Zooplankton response to organic carbon level in lakes of differing trophic states
Water eutrophication is associated with an increase in the organic carbon content (both particulate and dissolved forms), which may affect the functioning of the zooplankton community. Mesotrophic and eutrophic lakes in the Masurian Lake District (Poland) were selected to evaluate the relationship between the organic carbon level and the zooplankton community. The lakes differed significantly in most environmental variables. RDA analysis was performed to evaluate the impact of environmental variables on zooplankton. The variables that significantly explained the variance in the zooplankton community abundance (Monte Carlo permutation test) included dissolved and particulate organic carbon, Secchi disc visibility, soluble reactive phosphorus and total nitrogen. The response of zooplankton to an increasing level of organic carbon is functional rather than quantitative. In the mesotrophic system, the results of the redundancy analysis indicated relatively strong positive relationships between dissolved organic carbon and zooplankton biomass, and negative correlations between chlorophyll a and zooplankton biomass. The above suggests that indirect organic carbon utilization by zooplankton could partly compensate for the poor feeding conditions of planktonic animals (decreased phytoplankton availability). In the eutrophic lake, elevated organic carbon levels are partly limited by zooplankton, which is suggested by the positive relationship between particulate organic matter and the total zooplankton biomass (RDA results). The positive relationship between the biomass of copepods and organic carbon in particulate and dissolved forms implies that copepods benefit from the increased heterotrophic carbon flow that is activated in the eutrophic lake.