Abstract
Introduction: Lake productivity is determined by the amount of nutrients and light available. While phosphorus is the main limiting nutrient in freshwater systems light availability can be reduced by several factors, while the most important one in Scandinavian lakes is the amount of dissolved organic compounds (DOC). Primary productivity can affect zooplankton biomass and diversity by bottom-up driven mechanisms while zooplankton biomass and diversity can also be affected by fish via top-down control. The resource use efficiency of zooplankton gives an estimate about the realized amount of zooplankton biomass per available nutrients (total phosphorus). Methods: To investigate the effect of light and primary productivity in lakes on zooplankton diversity, biomass and resource use efficiency we sampled 75 lakes in southern Norway and Sweden during summer 2011. Total nutrients (total phosphorus and nitrogen), particulate nutrients (particulate organic carbon and particulate organic phosphorus), dissolved nutrients (dissolved organic compounds) were measured and estimates of gross primary productivity were used to determine the productivity of the lakes. Zooplankton samples were taken to determine zooplankton biomass (as dry weight) and zooplankton diversity (species richness). Additionally, existing data about fish diversity (species richness) were used. Results: Zooplankton biomass was positively affected by total phosphorus and negatively affected by total organic compounds (TOC). Additionally, a positive relationship between the estimated gross primary productivity (EPP) and zooplankton biomass was observed. Zooplankton diversity increased with longitude but decreased with latitude while resource use efficiency (RUE) of zooplankton showed the opposite relation and decreased with longitude and increased with latitude. Resource use efficiency of zooplankton also decreased with increasing zooplankton and fish diversity. Discussion: While the amount of nutrients had a positive effect on zooplankton biomass, increasing DOC concentrations reduced the amount of light and thus had a negative effect of zooplankton. There was no significant effect of zooplankton diversity or fish diversity on zooplankton biomass, while they both gave a negative contribution to zooplankton resource use efficiency. Fish diversity and zooplankton diversity showed an increase among the longitudinal gradient and a decrease with latitude.