Fish manipulation as a lake restoration tool in shallow, eutrophic, temperate lakes 2: threshold levels, long-term stability and conclusions

Abstract

In order to evaluate short-term and long-term effects of fish manipulation in shallow, eutrophic lakes, empirical studies on relationships between lake water concentration of total phosphorus (P) and the occurrence of phytoplankton, submerged macrophytes and fish in Danish lakes are combined with results from three whole-lake fish manipulation experiments. After removal of less than 80 per cent of the planktivorous fish stock a short-term trophic cascade was obtained in the nutrient regimes, where large cyanobacteria were not strongly dominant and persistent. In shallow Danish lakes cyanobacteria were the most often dominating phytoplankton class in the P-range between 200 and 1000 pg P 1⁻¹. Long-term effects are suggested to be closely related to the ability of the lake to establish a permanent and wide distribution of submerged macrophytes and to create self-perpetuating increases in the ratio of piscivorous to planktivorous fish. The maximum depth at which submerged macrophytes occurred, decreased exponentially with increasing P concentration. Submerged macrophytes were absent in lakes > 10 ha and with P levels above 250−300 μg P 1⁻¹, but still abundant in some lakes < 3 ha at 650 μg P 1⁻¹. Lakes with high cover of submerged macrophytes showed higher transparencies than lakes with low cover above ca. 50 μg P 1⁻¹. These results support the alternative stable state hypothesis (clear or turbid water stages). Planktivorous fish > 10 cm numerically contributed more than 80 per cent of the total planktivorous and piscivorous fish (> 10 cm) in the pelagical of lakes with concentrations above 100 μg P 1⁻¹. Below this threshold level the proportion of planktivores decreased markedly to ca. 50 per cent at 22 μg P 1⁻¹. The extent of the shift in depth colonization of submerged macrophytes and fish stock composition in the three whole-lake fish manipulations follows closely the predictions from the relationships derived from the empirical study. We conclude that a long-term effect of a reduction in the density of planktivorous fish can be expected only when the external phosphorus loading is reduced to below 0.5−2.0 g m ⁻² y ⁻¹. This loading is equivalent to an in-lake summer concentration below 80−150 μg P 1⁻¹. Furthermore, fish manipulation as a restoration tool seems most efficient in shallow lakes.