Intra-annual variability of water quality and phytoplankton in the North Fork of the St. Lucie River Estuary, Florida (USA): a quantitative assessment

Water quality and phytoplankton were examined within the North Fork of the St. Lucie River Estuary, Florida (USA) from March 2000 to March 2001. Alterations in water-quality parameters and phytoplankton assemblages mirrored salinity regimes resulting from the ‘wet’/‘dry’ seasonality of south Florida. Salinity was greatest during the ‘dry’, winter months whereas water temperature and nitrogen and phosphorus concentrations were greatest during the ‘wet’, summer months. A seasonal dissimilarity in phytoplankton occurred; cell abundance of cyanobacterial picoplankton (Synechocystis sp., Synechococcus sp.) and a diatom (Skeletonema costatum (Greville) Cleve) and cell carbon of a dinoflagellate (Gyrodinium sp.) and S. costatum were greatest during summer whereas abundance of chrysophytes (Chrysochromulina parva Lackey, Chromulina sp.) and carbon of dinoflagellates (Gymnodinium spp., Heterocapsa rotundata (Lohmann) Hansen) and a diatom (Cyclotella sp.) were greatest during winter. Watercolumn chlorophyll a concentrations reached up to 29 mg L 1 and were negatively associated with salinity. Diatom chlorophyll comprised the majority and at times, greater than 90% of the total chlorophyll a. Picoplanktonic cyanobacteria comprised up to 5% and 1.4% of total phytoplankton carbon and chlorophyll a, respectively throughout the estuary; as such, its impact on overall assemblage rate processes and system-level function appeared minimal. Sediment and whole-water incubations confirmed the presence of heterotrophic dinoflagellates within the estuary. Dissolved oxygen concentrations did not correspond with total chlorophyll a concentrations and salinity, indicating that hypoxia within bottom waters during summer was not solely attributable to phytoplankton biomass or water-column stratification but likely, a result of multiple, interacting physical/chemical and biological factors.