Effects of fish stocking and removal on plankton communities and trophic status in deep alpine-type lakes of the Five Lakes Valley (Tatra Mountains)

Most alpine-type lakes in the Tatra Mountains were stocked with salmonid fish during the 19th and 20th centuries, triggering profound ecological shifts. In the Five Lakes Valley, trout introductions into Przedni Staw Polski, Czarny Staw Polski and Wielki Staw Polski had contrasting effects on pelagic ecosystems. In the smaller and shallower Przedni Staw Polski and Czarny Staw Polski, fish introduction was associated with the loss of large-bodied zooplankton, increased trophic status and a rapid decline in water transparency-from 13-18 m to 5-7 m. Between 2021 and 2023, partial fish removal in Przedni Staw Polski resulted in a marked increase in water transparency (from 7 to 17 m), coinciding with reduced surface phytoplankton biomass. This improvement likely reflected decreased phosphorus input from fish excretion rather than enhanced zooplankton grazing. Large-bodied cladocerans did not recolonize the lake, although small-bodied taxa increased in abundance. In contrast, Czarny Staw Polski-where fish remain abundant-showed no signs of recovery: water transparency declined further and zooplankton community structure remained unchanged. Unlike the other stocked lakes, Wielki Staw Polski-the largest and deepest-retains high transparency and large-bodied zooplankton, likely due to low fish density or availability of predator-free refugia. While partial fish control in Przedni Staw Polski yielded rapid benefits, restoring natural ecosystem functioning in these fragile alpine ecosystems will require complete fish eradication.