Multiple external and internal phosphorus (P) sources to an urban lake, Half Moon Lake in Wisconsin, were examined during the summer of 1999 in order to develop management strategies for effective P control and reversal of eutrophication (Trophic State Index=74). Internal recycling ofP accounted for 80% of the summer P budget of the lake. F1ux ofP from the sediment accounted formostofthe internal P loading ( 42% of total budget). However, decomposition of Potamogeton crispus and recycling of macrophyte P during the middle of the summer growing season, and P resuspension due to motor boat activity, accounted for 20% and 17% of the P budget, respectively, representing additional important sources to be controlled. In contrast, summer P loading via the watershed (storm sewers and precipitation) was much less. Using a water quality model (Bath tub), we found that reduction of internal P sources could substantially reduce by greater than 70% the high concentrations of algae in the lake (mean summer chlorophyll = 82 mg • m3 ). Suggested internal P control measures included a sediment chemical treatment to bind P, greater harvesting of P. crispus to reduce the macrophyte P pool at the time of senescence, and limiting motor boat activity when the lake is weakly stratified.

Lakes Planning Report

Submersed aquatic macrophytes can play an important role in the phosphorus budget of aquatic systems; thus, their impacts need to be considered in lake management and rehabilitation plans. In particular, macrophytes can directly recycle phosphorus from the sediment via root uptake, incorporation into tissue, and subsequent senescence (Barko and Smart 1980; Carpenter 1980; Landers 1982; Smith and Adams 1986; Barko and James 1998). They can also indirectly recycle phosphorus from the sediment via increasing pH in the water column through photosynthetic activities. Phosphorus release from sediments can be enhanced at high pH as a result of ligand exchange on iron oxides contained in the sediment (Drake and Heaney 1987). These processes can lead to phosphorus enrichment of aquatic systems and eutrophication. The objectives of this study were to evaluate direct and indirect impacts of a near monotypic stand of Potamogeton crispus L. on the phosphorus economy of Half Moon Lake.

The City of Eau Claire proposes to continue its study of internal and external nutrient loading in Half Moon Lake. Phase Three and Four activities include "bathtub" modeling and reporting to determine the role macrophytes play in phosporus recycling, and the role Ski Sprites waterski activities play in phosporus recycling and transport. The Department of Natural Resources will be provided both paper and electronic copies of the final report. The project results will be disseminated to the public through mailings, meetings, newspaper, radio, television and the completion and dissemination of a comprehensive lake management plan.

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