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September 16, 1999 DES MOINES, Iowa -- Ever wonder what the dangers of fertilizing your lawn or crop are? Iowans living by a not-so-clear Clear Lake are starting to see some real consequences to these environmental concerns. For the past four years, a water improvement project has been going on at Clear Lake in Cerro Gordo County. Over time, visibility in the lake deteriorated due to excess phosphorus and nitrogen levels. This overabundance of nutrients has caused algae blooms that could lead to fish kills. "Eventually the algae blooms would use up enough oxygen that fish would begin to suffocate, if nothing were done," said Jim Wahl, Iowa Department of Natural Resources fisheries biologist. "In an effort to reduce these phosphorus and nitrogen levels, we started a tremendous project this August," Wahl said. While searching for the source of the excess nutrients, DNR investigators found that more phosphorus was leaving Ventura Marsh, which empties into Clear Lake, than entering it. "This was very puzzling to us. I had always been taught that a marsh acts as a filter, but in this case it was quite the contrary," Wahl said. A lack of submergent vegetation in the 400-acre marsh was to blame. Vegetation on the marsh's floor should have been filtering the phosphorus, but due to a surplus of carp and bullhead, this vegetation was unable to grow. These fish swim at the bottom, churning the floor, and prevent vegetation from growing. On Aug. 17, the DNR and an Iowa State University water research group dumped a chemical called Rotenone into the marsh to kill the carp and bullhead. The dead fish were removed from places visible to and used by the public, resulting in more than 15,000 pounds of fish being hauled to the dump, said Jeff Kopaska, a member of the ISU research team. Improvements have been seen since the fish kill, Kopaska said. Phosphorus measurements flowing from the marsh into Clear Lake previously measured 625 parts per million but have dropped to 359 parts per million. "A lot is left to be done, though. Ventura Marsh isn’t the only place phosphorus is getting into Clear Lake, and we still hope to improve the marsh's [phosphorus] output," Wahl said. Despite improvements in the marsh, not all of the carp and bullhead were killed. The DNR plans to lower the water level of the marsh and induce a winter kill. By reducing the water level, most of the remaining fish will be killed if the weather cooperates. An early freeze with snow cover on the ice will lower oxygen in the marsh and kill the fish. Next spring, 500,000 northern pike will be added to the marsh. Northern pike feed on carp and bullhead and will eliminate any remaining. The sources of the excess nutrients in Clear Lake vary. For example, more than 70 storm drains dump into the lake. "People don't realize whatever they dump in those drains ends up here," Wahl said. Other sources include lawn clippings, leaves burned by the shore and fertilizer run-off from fields. With the help of a "non-source pollution grant," DNR workers at Clear Lake have tried to educate the public on the watershed and how their property affects it. Lawn testing in the town of Clear Lake showed excess phosphorus levels, so residents are being urged to use phosphorus-free fertilizers for their lawns. Work on an "integrated crop management project" is trying to reduce the nutrients farmers are putting into the soil. Currently, Clear Lake is being constantly monitored to look for improvement or deterioration, something never done before. The ISU water research group hopes to come up with a nutrient budget for the future and continue to locate and remedy high phosphorus input spots. "Hopefully, as underwater vegetation begins to grow back, we'll have fewer algae blooms and avoid a serious fish kill in the lake. We can't get the situation completely solved until people in the community acknowledge the problem and try to help us. People won't have to alter their lives much to really help our lake," Wahl said. |