Faithfull CL, Huss M, Vrede T, Bergström AK
bottom up, top down, Carbon assimilation, freshwater, glucose, mesocosm, 18 m3, Sweden
Human impacts such as eutrophication, overexploitation and climate change currently threaten future global food and drinking water supplies. Consequently, it is important that we understand how anthropogenic resource (bottom – up) and consumer (top – down) manipulations aff ect aquatic food web structure and production. Future climate changes arepredicted to increase the inputs of terrestrial dissolved organic carbon to lakes. Th ese carbon subsidies can either increase or decrease total basal production in aquatic food webs, depending on bacterial competition with phytoplankton for nutrients. This study examines the eff ects of carbon subsidies (bottom – up) on a pelagic community exposed to different levels of top – down predation. We conducted a large scale mesocosm experiment in an oligotrophic clear water lake in northern Sweden, using a natural plankton community exposed to three levels of glucose addition (0, 420 and 2100 ? g C l –1 total added glucose) and three levels of young-of-the-year perch Perca fl uviatilis density (0, 0.56 and 2 individuals m –3 ). Bacterioplankton production doubled with glucose addition, but phytoplankton production was unaff ected, in contrast to previous studies that have manipulated carbon, nutrients or light simultaneously. This suggests that carbon addition alone is not sufficient to reduce autotrophic production, at least in an oligotrophic lake dominated by mixotrophic phytoplankton. Larval perch grazing did not produce a classical trophic cascade, but substantially altered the species composition of crustacean zooplankton and ciliate trophic levels. Glucose addition increased the biomass of rotifers, thus potentially increasing energy transfer through the heterotrophic pathway, but only when fi sh were absent. Th is study illustrates that changes in community structure due to selective feeding by top-predators can determine the infl uence of bottom – up carbon subsidies