The role of stratification in determining phytoplankton community composition during summertime in the Chesapeake Bay

During the summertime in the Chesapeake Bay, the water column is highly stratified, with buoyant freshwater from rivers flowing over salty dense waters from the Atlantic Ocean. This stratification separates surface microbial populations from those at deeper depths, except when high wind speed events encourage mixing. In this study, we compare two highly resolved vertical profiles of microbial communities, along with hydrographic and chemical measurements, to demonstrate how physical processes affect the vertical distribution of microbial communities. We sampled near the Chesapeake Bay Bridge on one stratified day in July 2015, and again in August on a day when storms deepened the mixed layer. Phytoplankton species relative abundance was determined using a chemotaxonomic method from pigment concentrations and amplicon sequencing of the 18S rRNA gene (for eukaryotes) and the 16S rRNA gene (for phytoplankton plastid DNA). We compare these methods because the pigment composition of a single species can vary based on environmental conditions and physiological state. This can lead to biases in the resulting pigment-based taxonomic assignment for samples collected over the range of conditions observed within a water column of the estuary. Preliminary results from our study show the surface phytoplankton community is dominated by chlorophytes in July, with highest chlorophyll-a and nitrate concentrations near the surface which then decreases with depth. In August, the surface phytoplankton community is dominated by diatoms, chlorophyll-a concentrations are elevated from the surface to 5m, and nitrate concentrations are relatively constant with depth. We examine the factors that contribute to the differences in phytoplankton community composition, and how stratification can affect the outcomes from chemotaxonomic and molecular methods. Given the uncertainties inherent in traditional pigment-based methods for determining phytoplankton community composition, we consider novel statistical techniques to find associations between the observed suite of phytoplankton pigments and the microbial community composition.