Changes in the crustacean zooplankton communities of Lake Constance after the invasion of stickleback, quagga mussel and filamentous cyanobacteria

The crustacean zooplankton community of Lake Constance (Upper Lake and the Lower Lake basins Rheinsee, Zellersee and Gnadensee) was studied before and after the invasion of three-spined stickleback (Gaster-osteus aculeatus), quagga mussel (Dreissena rostriformis bugensis) and the appearance of filamentous cyanobacteria (Planktothrix rubescens) blooms. The three species are known to influence zooplankton community structures. A shift from larger Daphnia galeata / hyalina to the smaller species Daphnia cucullata was observed in all lake basins. This shift was pronounced in the Upper Lake Constance, where D. cucullata was only present occasionally before the invasions. A further shift was observed in copepods: Large Cyclops spp. decreased in abundance in the Upper Lake and the Rheinsee and calanoid copepods of the genus Eudiaptomus increased in abundance in Zellersee and Gnadensee. The small copepod Thermocyclops oithonoides invaded the Upper Lake in 2016, where it had not been documented before. Average cladoceran biomass decreased from about 6 - 8 & mu;g ind.-1 to 4 - 5 & mu;g ind. -1 in several years after the recent invasions, except in Gnadensee where it remained constant. Total crustacean zooplankton biomass did not change during the 13-year study period. The impacts of stickleback, quagga mussel and Plankto-thrix cannot be clearly differentiated, yet, we attribute changes in the community structure of crustacean plankton to predation by stickleback since the quagga mussel is known to affect smaller zooplankton as opposed to larger zooplankton species. Moreover, the abundance of quagga mussels was still low at the time when the changes in zooplankton community were observed. Planktothrix rubescens blooms were present for a short period in 2016/17 which suggests a minimal contribution of Planktothrix to the observed changes.