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, Denm ark. A stable pycnocline separated an upper water mass with low salini ty, low inorganic nutrient concentration (< 0.1 mu mol l(-1)) and low bacterial abundance (<10(6) ml(-1)) from a bottom water mass with high er salinity, inorganic nutrient concentration, and bacterial abundance (>10(6) ml(-1)). In the upper layer, bacterivorous, pigmented flagell ates (mixotrophs) accounted for 49% of the pigmented biomass. In addit ion to their function as primary producers, mixotrophic flagellates we re responsible for 86% of the entire flagellate bacterivory. The abund ance of bacterial food particles (<10(6) ml(-1)) was probably not suff icient to sustain growth of most bacterivorous, colourless flagellates , and the nutrient-depleted water body prevented the strict phototroph s from dominating the environment. Below the pycnocline, nutrients wer e present, bacterial abundance exceeded 10(6) ml(-1), and mixotrophic flagellates made up only 9% of the pigmented biomass and accounted for 19% of the flagellate bacterivory.