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

Shallow freshwater systems often exhibit two alternative stable states. The clear-water stable state is dominated by macrophytes, and the turbid stabl e state is dominated by phytoplankton, with fewer macrophytes. Two factors that may influence a shift in stable state are nutrient loading, and presen ce of macrophytes. Enclosures were used to manipulate nutrient loading and exclude macrophytes to determine their impact on the stable state and inver tebrate communities at Delta Marsh, Manitoba, a large, freshwater lacustrin e wetland. The scope of the study was to provide a comprehensive examinatio n of the roles of all major players in the food web in both water column an d among macrophytes, including macroinvertebrates. Turbid conditions with p hytoplankton blooms were established when nutrients were added. Zooplankton density was low in all treatments most likely reflecting predation by fath ead minnows. Macrophytes did not maintain the clear-water conditions with i ncreased nutrient loading and did not provide a refuge for zooplankton. Mac rophyte exclusion alone did not produce a shift to turbid conditions. Densi ty of herbivorous macroinvertebrates that feed on epiphyton showed no respo nse to nutrient addition and did not control algal biomass.