The impact of nutrients and submersed macrophytes on invertebrates in a prairie wetland, Delta Marsh, Manitoba.

Abstract

Shallow freshwater systems often exhibit two alternative stable states. The clear-water stable state is dominated by macrophytes, and the turbid stable state is dominated by phytoplankton, with fewer macrophytes. Two factors that may influence a shift in stable state are nutrient loading, and presence of macrophytes. Enclosures were used to manipulate nutrient loading and exclude macrophytes to determine their impact on the stable state and invertebrate communities at Delta Marsh, Manitoba, a large, freshwater lacustrine wetland. The scope of the study was to provide a comprehensive examination of the roles of all major players in the food web in both water column and among macrophytes, including macroinvertebrates. Turbid conditions with phytoplankton blooms were established when nutrients were added. Zooplankton density was low in all treatments most likely reflecting predation by fathead minnows. Macrophytes did not maintain the clear-water conditions with increased nutrient loading and did not provide a refuge for zooplankton. Macrophyte exclusion alone did not produce a shift to turbid conditions. Density of herbivorous macroinvertebrates that feed on epiphyton showed no response to nutrient addition and did not control algal biomass.