Spring dynamics in shallow lakes: the role of plankton, fish and macrophytes in a changing climate

Abstract: Shallow lakes such as Lake Krankesjön are characterized by their extended littoral zones, usually covered by submersed and emergent vegetation. In paper 1, I investigated the relationship between turbidity and macrophyte growth (Chara spp.) during spring and summer in Lake Krankesjön and found that the colonization of the lake bed by macrophytes was dependent on turbidity on a non-linear way. Also, within-year macrophyte growth was dependent on an interplay between temperature and turbidity. Light availability for macrophytes in spring and summer is to a large extent governed by phytoplankton abundance, which is in turn controlled by the grazing intensity of herbivorous zooplankton. In paper 2, 3 and 4 I investigated how zooplankton abundance was affected by adult fish predation in the pelagic and by 0+ fish predation at different algal concentrations, in different habitats and during different stages of fish ontogeny in Lake Krankesjön and an enclosure study. My results indicate that adult fish reduced large zooplankton in the open water during spring and in turn decreased grazing pressure of zooplankton on algae. 0+ fish then reduced smaller size groups in the pelagic, as well as large zooplankton in the littoral, vegetated habitats that are difficult to access for adult fish. Only small cladocerans in the dense Chara vegetation could maintain stable populations throughout the summer and in turn reduce algal concentrations. Also, 0+ fish predation effects on zooplankton in the enclosures depended on the algal resource situation of zooplankton, indicating, that the timing of fish hatching and algal decline will determine 0+ fish predation effects in the lake. In paper 5 I explored possible effects of a warmer climate and increased water color on the timing of plankton spring succession. I found that increased temperature led to an earlier peak of phyto- and zooplankton, and the relative timing of peak densities of different zooplankton groups changed. Also, increased temperature and the combination of increased water color and temperature were beneficial for densities of cladocerans and calanoid copepods, while increased water color decreased chlorophyll-a concentrations. Future browning and warming might therefore lead to a strong top down control of algae by zooplankton.