RESPONSE OF BENTHIC AND PLANKTONIC ALGAL BIOMASS TO EXPERIMENTAL WATER-LEVEL MANIPULATION IN A PRAIRIE LAKESHORE WETLAND

The quantitative contribution of benthic (periphytic) and planktonic a lgae to primary production in prairie wetlands is largely unknown, as is their response to the fluctuations in water level that characterize such systems. We measured the biomass (chlorophyll-a m(-2) of wetland area) of phytoplankton, epipelon, epiphyton, and metaphyton in Delta Marsh, Manitoba as part of a five-year study in which diked, drawn dow n cells were reflooded to the normal level of the wetland, or to a dep th 30 cm or 60 cm deeper. Our objective was to investigate the effects of flooding depth on algal biomass and the relative contributions by each of the four algal assemblages. Floating metaphyton mars flourishe d in all cells after flooding, contributing about 87% of total algal b iomass. Epiphytes contributed 11% of biomass, and epipelon and phytopl ankton each contributed 1%. Emergent macrophyte density was reduced by flooding, leading to increases in open water area. The wetland cells changed gradually over the study period from an early ''open wetland'' to a ''sheltered wetland.'' In late stages of the study, phytoplankto n became more abundant as the cells proceeded to a ''lake wetland'' st ate.