Analysis of phytoplankton and mixotroph diversity and abundance in an estuarine system using complementary methods

Chlorophyll a concentrations, a proxy for phototrophic biomass, is used as a primary indicator of water quality. While chlorophyll a measurements can provide information on where phototrophic biomass is highest, it cannot provide information on the species present within the phototrophic community. Waquoit Bay, Massachusetts is part of the NOAA National Estuarine Research and Reserve program and has decades of chlorophyll a data that demonstrates a specific region has significantly higher phototrophic biomass compared to the rest of the system. However, it is not known how the diversity of phototrophs compares between the high phototrophic biomass and low. We used a combination of microscopy, DNA sequencing, and bromodeoxyuridine (BrdU) labeled bacteria ingestion experiments to examine annual phytoplankton and mixotroph diversity within the phototrophic community at two different regions with Waquoit Bay, and identified environmental factors associated with the variability. We discovered that the region with higher phototrophic biomass had a higher occurrence of flagellates and ciliates, while the region with lower phototrophic biomass had a higher occurrence of diatoms. The difference in biomass was the result of variability in inorganic nitrogen concentrations, while the difference in species was likely the result of variability in stratification. Through the use of BrdU labeled bacteria we were able to identify active mixotrophs within the phototrophic community and estimated that a large proportion of the phototrophs were potential mixotrophs. The variability in phototrophic species in different regions of Waquoit Bay and the high occurrence of potential mixotrophs will affect how efficiently carbon is transferred through the planktonic food-web.