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Phosphorus Removal by Plant Harvesting on Lake Minnetonka October 22, 2004 John M. Barten, Three Rivers Park District Phosphorus Removal by Plant Harvesting on Lake Minnetonka Introduction In 1986, the exotic plant Myriophyllum spicatum (Eurasian watermilfoil) was discovered in Lake Minnetonka. The initial discovery of the plant was followed almost immediately by nuisance growth that impeded recreational boating in many areas of the lake. In response to these concerns, the Lake Minnetonka Conservation District initiated a plant harvesting program on the lake to minimize impacts to boating. Annually, the LMCD program harvests between 400 and 1000 acres of the lake, and removes approximately 200 truckloads of plants. The annual removal of large quantities of plants from the lake has the potential to remove significant amounts of phosphorus from the lake and improve water quality. Previous studies, primarily in Wisconsin, found that aquatic plants are approximately 0.2 percent phosphorus by dry weight. Unfortunately, the dry weight of plant material removed from Lake Minnetonka was unknown, and it was thus very difficult to estimate the quantity of phosphorus removed from the lake. Estimates of phosphorus removal from the lake completed by the Minnehaha Creek Watershed District ranged from 2000 to 20,000 pounds annually. In an effort to accurately quantify the actual removal of phosphorus from the lake, the LMCD initiated a study during the 2004 -harvesting season. Representative truckloads of harvested plant material were weighed, the plants dried and then analyzed for phosphorus content. The study determined the ratio of dry to wet plant weight. Project Objectives The objective of the study project was to determine the quantity of phosphorus removed from Lake Minnetonka by the LMCD milfoil-harvesting program. A major component of the determination was to calculate the ratio of dry to wet plant weight in truckloads of material removed from the lake. Methods The mass of harvested plant material in a representative sample of truckloads was obtained by weighing two truckloads of material each week during the harvesting season. Trucks were weighed on a truck scale at a bituminous mixing facility near Lake Minnetonka. Prior to beginning the study, the truck was weighed empty with a full tank of fuel. The volume of fuel in the tank during each subsequent weighing was measured from the fuel guage. The fuel weight was then estimated assuming 7.1 -lbs/ gallon for diesel fuel. Typically, the final load of the day was selected for sampling to maintain the operational efficiency of the harvesting program. C.IllserslGabriellDownloadslMinnetonkaPremovharvesting.doc p. 2 of 6