Lake food web structure in Teici Nature reserve, Latvia: fish presence shapes functioning of pristine bog lake food webs

Studies on the effects of fish presence on lake ecosystems are widespread but only a few have been conducted in pristine aquatic environments. We employed Ecopath model for assessing food web structure in two fish-inhabited and one fishless lake in a pristine bog area. We hypothesized that: (a) fish absence will raise trophic positions of macroinvertebrate predators; (b) fish predation will lead to higher overall predation rates on zooplankton; (c) fish predation on large bodied zooplankton will result in top-down cascading effect, increasing phytoplankton biomasses. We found that fish have direct and indirect effects on zoobenthic communities. Chironomid biomass was greater and predatory macroinvertebrate groups had a higher trophic level in the fishless lake than in fish-inhabited lakes. Consumption rates of the benthic consumer fraction were greater than that of the planktonic fraction in the fishless lake; the opposite was found in the two lakes with fish. No effects of fish presence on zooplankton were found and we explain this partly by the low water transparency masking the impact of fish. Terrestrial insects constituted a crucial part of adult fish diet and we conclude that terrestrial secondary production contributes to the trophic support of fish communities in pristine bog lakes.