Nutrient Bioassays of Plankton Biomass and Metabolism in an Urbanized Drowned River-mouth Lake (Mona Lake, Michigan)

Abstract

ABSTRACT A variety of short-term responses were observed in inorganic nutrient enrichment bioassays on the plankton of Mona Lake—an urbanized and eutrophic drowned river-mouth lake that drains directly to Lake Michigan (USA). Chlorophyll a and phytoplankton production data indicated phosphorus (P) limitation in spring, nitrogen (N) and P co-limitation in summer, and no N or P limitation in autumn. Heterotrophic bacterial abundance, dissolved organic carbon levels, and plankton respiration were not stimulated by nutrient enrichment. Autotrophic biomass and production, but not heterotrophic biomass or respiration, were seasonally stimulated by the availability of P and/or N. Such imbalance between autotrophic and heterotrophic processes may contribute to the accumulation of phytoplankton biomass during the growing season. Cyanobacterial abundance in the lake increased from 22% of the plankton biomass in the spring when the TN:TP ratio was high to 68% in the autumn when the TN:TP ratio was low, suggesting that P availability helps drive a seasonal succession towards cyanobacterial dominance.