Phosphorus Kinetics in Lake Superior: Light Intensity and Phosphate Uptake in Algae

Abstract

Epilimnetic phytoplankton in Lake Superior in September, 1979, had low I_k values (75–190 μE∙m⁻²∙s⁻¹), low N/P ratios (8 to 13:1) and ³²PO₄–P uptake kinetics that were not consistent with a state of extreme phosphorus limitation. Parallel laboratory experiments with Chlorella pyrenoidosa indicated that phosphorus content per cell was higher and uptake rates of phosphate were lower in cells grown under low light (57 μE∙m⁻²∙s⁻¹) than those under high light (340 μE∙m⁻²∙s⁻¹). Maximum ³²PO₄ uptake occurred at about 50–60 μE∙m⁻²∙s⁻¹ in both cultures indicating ³²PO₄ uptake kinetics are light dependent at very low light levels, below or close to their I_k values. It appears that light and not phosphorus limited phytoplankton growth in Lake Superior at the time of our experiments. Antecedent solar radiation prior to our experiments coupled with complete mixing of the top 20–25 m of the Lake possibly resulted in a low light-adapted phytoplankton population. We suggest that phosphorus control need not be the correct management strategy to maintain oligotrophy in Lake Superior. Key words: phosphorus, kinetics, light, primary production, mixing, management