RESPONSES OF A PRAIRIE WETLAND TO PRESS AND PULSE ADDITIONS OF INORGANIC NITROGEN AND PHOSPHORUS - PRODUCTION BY PLANKTONIC AND BENTHIC ALGAE

Enclosures deployed in Delta Marsh, Canada, were treated with inorgani c N and P either as two discrete additions (''pulse'') or as 29 smalle r additions (''press'') comprising the same cumulative nutrient load o ver a three-month period. No effects on photosynthesis by phytoplankto n, epiphyton or epipelon were detected. Similarly, phytoplankton and e pipelon biomass was unaffected by any treatment. However, epiphyton an d metaphyton biomass increased significantly. Absent from controls exc ept after macrophyte senescence, metaphyton represented > 80 % of tota l algal biomass and total algal P in treated enclosures. No difference s in metaphyton biomass between press and pulse treatments were observ ed although its composition varied; diffuse masses of Cladophora devel oped in pulse enclosures whereas floating carpets of Enteromorpha occu rred in press enclosures. The latter decreased water column irradiance markedly, leading to premature macrophyte senescence. Sediment and me taphyton were probably the primary sinks for added nutrients. These re sults illustrate that nutrient enrichment can shift a stable epiphyton -dominant system (''open wetland'') to metaphyton dominance (''shelter ed wetland'') if macrophytes remain sufficiently abundant to provide s ubstratum for metaphytic algae. Development of the phytoplankton-domin ant ''lake wetland'' state presumably occurs only when there are few o ther algal and macrophytic competitors for nutrients.