Pace, M. L. Institute of Ecosystem Studies, pacem@ecostudies.org
Cole, J. J. Institute of Ecosystem Studies, colej@ecostudies.org

 
BACTERIAL RESPONSES TO NUTRIENT LOADING DEPEND ON FOOD WEB STRUCTURE: RESULTS FROM WHOLE LAKE EXPERIMENTS
 
Nitrogen and phosphorus were added to three experimental lakes over five summer seasons after observing the lakes for two years at nominal loading. A fourth lake served as an unmanipulated reference system. The experiments provided 28 lake-years of data at a range of phosphorus loading varying from background to conditions sufficient to cause eutrophication. Previous studies demonstrated bacteria were strongly phosphorus limited in these lakes suggesting nutrient fluxes are critical to bacteria. Nevertheless, bacterial responses in terms of abundance and production could not be predicted based on rates of nutrient loading. Instead, bacteria were related both to nutrient loading and top-down controls determined by food web structure. For example, at the highest nutrient loading, lakes dominated by large-bodied zooplankton (i.e., Daphnia spp.)., had 3x lower production as measured by leucine incorporation. Mean annual bacterial production was strongly correlated with mean primary production suggesting that bacterial responses were determined by limitations imposed on primary producers via grazing, by the direct effects of grazers on bacteria, or both. Since Daphnia abundance was controlled by fish community structure, bacterial dynamics were strongly related to trophic cascades mediated by top-predators.
 
Day: Thursday, Feb. 4
Time: 11:15 - 11:30am
Location: Sweeney Center
 
Code: SS39TH1115S