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

Abstract

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 an upper water mass with low salinity, low inorganic nutrient concentration (< 0.1 μmol l-1) and low bacterial abundance (< 106 ml-1) from a bottom water mass with higher salinity, inorganic nutrient concentration, and bacterial abundance (>106 ml-1). In the upper layer, bacterivorous, pigmented 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 ml-1) 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-1, and mixotrophic flagellates made up only 9% of the pigmented biomass and accounted for 19% of the flagellate bacterivory.