Microbial gradients in a turbid estuary: Application of a new method for protozoan community analysis

Abstract

A method was developed to assess the planktonic flagellate, ciliate, and amoeboid populations across the transition from fresh to salt water in the upper St. Lawrence estuary. As in many estuarine environments the water is too turbid for examination by standard concentration and microscopy techniques. A combined system of fluorescence, Nomarski interference, and Utermohl sedimentation (FNU) allowed us to resolve a community of 57 protozoan species across the freshwater-salt-water gradient. Colorless nanoflagellate concentrations decreased from 4.1 × 106 cells liter−1 (247 × 106 µm3 liter− 1) in the freshwater zone to 1.7 × 106 cells liter−1 (139 × 106 µm3 liter−1) in the estuarine frontal region; on average this group contributed at least 45% of the total protozoan cell numbers. Nonpigmented ochromonads (Spumella spp.) were the consistent dominants. The heterotrophic picoplankton increased from 1.7 to 2.8 × 109 cells liter-1 across the salt-water gradient, whereas the autotrophic picoplankton decreased from 8.4 to 2.8 × 106 cells liter . The frontal region of maximum fish larval biomass contained a diverse protozoan assemblage and was a transitional zone of changing microbial food-web structure.