SEASONAL-CHANGES IN CHLOROPHYLL-ALPHA RESPONSE TO NUTRIENT AMENDMENTSIN A NORTH SHORE TRIBUTARY OF LAKE-SUPERIOR

Four experiments were conducted to determine whether nutrient amendmen t with nitrogen (N), phosphorus (P), or birth N and P had seasonally f luctuating effects on algal biomass, as chlorophyll a, in the French R iver, Minnesota, a North Shore tributary to Lake Superior. During spri ng, early summer, late summer, and fall seasons, clay pots containing agar supplemented with N, P, N+P, or control (no nutrients) were place d in the stream for 3-wk periods. In spring, with meltwater in the str eam, P-diffusing pots had significantly higher algal biomass than cont rols (p < 0.05). N-diffusing pots supported significantly higher algal biomass in early summer (p < 0.05) and late summer (p < 0.05) than co ntrol pots. In fall, algal biomass was not significantly different amo ng treatments. Algal biomass on control pots was similar for ail 4 exp eriments. In the spring, there was a significant accumulation of algae on stream rocks during the experimental period (p = 0.002). However, in fall,algal biomass on rocks was much higher than in thr spring and showed a trend toward increasing (p = 0.06). Nutrient concentrations a nd the ratio of dissolved inorganic nitrogen to soluble reactive phosp horus also fluctuated seasonally, but were not consistent predictors o f algal biomass responses on pots. These results show that nutrient li mitation of algal biomass is seasonally dynamic, thus must be determin ed by a variety oi processes within the stream and watershed.