Abstract
Restoration of eutrophic lakes is a significant challenge in the management of freshwater globally. Despite considerable efforts, restoration by nutrient reduction has often failed because in-lake chemical and biological mechanisms prevent or delay recovery. Eutrophic Lake Hayes in the Central Otago region of New Zealand’s South Island has experienced ongoing nuisance algal blooms and poor water clarity despite some recent reductions in nutrient loading. This study aimed to determine how thermal stratification, chemical dynamics and seasonal changes in the plankton community interact to undermine restoration efforts by nutrient reduction, and to investigate the relative importance of bottom-up and top-down controls in order to test the hypothesis that classical pelagic biomanipulation could be used to achieve improved water clarity. Chemical and physical variables along with phytoplankton and zooplankton communities were monitored and compared over two successive stratified periods with contrasting water clarity (2015-16 and 2016-17). Ontogenetic dietary shifts of European perch (Perca fluviatilis) were examined using stable isotope analysis.
Stratification dynamics, particularly an extended period of anoxia, early-season growth of grazing resistant phytoplankton taxa and total nutrient concentrations were key drivers of high phytoplankton biomass and poor water clarity observed in 2015-16. Zooplankton community composition was significantly different between years and was dominated by Bosmina during the summer of 2015-16 when there was high phytoplankton biomass and poor water clarity. In contrast Daphnia were dominant in 2016-17 and a significant improvement in water clarity was observed. Fisheries surveys found the fish community was dominated by young-of-the-year perch which were predominantly planktivorous and the perch recruitment period was associated with a marked decline in Daphnia abundance and increased phytoplankton abundance in early summer.
Bottom-up controls were found to be key of drivers of water clarity in Lake Hayes; however, the short-term involvement of top-down control was evidenced by observed differences in the plankton community and variable perch feeding in years with contrasting summertime water clarity. While continued nutrient reduction will be critical to the long-term recovery of the lake, careful application of food web biomanipulation to enhance Daphnia grazing on phytoplankton through sustained reduction of the perch population could be expected to increase the duration and frequency of clear water phases in Lake Hayes, enhancing recovery from eutrophication.