Factors regulating primary production and phytoplankton dynamics in western Lake Erie

Abstract

Phosphorus management plans in the Great Lakes were based on an empirical relationship between total phosphorus loads and primary production. The aim of this study was to determine if primary production in western Lake Erie had declined as predicted by these original models. Annual primary production, estimated using both in situ and constant light incubations, ranged from 320-370 g C m -2 between 2000 and 2002. The current rate of annual primary production in the west basin was comparable with the 340 g C m -2 reported for 1970 when phosphorus abatement programs were initiated. Phytoplankton standing crop, measured as chlorophyll-a, has declined however, from annual mean (i.e. growing season) concentrations of 12 mg m -3 in 1970 to 4-7 mg m -3 in 2000-2002. Phytoplankton biomass ranged from 4-5 g m -3 in the spring and summer of 2000-2001, similar to the 5-7 g m -3 reported in 1970. Taxonomic analysis of the phytoplankton community, however, revealed an increase in the relative abundance of mesotrophic genera (e.g. Chlamydomonas, Cyclotella, Microcystis) as compared with the more eutrophic assemblage observed in 1970. It is concluded that primary production in the western basin of Lake Erie has not declined as predicted by the original phosphorus-chlorophyll-a-primary production models. Multiple stressor models linking nutrient dynamics to species invasions and subsequent changes in the underwater light climate are required for the future management of primary production in Lake Erie.