Phytoplankton nutrient status and mean water column irradiance in Lakes Malawi and Superior

Abstract

Abstract Phytoplankton growth in Lake Malawi was moderately nitrogen and phosphorus-deficient according to indicators of phytoplankton nutrient status (particulate C:N, C:P, N:P composition ratios, nitrogen and phosphorus debt assays) and occasionally light-deficient during the period of deepest mixing (July and August). Phytoplankton in Lake Superior was light-deficient during most of the year because of the deeply mixed water column. However, during the stratified period when the mean water column irradiance increased, phytoplankton in Lake Superior became severely phosphorus-deficient according to the same nutrient status indicators used in Lake Malawi as well as alkaline phosphatase activity. Specific rates of carbon uptake normalized to particulate carbon, calculated from photosynthesis at optimum light, were on average three times greater in Lake Malawi than in Lake Superior. We calculated that nitrogen and phosphorus inputs from rivers and precipitation supplied < 15% of the demand for these elements due to photosynthesis for both Lake Malawi and Lake Superior and could not explain the observed difference in phytoplankton nutrient status or specific rate of carbon uptake normalized to particulate carbon. The ratio of nitrogen to phosphorus supplied in Lake Malawi is lower and closer to Redfield ratios than that in Lake Superior. We speculate that the more balanced supply ratio of these nutrients, combined with higher rates of internal regeneration in the warmer deeper mixed layer of Lake Malawi, result in phytoplankton that is less nutrient-deficient and has higher specific rates of carbon uptake normalized to particulate carbon than Lake Superior.