In-lake measures for phosphorus control: The most feasible and cost- effective solution for long-term management of water quality in urban lakes Brian J. Huser a, * , Martyn Futter a , Jeff T. Lee b , Mike Perniel c a Swedish University of Agricultural Sciences, Department of Aquatic Sciences and Assessment, Box 7050, 75007 Uppsala, Sweden b Barr Engineering Company, 325 South Lake Avenue, Suite 700, Duluth, MN, USA c Minneapolis Park and Recreation Board, 3800 Bryant Ave South, Minneapolis, MN, USA article info Article history: Received 11 May 2015 Received in revised form 14 July 2015 Accepted 21 July 2015 Available online xxx Keywords: Aluminum sulfate Sediment Lake restoration Best management practices Alum abstract Both in-lake and catchment measures designed to reduce phosphorus (P) loading were implemented as part of a 12.3 million USD restoration project for the Minneapolis Chain of lakes in Minnesota (USA). Treatment wetlands, ‘in-pipe’ measures, and in-lake aluminum sulfate (alum) treatment were applied to restore water quality in the four urban lakes. Different alum dosing methods led to between 4 and 20þ (modeled) years of water quality improvements in these lakes after treatment and only one of the four lakes continues to meet water quality goals approximately 25 years after the project started. Due to limited space and poor performance, reduction of total external loads was low (1e13%) for three lakes. Changes to internal P sediment release rates after application of alum correlated well with epilimnetic total P (TP) concentrations in these lakes, indicating that improvements in water quality were mainly driven by reduced internal loading via in-lake measures. Substantial reductions to external P loading were only achieved at Cedar Lake (49%) via conversion of an existing natural area to a treatment wetland, but even Cedar Lake is no longer meeting management goals. When expressed in terms of dollars spent per unit P removed, in lake alum treatment was on average 50 times more effective than in-catchment measures. The results of this study indicate that substantial external nutrient reductions may not be adequate to sustainably maintain water quality in urban lakes and that continued in-lake management of P accumulated in lake sediment will not only be necessary, but will also be more cost efficient relative to in-catchment measures. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Lakes and parks in urban areas have high societal value as they provide recreational opportunities and aesthetic benefits to large numbers of people. However, high population density and urban encroachment impose significant challenges to water quality management. Many lakes in urban areas experience algal blooms and reduced water clarity associated with cultural eutrophication due to excess nutrient inputs (Welch, 1992; Carpenter et al., 1998). If not controlled, eutrophication can cause a breakdown in resil- ience and seriously impair the delivery of ecosystem services by urban lakes. While excessive inputs of nitrogen, organic matter, and pathogens can impair water quality, excessive inputs of phosphorus (P) are the main cause of cultural eutrophication in lakes (Schindler, 1974). Inputs of P can come from external sources including the local catchment and upstream lakes or internal sources, primarily legacy P in lake sediments (Pilgrim et al., 2007; Søndergaard et al., 2013). Although there have been some successful cases of lake restoration where only external P loads were reduced, the continued release of excess legacy P from lake sediment can continue to fuel algal blooms after external nutrient reduction (Welch and Jacoby, 2001) and delay recovery for decades or more (Sas, 1990; Chapra and Canale, 1991). The cumulative effect of excessive P inputs and increased in- ternal cycling can lead to a loss of resilience as aquatic plant communities are pushed from a clear water to algal dominated state. Maintaining or restoring the desired clear water state may require ongoing management intervention in the lake or * Corresponding author. Tel.: þ46 (0)18 673128; fax: þ46 (0)18 673156. E-mail address: brian.huser@slu.se (B.J. Huser). Contents lists available at ScienceDirect Water Research journal homepage: www.elsevier.com/locate/watres http://dx.doi.org/10.1016/j.watres.2015.07.036 0043-1354/© 2015 Elsevier Ltd. All rights reserved. Water Research xxx (2015) 1e11 Please cite this article in press as: Huser, B.J., et al., In-lake measures for phosphorus control: The most feasible and cost-effective solution for long-term management of water quality in urban lakes, Water Research (2015), http://dx.doi.org/10.1016/j.watres.2015.07.036