Annual phytoplankton cycle in a meromictic anoxic basin of a Rhode Island (USA) estuarine river

Estuarine Pettaquamscutt River is a unique habitat 10 km in length with physical and biogeochemical characteristics analogous to a miniature fjord. Its meromictic upper basins are characterized by a permanent oxic-anoxic vertical gradient in which a well-oxygenated upper layer overlies a deeper, anoxic reservoir, with persistent blooms of phototrophic anoxygenic bacteria (Chromatium, Chlorobium) at the oxic-anoxic transition layer. A diverse assemblage of nanoplanktonic, centric diatoms (Cyclotella caspia, Thalassiosira pseudonana, cf. Cyclotella cryptica) dominated the seasonal phytoplankton cycle in the aerobic layer, similar to comparable meromictic habitats elsewhere. This assemblage of nano-centric diatoms appears to be trait-separated from other species clusters and is potentially useful as a functional group flora with ecophysiology diagnostic of marine estuarine rivers and meromictic habitat niche structure. The most conspicuous phytoplankton feature, however, was the year-round occurrence of the photoautotrophic euglenid Euglenaformis (Euglena) proxima, restricted in its upper water column distribution to the O-2-H2S boundary layer where it formed a consortium with the photosynthetic green and purple sulfur bacteria community. The association of E. proxima with meromixis, H2S, and phototrophic anoxygenic bacteria is similar to that reported previously in a brackish Norwegian oyster poll and a brackish loch in Scotland.