Response of phytoplankton, zooplankton, and fish to re-oligotrophication: An 11 year study of 23 Danish lakes

Abstract

The response of the biological community to total phosphorus loading reductions was studied by sampling fortnightly during the summer for 11 years in 18 lakes with significant in-lake total phosphorus reductions, as well as in five reference lakes. Fish manipulation was conducted in four of the total phosphorus-reduced lakes during the study period. For the majority of lakes, a fast response to total phosphorus reduction (but not necessarily to TP loading reduction due to internal loading) was found for phytoplankton (as measured by chlorophyll a) and fish, while less pronounced changes were observed for the zooplankton. In contrast to zooplankton biomass, phytoplankton, and fish biomass generally declined, leading to an overall higher ratio of zooplankton to phytoplankton, which suggests enhanced grazing pressure on algae. A response at the community level also was found. Generally, the share of non-heterocystous cyanobacteria declined substantially, while the contribution of heterocystous cyanobacteria in particular, and in some lakes also dinophytes, cryptophytes and chlorophytes, increased. The biomass of planktivorous fish declined and the share of potential piscivores increased. Accordingly, in some, but not all, lakes the share of Daphnia spp. among cladocerans and the body weight of Daphnia spp. and cladocerans increased, suggesting a reduced predation pressure on zooplankton. The biological response to TP loading reduction was not restricted to low-total phosphorus lakes, but was also found in lakes in the total phosphorus range 0.2–0.4 mg P l−1. However, only minor changes to a total phosphorus reduction from 1.2 to 0.4 mg P l−1 were recorded in the most hypertrophic lake. In the biomanipulated lakes, an overall stronger response was found, now also including zooplankton. In general, no changes were found in the reference lakes, suggesting that the changes in the total phosphorus-reduced lakes were not attributable to variations in climate during the study period. We use the results for discussion of various recently proposed hypotheses of lake response to re-oligotrophication.