The St. Johns River, a 300-mile riverine and estuarine system located along the northeastern coastline of Florida, has undergone extensive eutrophication through point and non-point source nutrient inputs. Moreover, recent reports of sudden fish kills and a high incidence of fish with lesions suggest the potential for harmful algal blooms. As part of a NOAA-funded project involving the characterization of water quality parameters in relation to water inflows and nutrient inputs and the development of species-specific markers/probes for instrumental-based monitoring efforts, we used Primer-based multivariate analyses to examine the relationship between abiotic variables and both total and phylogenetic-group chlorophyll a concentrations (derived from photopigments and ChemTax matrix factorization of diagnostic carotenoids) during 2001. Seven sampling sites (identified through principle components ordination of physical/chemical parameters as ranging from oligo- to mesohaline) were sampled intensively over two-week periods on a seasonal basis. Chlorophyll a concentrations typically ranged from 5 to 35 ug/L with the greatest concentrations occurring at the oligohaline sites. Phytoplankton assemblages were dominated by diatoms, cryptophytes, and cyanobacteria, and together typically comprised up to 90% of the total chlorophyll a. Temporal variability in phytoplankton assemblages followed seasonal trends impacted by meteorological and hydrological forcing. Spatial variability in phylogenetic-group abundance (illustrated through multi-dimensional scalar ordination of sample dissimilarity) was dramatic and associated with differences in abiotic variables along the estuarine gradient.

Key words: estuary, microalgae, multivariate, phytoplankton, Primer