Impacts of chemical precipitation of phosphorus with polyaluminum chloride in two eutrophic lakes in southwest Finland

AbstractIn an attempt to improve water quality in two eutrophic shallow Finnish lakes, Kirkkojärvi and Littoistenjärvi, phosphorus precipitation with polyaluminum chloride was performed in June 2002 and May 2017, respectively. Here we compare the effects of the chemical treatment between the lakes to enhance our understanding of the mechanisms involved and to improve the predictability of similar management actions in the future. All plankton was killed in the treatment, but phytoplankton recovered in four weeks and crustacean zooplankton in two months. Because removal fishing had not been successful, the chemical dosage in Kirkkojärvi was intentionally set so high that the treatment killed all fish. In Littoistenjärvi pH was adjusted so that most fish survived. In Kirkkojärvi, the summer phosphorus (TP) and chlorophyll (Chl) concentrations in three years after the treatment dropped by 85 and 88% compared to those recorded three years before the treatment. Cyanobacterial biomass declined by 88%, only occasional blooms appearing in three out of twenty years. The average TP and Chl of the post-treatment period 2006–2020 indicated substantial improvement in the ecological state from “bad” to “moderate” rating of the EU Water Framework Directive (WFD). In Littoistenjärvi, the corresponding declines due to the Al treatment were 72% in TP and 87% in Chl concentration, and 92% in cyanobacterial biomass. Longevity of treatment effects was estimated using the upper boundaries of the WFD quality classes as the target values. Water quality changes followed the internal loading of TP, affected by temperature and pH.Keywords: total phosphoruschlorophyll aeutrophicationlake managementshallow lakesDisclaimerAs a service to authors and researchers we are providing this version of an accepted manuscript (AM). Copyediting, typesetting, and review of the resulting proofs will be undertaken on this manuscript before final publication of the Version of Record (VoR). During production and pre-press, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal relate to these versions also. Acknowledgements:Thanks are due to the numerous people involved in the monitoring and management of the study lakes. The actual chemical treatments became possible through the dedicated efforts of Jukka Heikkilä (Littoistenjärvi) and Eeva Ståhle (Kirkkojärvi). Funding was provided by the local municipalities, the Ministry of the Environment, and the Academy of Finland.Disclosure statement:The authors report there are no competing interests to declare.Data availability statement:Water chemistry, phytoplankton and fish data are available through Finnish Environment Institute’s open data service. Other data are available from the authors on request.