¹University of Wisconsin – Green Bay, Natural and Applied Sciences, 2420 Nicolet Dr., Green Bay, WI 54311.
²National Center for Water Quality Research, Heidelberg College, 310 E Market St., Tiffin, OH 44883, USA

*Corresponding author: prichard@heidelberg.edu

Summary

   The total phosphorous (TP) load estimates to Lake Erie for each year have been compared to the target load from the Great Lakes Water Quality Agreement of 11,000 metric tons per year. The estimates for 1997 and 1998 exceeded the target load mainly due to elevated tributary loads. Five tributaries in Northern Ohio and Southeast Michigan have been monitored intensively for several years before and after this “pulse” in loading. While increased rainfall in these years was a major contributor to the unusually large loading, another factor was that the slopes in the TP-flow relationships for each of these five tributary sub-basins were usually larger in the winter. This suggests that, in addition to frequency and duration, the timing (i.e., winter vs. summer) of storms may have been responsible for this loading “pulse”. Various statistical models have been applied to the Lake Erie tributary data for the years 1996 through 2001 for these sub-basins. The results show different seasonal relationships within each tributary.