The distribution of protozoa across a trophic gradient, factors controlling their abundance and importance in the plankton food web

The relative contribution of protozoan biomass to whole planktonic communities (phytoplankton, picophytoplankton, bacterioplankton, protozoa and zooplankton) and factors important in controlling protozoan abundance were investigated at two eutrophic coastal sites and two meso-oligotrophic offshore sites in the central basin of Lake Erie, USA, from May through August in 1993 and 1994. The abundance and biomass of heterotrophic nanoflagellates (HNAN) and ciliates (and also other plankton components) were significantly higher at the coastal sites than at the offshore sites. HNAN dominated numerically at all sites most of the time, but the biomass of phototrophic nanonagellates (PNAN) was as high as that of HNAN, indicating that the average size of PNAN was larger. Percent protozoan carbon content was always higher at the offshore sites than the coastal sites, due to relatively lower phyto- and zooplankton biomass at the offshore sites. The percent contribution of heterotrophic protozoans (both HNAN and ciliates) also showed the same trend. Correlations between protozoan abundance and other parameters were stronger at the offshore sites than the coastal sites. When correlating data over the coastal to offshore transect, both HNAN and ciliate abundances were significantly correlated with total phosphorus (TP) and the abundance of bacteria, nauplii and copepods. These results suggest that both bottom-up and top-down factors may be important in controlling protozoan abundance, and suggest that protozoans are important as a carbon link in the microbial food web of Lake Erie.