Reduced river phosphorus following implementation of a lawn fertilizer ordinance. Lake and Reservoir Management

showing statistically significant reductions in total phosphorus (TP) and a trend of reduction in dissolved phosphorus following implementation of a municipal ordinance limiting the application of lawn fertilizers containing phosphorus. No reductions were seen at an upstream control river site not affected by the ordinance. Non-target analytes including nitrate, silica, colored dissolved organic matter, specific conductance, and pH did not change systematically as did P. The data were compared with a multi-year historical data set at weekly and sub-weekly resolution that preceded the ordinance. The average reduction in TP from May to September was 17 percent. In response to a state-imposed phosphorus TMDL (Total Maximum Daily Load) that called for a 50 % reduction in phosphorus discharges to the Huron River, the city of Ann Arbor, Michigan enacted an ordinance that went into effect in 2007 (Ann Arbor 2006) to limit phosphorus application to lawns. The projected effect of full compliance was a 22 % reduction in phosphorus entering the river. The prediction was made by measuring the lawn fertilizer runoff from a creekshed within the city and extrapolating that result to all other creeksheds. Statistical modeling (Ferris and Lehman 2008) indicated that reductions of that specified magnitude for total P (TP) could be detectable using an existing historical set of Huron River water quality data within two years by sampling four times per month. Similar percentage reductions in dissolved P (DP) would likely take two or three years, and for soluble reactive P (SRP), the time could be as long as 8 years. After just one year, 2008, Lehman et al. (2009) were able to report a statistically significant reduction in TP and a trend of decreasing concentration for DP. This report summarizes the results of our Year 2 (2009) sampling season. Study site−−Our field site (Fig. 1) is a portion of the Huron River catchment in southeaster