Erik Jeppesen

Effects of fish predation on consumers tend to be particularly strong in oligotrophic Arctic and sub-Arctic lakes. However, it remains unclear whether the fish influence the trophic structure and dynamics of naturally eutrophic lakes in these cold environments with simple food web structures. To study this, we conducted a 3-month in situ-controlled experiment in sub-Arctic Lake MA 1/2 vatn, Iceland. We used the planktivorous fish three-spined sticklebacks (Gasterosteus aculeatus) as the main top predator. The cladocerans Eurycercus lamellatus and Acroperus harpae were significantly associated with fishless enclosures, whereas the rotifers Polyarthra sp. and Filinia terminalis were significantly associated with the fish enclosures. Fish predation led to a significant increase in phytoplankton biomass and a reduction in the zooplankton:phytoplankton biomass ratio, the mean zooplankton length and cladoceran mean biomass. Fish effects might have been stronger if a bloom of Anabaena in August had not overridden potential cascading effects. We argue that both top-down and bottom-up forces are important for structuring the communities in the lake. Our results suggest that Arctic and sub-Arctic lakes may undergo important changes in trophic dynamics if they get warmer and more nutrient rich as expected with the global climate change.