Eutrophication and pelagic-benthic coupling in a deep lake: What is different from shallow lakes?

To examine the long-term effect of eutrophication on pelagic-benthic coupling in deep lake food-webs, we analyzed archive data on annual variation in pelagic and profundal productivity since 1960 in Lake Biwa with a maximum depth of 104m, Japan. We also traced trophic dynamics of a predator fish, Gymnogobius isaza, which exhibits diurnal vertical migration for foraging, using nitrogen isotope analysis of its specimens collected from 1962. Settling particulate organic matter as profundal subsidies increased due to eutrophication, resulting in an increased availability of a  herbivorous-detritivorous amphipod, Jesogammarus annandalei, and foraging shift by the predator fish from pelagic to profundal preys (i.e., from zooplankton to the amphipod) in the early 1970s. Consequently, top-down control operated in the profundal zone, depressing the amphipod population extremely. Such a cascading effect accelerated organic loadings and subsequently oxygen deficiency in the profundal layer, which might have caused population collapse of the predator fish. Free from the predation, the amphipod increased its density again, with marked annual fluctuations. Such a food-web dynamics is apparently consistent with recent models predicting that systems will be unstable under eutrophic conditions. However, we stress the pelagic-profundal coupling in deep lakes contrasts with pelagic-littoral coupling in shallow lakes where eutrophication retards the benthic productivity through shading effects and consequently benthic fish shift their feeding habit from benthic to pelagic preys. It is important to view how ecosystem response to the eutrophication differs between shallow and deep lakes, which will help us understand mechanisms determining food-web dynamics and stability in lake ecosystems.