Control of internal phosphorus loading in two shallow lakes by alum and aquatic plant harvesting

Abstract

Erie and Campbell Lakes are two shallow, nonstratified lakes located in Skagit County, Washington. Both lakes suffered from dense blooms of blue-green algae during late summer. Erie Lake also suffers from occasional fish kills. A Phase I diagnostic study conducted in 1981–82 identified phosphorus as the nutrient controlling algal growth, and internal phosphorus loading via sediment release and aquatic plants was viewed as the most significant controllable sources. The restoration plan recommended aluminum sulfate (alum) to control sediment P release and mechanical harvesting to remove plant phosphorus. Harvesting would also provide recreational benefits and control any increased plant colonization made possible by improved water clarity. Both lakes were treated with liquid alum during Fall 1985, and the harvesting program was implemented in Summer 1986. The 1986 post restoration values indicate a dramatic improvement in the quality of Erie Lake. Mean summer total phosphorus concentrations were reduced from 115 to 26 μgP/L, chlorophyll a concentrations were reduced from 58 to 5 μg/L, while water clarity increased from 1.7 to 2.5 + meters. Improvements in Campbell Lake were significant but not as dramatic as Erie Lake; mean summer total phosphorus concentrations were reduced from 49 to 28 (μgP/L, chlorophyll a concentrations were reduced from 18 to 10 μg/L, but water clarity remained similar to pre-restoration values (1.8 meters). © 1987 Taylor & Francis Group, LLC.