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Seasonal Changes in Chlorophyll a Response to Nutrient Amendments in a North Shore Tributary of Lake Superior

Four experiments were conducted to determine whether nutrient amendment with nitrogen (N), phosphorus (P), or both N and P had seasonally fluctuating effects on algal biomass, as chlorophyll a, in the French River, Minnesota, a North Shore tributary to Lake Superior. During spring, early summer, late summer, and fall seasons, clay pots containing agar supplemented with N, P, N+P, or control (no nutrients) were placed in the stream for 3-wk periods. In spring, with meltwater in the stream, P-diffusing pots had significantly higher algal biomass than controls (p < 0.05). N-diffusing pots supported significantly higher algal biomass in early summer (p < 0.05) and late summer (p < 0.05) than control pots. In fall, algal biomass was not significantly different among treatments. Algal biomass on control pots was similar for all 4 experiments. 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 the spring and showed a trend toward increasing (p = 0.06). Nutrient concentrations and the ratio of dissolved inorganic nitrogen to soluble reactive phosphorus also fluctuated seasonally, but were not consistent predictors of algal biomass responses on pots. These results show that nutrient limitation of algal biomass is seasonally dynamic, thus must be determined by a variety of processes within the stream and watershed.