Phytoplankton assemblages across the marine to low-salinity transition zone in a blackwater dominated estuary

Along the salinity gradient between the freshwater reach ofthe Suwannee River and the marine waters of the Gulf of Mexico,the spatial and temporal composition and abundance of phytoplanktonwas examined in relation to physical and chemical water-columnvariables (i.e. salinity, nutrient and light availability andtemperature). The goal of this study was to explore the applicabilityof the concepts of ecotone and ecocline to the distributionof phytoplankton. This was undertaken in a relatively open estuarydominated by a tannin-colored river, the Suwannee River, alongthe central northwest coast of Florida, USA. Hierarchical clusteranalysis and non-metric multi-dimensional scaling (MDS) wasused to define biologically distinct regions within the estuarybased on the composition of phytoplankton. Variations in phytoplanktonassemblages in relation to environmental variables were investigatedusing a canonical correspondence analysis. The results indicatethat in addition to the alignment of phytoplankton compositionalong a salinity gradient, several key environmental factorsinfluence phytoplankton assemblages within hydrologically distinctregions of the estuary supporting an ecocline view. However,the distribution of dinoflagellate assemblages were in contrastto the more temporally fluid spatial distributions of cyanobacteriaand diatoms in the Suwannee River estuary. The spatially definednature of the dinoflagellate assemblages was more characteristicof an ecotone rather than an ecocline.