Plankton communities in a changing world - responses to temperature, brownification and lake restoration

Sammanfattning: The effects of increasing temperatures, in light of climate change, have been a well-studied topic during the past decades. However, aquatic ecosystems are also faced with additional challenges such as increasing water colour, known as “brownification”. Although we know that temperature and water colour will increase simultaneously this has been given little attention in ecological research. In this thesis I investigated how an increasing temperature and increasing water colour affect planktonic communities. More specifically I studied the effects of these drivers on zooplankton spring dynamics and cyanobacterial bloom formation. My studies show that zooplankton will start to establish earlier in spring in response to an increasing temperature but that the effects of water colour were small. However, cladoceran zooplankton seemed to benefit from the combination of increasing water colour and temperature. An increasing temperature did not lead to larger algal blooms in our experiment. However, the combination of increasing temperature and water colour showed to favour one cyanobacterial species, namely Microcystis botrys. This showed to have a large impact on the amount of cyanobacterial toxins in the water (microcystins), which increased by 300% in the combined elevated temperature and brownification treatment in comparison to the control scenario. This clearly highlights the importance of studying drivers together rather than in isolation if we expect then to occur simultaneously in nature. Also, as it was the changes in one species that explained the majority of the changes in toxicity this also stress the need of high taxonomic resolution. As climate change is expected to increase the occurrence of cyanobacterial blooms I also investigated how a biomanipulation can aid in controlling cyanobacterial blooms. This was done by evaluating an ongoing biomanipulation in Lake Ringsjön, southern Sweden. The results from this study show that biomanipulation can help in reducing algal blooms but that this effect was strongest during early summer (June). Following biomanipulation, the relative proportion of large, efficient grazers (Daphnia spp.) has increased during early summer. At the same time, cyanobacterial biomasses have decreased and the amount of cyanobacterial toxins (microcystins) have gone down. Furthermore, to evaluate if the decrease in cyanobacterial biomass could be the result of increased grazing by Daphnia we performed a field grazing experiment. This showed that a zooplankton community dominated by Daphnia were in fact more efficient in controlling cyanobacterial biomass compared to a community dominated by small cladocerans and copepods. In conclusion my results show that we can expect significant effects of increasing temperatures and water colour on both zooplankton spring dynamics and cyanobacterial blooms. However, there are methods, developed in light of eutrophication, which can help in controlling the extent of cyanobacterial blooms.