Nutrients and biota in a lake system before and after restoration; a data analysis of the Swedish eutrophication case study Växjösjön

Sammanfattning: Eutrophication has proven to be a fundamental ecological problem for lakes and other bodies of water all around the world. The process of eutrophication can be defined as a lake containing increasing concentrations of nutrients from external and/or internal input over time. The increase of nutrients in the lake has several consequences for the lake ecosystem, such as the increase in algal blooms (sometimes containing toxic and harmful cyanobacteria) and the decrease of macrophytes. One nutrient that plays a key role in the eutrophication process is phosphorus. To restore eutrophic waters, the external and internal input of phosphorus needs to be reduced. External input can be decreased by reducing the run-off from industrial areas or agriculture. Internal input can be reduced by disrupting the in-lake phosphorus loading processes, which are connected heavily to the lake sediment. The internal phosphorus loading processes can be caused by several different processes. One is the mineralization of organic biomass on the sediment which releases phosphorus into the water, another is the release of previously iron-bound phosphorus from the sediment. Different treatments can be implemented in a lake system to disrupt these internal processes of phosphorus loading and consequently restore the water quality of the lake. Such treatments also influence the biota of the lake and the ecosystem services, because of their effect on water quality. Biomanipulation treatments and aluminum treatments were implemented in lake Växjösjön in Sweden to restore the lake to a more natural and balanced state. Both treatments were effective in reducing the eutrophic conditions of the lake, improving water quality, biota, and the ecosystem services. Local human populations benefit from these improvements, for example by receiving increased revenue from lake recreation. More research is however needed to discern the long-term effects of the treatments in the Växjö municipality, thereby aiding local government and policy makers in their future decisions regarding restoration.