Primary Production of the Big Eau Pleine Reservoir, Wisconsin

Abstract

Measurements of primary production and phytoplankton biomass were determined in 1975 and 1976 in the Big Eau Pleine Reservoir, Wisconsin. The data was collected to provide an information base of the phytoplankton dynamics in the reservoir for the calibration of a computer model. Primary production analysis was accomplished using the carbon-14 and oxygen light and dark bottle techniques. Phytoplankton biomass was measured by chlorophyll a analysis and by the determination of phytoplankton volume. The average rates of carbon-14 productivity for 1975 and 1976 were 1.3 and 2.8 g C/m^2/day, respectively. The estimated annual productivity was 300 g C/m^2 in 1975 and 605 g C/m^2 in 1976. The average surface phytoplankton volume measurements for the summer periods (June-September) of 1975 and 1976 were 32.2 and 53.0 mm^3/l, respectively. Correspondingly, the mean chlorophyll a concentrations for these periods were 53.2 and 103.0 mcg/1, respectively. Phytoplankton volume paralleled changes in chlorophyll a throughout the study. The observed increase in primary productivity and phytoplankton biomass in 1976 were attributed to increased solar radiation, water temperature, and internal phosphorus loading in the summer months of 1976 as compared to the previous summer period. Surface water temperatures were warmer in the summer of 1976 in response to increased solar radiation. The timing and magnitude of the summer drawdown influenced the release of phosphorus from the reservoir sediments. An earlier and more extensive summer drawdown in 1976 provided higher levels of total reactive phosphorus than the previous summer period. The release of phosphorus from the reservoir sediments during the summer months provided a significant source of phosphorus for phytoplankton production. The phytoplankton biomass was dominated by a bloom forming blue-green alga, Aphanizomenon flos-aguae. Results of nutrient enrichment bioassays and the determination of the chemical composition of A. flos-aguae indicated that phosphorus was the primary limiting nutrient. A discussion of computer model coefficients for algae is presented.