Top‐down or Bottom‐up Effects by Fish: Issues of Concern in Biomanipulation of Lakes

Abstract

A large‐scale biomanipulation trial was carried out on Lake Vesijärvi in Finland during 1989–1993. Following the mass removal of coarse fish the biomass of cyanobacteria collapsed from 1.4 g/m−3 to below 0.4 g/m−3, while total phosphorus concentration declined from 45 μg/L to 30 μg/L. No relevant changes in zooplankton communities were observed. The results suggest that the success of food web manipulation as a tool for lake restoration is not necessarily dependent on the grazing rate of zooplankton. The effects of reduced fish‐mediated internal loading and recycling of nutrients are in many cases stronger than those of reduced planktivory. Alternative stable states of water quality may also exist in lakes not covered by macrophytes, owing to the changes in the behavior of fish stocks. Year‐to‐year variation in the littoral zone may cause large oscillations in lake ecosystems—for example, through the recruitment of fish. In addition, the nutrients translocated by fish from the littoral zone may affect the nutrient dynamics of the pelagial plankton community. In terms of phytoplankton species composition and the ratio of phosphorus to chlorophyll a, the water quality in Lake Vesijärvi has improved in a stepwise fashion within the last 10 years. This is probably due to the fact that the five‐year mass removal of fish in Enonselkä fulfilled the requirement of sustained management of fish stocks in order to maintain nonequilibrial conditions between alternate stable states. The prediction of the water quality development is obscured, however, by spatial and temporal within‐lake variation, which sets high requirements for sampling programs.

Long-term management of Pyhäjärvi (southwest Finland): eutrophication, restoration – recovery?

“…Also, increasing water temperatures may accentuate oxygen deficits in near-bottom waters through longer stratification periods and faster decomposition and thus accelerate eutrophication (Elo et al 1998). Higher temperatures also have unfavourable effects on water quality through food web processes, such as impairing the ability of zooplankton to control phytoplankton (Vakkilainen et al 2004, Malve et al 2007) and enhancing the recruitment of warm-water fish species known to have deleterious water quality effects (such as roach; Horppila et al 1998). …”

3D-modelling of the thermal circumstances of a lake under artificial aeration

“…1). Enonselkä has a surface area of 26 km 2 and a mean depth 6.8 m. Originally, Enonselkä was a clear-water basin, but in the 1980s, the water quality was degraded by massive cyanobacteria blooms (Horppila et al 1998). In the 1990s, after a largescale biomanipulation, water quality improved and harmful blooms of cyanobacteria decreased, but in the 2000s, the blooms appeared again (Keto et al 2005).…”

Implications for the use of sedimentary invertebrate communities to infer past presence of fish

“…The ecological role of fish in lake ecosystems is invaluable, because they affect the abundance and diversity of their prey organisms, such as planktonic and benthic Cladocera and Diptera larvae (Kornijóv et al, 2005;Vašek et al, 2006;Davidson et al, 2007) through topdown control (Vanni et al, 1997). Fish are also significant due to their participation in internal loading and recycling of nutrients that may affect planktonic organisms (Horppila et al, 1998). The major structuring effect of fish predation on their prey is due to size-selectivity that leads to the dominance of small species and removal of large ones, thus changing community structures (Brooks and Dodson, 1965).…”