ROLE OF NUTRIENTS AND ZOOPLANKTON IN REGULATION OF PHYTOPLANKTON IN FLATHEAD LAKE (MONTANA, USA), A LARGE OLIGOTROPHIC LAKE

1. Increased primary production in Flathead Lake during the 1980s has been variously attributed to increased nutrient loadings and/or deceas es in zooplankton abundance resulting from the introduction of Mysis r elicta. In order to assess the importance of these two factors in regu lating the phytoplankton community in Flathead Lake, we manipulated zo oplankton abundance and nutrient availability in a series of 5-day enc losure experiments. 2. Chlorophyll a levels were stimulated by simulta neous addition of nitrogen and phosphorus. At ambient nutrient levels, alteration of zooplankton density had no effect on chlorophyll a leve ls. Top-down control through zooplankton grazing could only be demonst rated in treatments supplemented with nutrients. Under these condition s, there was a significant negative correlation between zooplankton ab undance and final chlorophyll a levels. 3. These results suggest that the phytoplankton community in Flathead Lake is regulated primarily by bottom-up controls. Consequently, future management activities aimed at preventing further increases in algal growth in the lake should foc us on nutrient abatement. Alteration of the upper trophic levels does not appear to have significantly affected phytoplankton abundance in t he lake. Should nutrient levels increase in the future, then top-down controls may become more important. 4. A conceptual model is presented illustrating the relative importance of top-down and bottom-up contro ls across a trophic gradient.