Biomanipulation an effective way of reducing algal blooms
For the first time, a systematic review has been undertaken of the use of biomanipulation to restore eutrophic lakes. Removing roach, bream and similar fish to increase populations of zooplankton, which in turn feed on phytoplankton, has been shown to be a successful method of improving water quality.
The extensive review has been conducted by the Mistra Council for Evidence-Based Environmental Management (EviEM).
‘By looking at a very large number of lakes, we’ve been able to show that this method is actually effective in improving water quality. We can also see under what conditions it works better, and in what types of lakes. This provides decision-makers with a good scientific evidence base,’ says Anna Gårdmark, professor of quantitative fish and fisheries ecology and a member of the team that carried out the review.
Many attempts have been made in recent decades to restore eutrophic lakes by biomanipulation, that is to say, by removing or stocking fish or other species.
The commonest form of biomanipulation is intensive fishing to reduce populations of fish that feed on zooplankton or benthic fauna. In European lakes, members of the carp family, such as roach and bream, are the main species in that category. If the numbers of such fish in a eutrophic lake are successfully reduced, the ecosystem may be modified in a way that makes the water more transparent and reduces the risk of algal blooms and oxygen deficiency, at least in the short term.
The review began with a search of the scientific literature that identified a total of 14,500 articles. After screening for relevance, 233 of these remained. Between them, they contained useful data on 128 biomanipulation interventions in 123 lakes. Of these, 85 per cent had been carried out in Europe and 15 per cent in North America. Among the Swedish lakes included in the review were Ringsjön and Finjasjön in Skåne, and Vallentunasjön north of Stockholm.
To sum up, the results indicate that removing zooplankton- or bottom-feeding fish is a useful means of improving water quality in eutrophic lakes.
• The beneficial effects of removing the fish normally lasted for at least three years. In some lakes that were monitored long-term, the effects persisted for ten years or more.
• Fish removal was particularly successful in relatively small lakes with short retention times and high phosphorus levels prior to biomanipulation.
• Removing larger numbers of fish increased the efficacy of the intervention.
• These two relationships could also reinforce each other – increasing fish removals had the most pronounced effect on chlorophyll levels in lakes that were small and/or had high phosphorus concentrations prior to manipulation.
• In some lakes, as an alternative to removing plankton- and bottom-feeding fish, predatory fish have been stocked, which then feed on and reduce populations of roach and similar fish. That method, however, was not found to significantly improve water quality.
A more detailed summary of the review is available from the EviEM website, where the full report can also be downloaded.
High-resolution photographs and illustrations can be obtained from www.eviem.se/en
See the film about the project at www.eviem.se/en
The report has also been published in the journal Environmental Evidence 2015, 4:13 (www.environmentalevidencejournal.org).
The Mistra Council for Evidence-Based Environmental Management (EviEM) works to promote environmental management built on a scientific foundation. Using systematic reviews of different issues relating to the environment, its aim is to improve the basis for environmental decision-making in Sweden.
The systematic review method, which sets out to synthesise the results of a large number of existing studies, can be used to bring clarity to issues on which consensus has previously been lacking.
EviEM is funded by the Swedish Foundation for Strategic Environmental Research (Mistra) and is based at the Royal Swedish Academy of Sciences. Its work is politically and financially independent.
Chair of the review team and professor at Linnaeus University, Kalmar.
Phone: +46 (0)706 22 37 79, +46 (0)480 44 73 11
Member of the review team and professor at the Swedish University of Agricultural Sciences, Öregrund
Phone: +46 (0)10 4784125
Project manager for the review, EviEM
Phone: +46 (0)8 673 97 55, +46 (0)72 246 38 89
Phone: +46 (0) 708 249431