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Phosphorus Removal by Plant Harvesting on Lake Minnetonka October 22, 2004 John M. Barten, Three Rivers Park District The weighed plant loads were transported to the Three Rivers Park District Gale Woods Park and dumped on a composting slab. Representative sub -samples were collected from the top, middle and bottom of each plant pile immediately after dumping, and mixed. The dominant plant species in each truckload were identified. The plant sample was placed in a plastic bag and weighed to determine the wet weight, transported back to the Park District laboratory and allowed to air dry. The air-dried material was then oven dried to a constant temperature of 105 C, and weighed. Sub -samples of the oven dried plants were taken, ground up, and digested. The phosphorus concentration of the dried plant material was determined by the ascorbic acid method, (Standard Methods, 18th Edition). Duplicate analyses were performed on all collected samples. Results and Discussion Sample collection began on June 29, 2004. Two samples per week, one on Tuesday and one on Thursday, were collected for the following seven weeks. Typically, the final truck load of the day was weighed, and the plants delivered to Gale Woods Park. A total of 14 truckloads of plants were weighed and sampled. There was considerable variability in the weight of plant material in the trucks. The plant loads ranged in weight from 6,200 to 14,118 pounds, with an average of 10,002 pounds/truck. Some of the variability was undoubtedly due to the load being the last of the day. Although the truck operator attempted to fill the truck completely, it was not possible to control the plant mass on each harvester at the end of the day. The time between unloading the final harvester load onto the truck and arrival at the weigh station was fairly consistent. Thus, water loss from the plants was also consistent for each harvester load. The percent dry weight of the plant material ranged from 9.7 to 14.3 percent, with an average of 11.8 percent. This is similar to wet to dry weight ratios for aquatic plants reported by Burton, King and Ervin, 1979. Loads containing mainly EWM tended to be very similar in percent of wet to dry weight. Samples with significant amounts of coon tail, however, tended to have a lower wet to dry weight ratio, and samples with mainly niads's or pondweeds tended to have a higher percent of dry weight (Table 1). Only 11 of the 14 samples were oven dried and analyzed for phosphorus concentration. Two of the samples developed a significant growth of a fungus during the air -drying process and were discarded. One sample, collected by staff from Gale Woods, was too large, and a stable oven -dried weight could not be obtained. This sample was also discarded. The percent of phosphorus per gram of dry weight of 11 samples ranged from 0.11 to 0.41, with an average of 0.23 percent. The average is comparable to the value reported by Carpenter and Adams for EWM from Wisconsin lakes, and by Burton, King and Ervin, 1979. Differences in the percent of phosphorus in plant tissue (dry weight) are C.I Usersl GabriellDownloadslMinnetonka Premovharvesting.doc p. 3 of 6