Carbon, nitrogen and phosphorus content of seston and zooplankton in tropical Lake Malawi: Implications for zooplankton nutrient cycling

Abstract

Differences in chemical composition among zooplankton and their food sources can have important consequences for nutrient cycling in lakes. To date, studies of the interaction between plankton stoichiometry and nutrient cycling have focused primarily on temperate lakes. Here we explore the implications of plankton stoichiometry for nutrient cycling in tropical Lake Malawi. Three seston size fractions (picoplankton, nanoplankton and net plankton) displayed seasonal variations in C:P, C:N and N:P ratios. On average, picoplankton displayed lower C:P, C:N, and N:P ratios than nanoplankton and net plankton. Average C:N and C:P ratios for all seston size classes were much higher than Redfield ratios (C:N:P = 106:16:1). The elemental composition of individual species of crustacean zooplankton showed little temporal variation. However, there were large inter-species differences in P and N content, which can be expected to result in differences in N:P recycling ratios. The zooplankton community of Lake Malawi is dominated by copepods (>80% of total zooplankton biomass), which have a relatively low P content and high N:P ratio, resulting in a relatively low N:P recycling ratio. This might be expected to promote N limitation of phytoplankton and dominance of N-fixing cyanobacteria, but in some seasons the effects of zooplankton nutrient cycling appears to be masked by nutrient inputs from rivers, the atmosphere and vertical mixing.