increases in the intensity of hurricanes and El Niño periods predicted by climate change models have focused attention on their role in stimulating harmful algal blooms (HABs). A series of hurricanes that recently impacted Florida (USA) provided a unique opportunity to explore the relationships between hurricanes, El Niño and HABs in two Florida estuaries subject to repeated intense ecosystem disruptive HABs, the Indian River Lagoon and the St. Lucie Estuary. The roles that hurricanes and El Niño play in contributing to HAB events are examined in the context of key structural and functional features of each estuary and their watersheds, including morphology, water residence time and hydrology, such as the influence of Lake Okeechobee discharges into the St. Lucie Estuary. The most direct impact was the increase in rainfall associated with hurricanes and El Niño, resulting in enhanced nutrient loads which drive HABs in the Indian River Lagoon and Lake Okeechobee. Major HABs in Lake Okeechobee also present an indirect threat of freshwater HAB blooms in the St. Lucie Estuary via mandated discharges from the lake into the estuary during high rainfall periods. Conversely, during the absence of HABs in Lake Okeechobee, short water residence times produced by discharges into the St. Lucie Estuary can result in lower bloom intensities.

The abundance and composition of the phytoplankton community off the coast of Cape Canaveral, Florida was examined from Fall 2013 to Summer 2015. The region is a shallow shelf environment. Water samples were collected quarterly at 24 sites from the surface and bottom of the water column; temperature and nutrient concentrations were determined. Photoautotrophic picoplanktonic prokaryotes (PPP) were consistently important in terms of numerical abundance and biomass throughout the study. Among the cyanobacteria, a surface bloom of the nitrogen-fixing filamentous species Trichodesmium was observed in Fall 2013. Dinoflagellates and diatoms were also major contributors to phytoplankton biomass. Many of the dominant dinoflagellates were mixotrophic or heterotrophic species. Nanoplanktonic eukaryotic algae were also periodically an important group in terms of biomass, such as prasinophytes (Chlorophyta). No consistent and reoccurring spatial patterns were observed, likely due to the dynamic water circulation in the open Cape Canaveral shelf environment. Depth-related differences in biomass were observed for dinoflagellates and diatoms. Dinoflagellate biomass was on average higher in surface samples, related to their motility. Conversely, diatom biomass was on average higher in bottom waters, reflecting the negative buoyancy of most species and the potential for re-suspension of cells from the sediment surface. The strong increase in the importance of PPP biomass observed in year 2 of the study may reflect the influence of high rainfall resulting from El Nino conditions on nutrient loads from regional land masses, as indicated by increases in phosphorus concentrations.