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

Abstract

Along the salinity gradient between the freshwater reach of the Suwannee River and the marine waters of the Gulf of Mexico, the spatial and temporal composition and abundance of phytoplankton was examined in relation to physical and chemical water-column variables (i.e. salinity, nutrient and light availability and temperature). The goal of this study was to explore the applicability of the concepts of ecotone and ecocline to the distribution of phytoplankton. This was undertaken in a relatively open estuary dominated by a tannin-colored river, the Suwannee River, along the central northwest coast of Florida, USA. Hierarchical cluster analysis and non-metric multidimensional scaling (MDS) was used to define biologically distinct regions within the estuary based on the composition of phytoplankton. Variations in phytoplankton assemblages in relation to environmental variables were investigated using a canonical correspondence analysis. The results indicate that in addition to the alignment of phytoplankton composition along a salinity gradient, several key environmental factors influence phytoplankton assemblages within hydrologically distinct regions of the estuary supporting an ecocline view. However, the distribution of dinoflagellate assemblages were in contrast to the more temporally fluid spatial distributions of cyanobacteria and diatoms in the Suwannee River estuary. The spatially defined nature of the dinoflagellate assemblages was more characteristic of an ecotone rather than an ecocline.