Fish manipulation as a lake restoration tool in shallow,eutrophic, temperate lakes 2: threshold levels,long-term stability and conclusions |
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Authors: | Jeppesen E Jensen J P Kristensen P Søndergaard M Mortensen E Sortkjær O Olrik K |
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Institution: | 1.National Environmental Research Institute, Division of Freshwater Ecology, 52 Lysbrogade, DK-8600, Silkeborg, Denmark ;2.Environmental Biology Laboratory, 5 Baunebjergvej, DK-3050, Humleb?k, Denmark ; |
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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 1 000μg P l?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 250–300μg P l?1, but still abundant in some lakes<3 ha at 650μg P l?1. Lakes with high cover of submerged macrophytes showed higher transparencies than lakes with low cover aboveca. 50μg P l?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 100μg P l?1. Below this threshold level the proportion of planktivores decreased markedly toca. 50 per cent at 22μg P l?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 80–150μg P l?1. Furthermore, fish manipulation as a restoration tool seems most efficient in shallow lakes. |
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Keywords: | biomanipulation shallow eutrophic lakes threshold levels long-term stability submerged macrophytes fish phytoplankton empirical relationships |
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