MACROPHYTE GROWTH AND SEDIMENT PHOSPHORUS AND NITROGEN IN A CANADIAN PRAIRIE RIVER

1. The role of sediment phosphorus (P) and nitrogen (N) in regulating growth of rooted macrophytes in a Canadian prairie river was investiga ted by means of in situ observations and artificial stream fertilizati on experiments. 2. Biomass and percentage cover of rooted macrophytes in the South Saskatchewan River increased downstream of a municipal se wage treatment plant, with maximum abundance occurring between 25 and 100 km downstream of the outfall. Biomass in the river was related to sediment P but not N concentration, although sites of maximum biomass did not coincide with sites of maximum sediment P concentration. 3. Ar tificial stream experiments revealed that while biomass was unaffected by addition of N to the sediment, it was enhanced by the addition of P to sediments, and further enhanced with the addition of N and P toge ther, indicating a primarily P-limited system, with secondary N limita tion when P is in excess. 4. Macrophyte biomass increased linearly wit h increasing sediment P concentration in the artificial streams, and t issue P concentration peaked at approximate to 400 mu g g(-1). Biomass did not respond to increasing sediment N concentration, and only a we ak relationship was observed between tissue N and sediment N, with max imum tissue N corresponding to approximate to 140 mu g g(-1) sediment exchangeable N. 5. A lack of concurrence between the sediment P concen tration producing maximum biomass and tissue P concentration in situ v s, under experimental conditions indicates that other environmental fa ctors have an important role in regulating macrophyte growth in rivers . Thus, while nutrient control may be one element in a river macrophyt e control programme, a holistic ecosystem approach should be adopted t o account for the other factors that may affect the growth of rooted p lants.