A 2 year study of the phytoplankton community was carried out in the Indian River Lagoon, USA. In terms of biovolume, the phytoplankton community was generally dominated by dinoflagellates, diatoms or cyanobacteria. Mean phytoplankton standing crops were highest in the most flow-restricted regions of the lagoon, which had the lowest mean salinity values and comparatively high total nitrogen:total phosphorus ratios. In this region, blooms of dinoflagellates were common in the first year of the study, which was characterized by an El Niño event that yielded exceptionally high rainfall levels and freshwater outflow. Picoplanktonic cyanobacteria blooms became more prominent in the second year of the study, which was characterized by below average rainfall conditions. In unrestricted flow regions of the lagoon, located near inlets to the Atlantic Ocean, diatoms were most often the dominant taxa. Regions of intermediate water turnover rates and high external loading of phosphorus had a prevalence of diatom blooms. However, the average phytoplankton standing crops in the latter regions did not reach the levels experienced in the flow-restricted parts of the lagoon. In terms of individual phytoplankton taxa, the most common bloom-forming diatoms in the Indian River Lagoon system included: Skeletonema costatum, Dactyliosolen fragilissimus, Skeletonema menzelii, Cerataulina pelagica, Odontella regia, Chaetoceros lorenzianus, Rhizosolenia setigera and Thalassionema nitzschioides. The major bloom-forming dinoflagellate species included: Pheopolykrikos hartmannii, Akashiwo sanguinea, Prorocentrum micans, the potentially toxic species Pyrodinium bahamense var. bahamense and Prorocentrum minimum. Several picoplanktonic cyanobacteria were also prominent members of the phytoplankton community, including Synechococcus elongates. The spatial and temporal patterns observed in some of these dominant species were attributable to patterns in key environmental variables, including salinity, temperature and nutrient concentrations.
