Ecological importance of bacterivorous, pigmented flagellates (mixotrophs) in the Bay of Aarhus, Denmark

Abundance and bacterivory of mixotrophic flagellates were examined in a vertical profile during 1 wk in June 1992 in the Bay of Aarhus, Denmark. A stable pycnocline separated a n upper water mass with low salinity, low inorganic nutrient concentration (< 0.1 pm01 1-l) and low bacterial abundance (<106 ml-') from a bottom water mass with higher salinity, inorganic nutrient concentration, and bacterial abundance (>106 1111-l). In the upper layer, bacterivorous, pign~ented flagellates (mixotrophs) accounted for 49% of the pigmented biomass. In addition to their function as primary producers, mixotrophic flagellates were responsible for 86% of the entire flagellate bacterivory The abundance of bacterial food particles (<106 m]-') was probably not sufficient to sustain growth of most bacterivorous, colourless flagellates, and the nutrient-depleted water body prevented the strict phototrophs from dominating the environment. Below the pycnocline, nutrients were present, bacterial abundance exceeded 106 ml-l, and mixotrophic flagellates made up only 9 % of the pign~ented biomass and accounted for 1 9 % of the flagellate bacterivory.