Growth and production of heterotrophic nanoflagellates in a meso-eutrophic lake

Abstract

The growth of heterotrophic nanoflagellates (HNF) in mesotrophic Lake Constance was measured in situ during a 13 month period. Experiments were conducted with 10 urn pre-filtered late water incubated in diffusion chambers at 3 m water depth at the sampling location for 24 h. Growth rates were calculated from changes in cell numbers occurring during the period of incubation. Growth rates of all dominant taxa showed pronounced seasonal variation (-0.13 to 1.76 day') and were gener- ally highest in summer at high water temperatures. In situ growth rates were well below maximum growth rates known for the respective and similar species from laboratory experiments. While water temperature was a key parameter positively related to the growth of all HNF species, the effect of various potential food items was taxon specific and less clear. Bacterial abundance was equally impor- tant as temperature for growth in the small bacterivorous Spumella sp., but was insignificant for growth rates of the larger omnivorous Kathablepharis sp. In Spumella sp., 84% of the observed seasonal variation of its gTowth rate could be explained by temperature and bacterial food supply. Based on these results, a multiple linear regression equation with temperature and bacterial concen- tration as dependent variables was calculated for the growth rate of Spumella. Taxon-specific pro- duction rates were derived from growth rates and average biomass of these two species, and compared to total HNF production estimated from previously measured community growth rates and biomass in Lake Constance. Production peaks of Spumella sp. and Kathablepharis sp. alternated seasonally. Total HNF production ranged from -0.01 to 10 mg C m~3 day1. The average seasonal production varied between 1.4 and 3.5 mg C nr3 day-1 over 6 consecutive years. These small protozoa thus con- tribute a substantial amount to total zooplankton production in Lake Constance.