Carp exclusion, food-web interactions, and the restoration of Cootes Paradise Marsh

Carp were excluded from Cootes Paradise Marsh (Lake Ontario) in 1997 in order to improve water clarity and promote submerged plant growth. On average, turbidity at open water and vegetated areas was reduced by 40 and 60 percent, respectively, following carp exclusion. However, responses by plants and other trophic levels have been both spatially and temporally variable due in part to annual variation in environmental conditions and fish-zooplankton interactions. In 1997, an unusually cool spring delayed the migration of spawning fish, including a usually abundant planktivore population, into the marsh. This had three main effects: 1) large Daphnia were released from predation in early summer and reached unprecedented numbers (530 Daphnia/L) in open water areas, 2) despite the lack of vegetated marsh habitat, larval fish production was high due to reduced predation and abundant zooplankton prey, and 3) zooplankton grazing initiated a spring clear-water phase which, together with carp exclusion, promoted submerged plant growth in shallow areas previously devoid of vegetation. These newly vegetated areas showed the greatest improvements in clarity and macrophyte growth in the first 2 years following exclusion. Currently, however, the future of the biomanipulation remains uncertain, due in part to natural climatic confounding factors, and further remedial actions will be required before this wetland represents a stable, clear-water, macrophyte dominated state.