Holz, J. C. University of Nebraska, jholz@unl.edu
Hoagland, K. C. University of Nebraska, khoagland@unl.edu
Joern, A. University of Nebraska, tonyj@niko.unl.edu

 
EXPERIMENTAL MESOCOSM STUDY OF PHYTOPLANKTON SPECIES COMPOSITION, ALTERNATE LAKE STATES, AND COMMUNITY BREAKPOINTS ALONG A PHOSPHORUS GRADIENT
 
A model combining algal mortality due to selective zooplankton grazing with the Monod model of phytoplankton growth was used to predict phytoplankton species composition along a gradient of phosphorus (P) concentration and zooplankton grazing pressure. Model predictions were compared to the results of a mesocosm experiment of phytoplankton species composition along a range of P concentrations of 5-115 ug/l and potential grazing loss rates of 0.01-27% per day. The effect of the P gradient was measured by monitoring phytoplankton and zooplankton species composition and biomass, and physical and chemical parameters for 7 wk in 12 fiberglass tanks (5500 L) filled with lake water and associated plankton. P concentrations were manipulated so that tanks were evenly distributed along the gradient. Using laboratory derived Monod growth constants for P and literature values of phytoplankton selectivity coefficients, zooplankton filtering rates, and zooplankton assimilation efficiencies, the model correctly predicted 1) the existence of three alternate states, 2) breakpoints between the alternate states which are similar to the P concentrations defining Vollenweider's lake trophic states, and 3) phytoplankton species composition along the P gradient in the mesocosm study. These results suggest that the trade-off between competitive ability for P and edibility can be a primary factor structuring phytoplankton communities, and that multiple states exist which are consistent with the long-standing P-based lake trophic classification.
 
Day: Thursday, Feb. 4
Time: 12:00 - 12:15pm
Location: Eldorado Hotel
 
Code: SS51TH1200E