Lake restoration: successes, failures and long-term effects

Published source details Søndergaard M., Jeppesen E., Lauridsen T.L., Skov C., van Nes E.H., Roijackers R., Lammens E. & Portielje R. (2007) Lake restoration: successes, failures and long-term effects. Journal of Applied Ecology, 44, 1095-1105.
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Summary

Eutrophication is a serious threat to many European lakes and many approaches have been used over the past 20-30 years in attempts to improve water quality. The aim of this study was to give an overview of the different types of restoration projects carried out in Denmark and the Netherlands, where restoration techniques have been widely used to improve lake water quality since the 1980s.

Results from these lake restoration initiatives are diverse and long-term effects have not been well described. Biomanipulation (removal of zooplanktivorous and benthivorous fish) has been the primary method and results from such initiatives was the main focus.

Summer averages of total phosphorus, total nitrogen, chlorophyll a, Secchi depth, and sometimes suspended solids, were used as key variables to evaluate lake restoration projects in Denmark and the Netherlands . For some of the Danish lakes, data on fish, macrophytes and zooplankton were also available. Data were evaluated for more than 70 restoration projects conducted mainly in shallow eutrophic lakes.

In more than half of the lake biomanipulation projects, Secchi depth increased and chlorophyll a decreased to less than 50% within the first few years, indicating a clearer water column and less planktonic algae. In some of the shallow lakes, total phosphorus and nitrogen levels decreased considerably, indicating an increased retention or loss by denitrification. The strongest effects seemed to be obtained 4-6 years after commencement of fish removal.

Due to lack of data, it was only possible to describe the longer-term effect of restoration initiatives for a few lakes; data from biomanipulated lakes indicate a return to a turbid state within 10 years or less in most cases. One of reasons for the lack of long-term effects may be internal phosphorus loading (from accumulated sediment) leading to renewed eutrophication of the water.

Conclusions: Lake restoration, and in particular fish removal in shallow eutrophic lakes, has been widely used in Denmark and the Netherlands. It has had marked positive short-term effects on improving lake water quality in many lakes. Longer-term effects (> 8-10 years) are less obvious and a return to turbid conditions is often seen unless fish removal is repeated. Insufficient external nutrient loading reduction, internal phosphorus loading and absence of aquatic macrophytes are considered the most probable causes for this relapse.

Note: The compilation and addition of this summary was funded by the Journal of Applied Ecology (BES). If using or referring to this published study, please read and quote the original paper, this can be viewed at: http://www.blackwell-synergy.com/doi/full/10.1111/j.1365-2664.2007.01363.x