Brunifiering av akvatiska ekosystem: En experimentell studie som undersöker tröskelvärde för predator- och bytesförhållande vid en gradient från klart till brunt vatten

Sammanfattning: Browning of water has increased over the last couple of decades on the northern hemisphere. Research show that possible drivers for this change are land use change, climate change, increased precipitation and increased temperatures. Browning of water can affect important interactions in the aquatic ecosystem which in turn can affect the whole structure of the ecosystem. Furthermore the loss of a top predator can result in a similar situation as eutrophication and brownification can thus enhance the effect of eutrophication in the future. The aim of this study was to investigate how predator-prey interactions between pike (Esox lucius) and roach (Rutilus rutilus) was affected by a gradient from clear to brown water; and specifically, to identify a threshold value, where the foraging of pike was clearly affected by increased browning of the water. To do this, I preformed a factorial laboratory feeding experiment were young-of-the-year pike were allowed to feed on juvenile roach at two different prey densities (5 or 10 roach individuals in 700L tanks) at five brown water treatments (from clear to brown) and quantifying the number of prey consumed over a period of 24 hours in each treatment. This study found no threshold values, but tendency to a reduced amount of eaten roach in the clearest and darkest experimental tanks at low prey densities. Larger pike ate more individuals and a larger prey density resulted in a larger number of eaten prey. The pike seemed to reach a saturation level at around five consumed roach a day. The results highlights that predator-prey relationships are not only affected by impaired visibility, but depend on individual differences. Identifying threshold values can provide stakeholders with important information that may help develop more efficient strategies to mitigate the ecosystem consequences of climate change.