Case Study: Reduction of Total Phosphorus Loads to Big Platte Lake, MI Through Point Source Control and Watershed Management.

The Platte River watershed is located in the northwest part of Michigan's Lower Peninsula. The watershed has a drainage area of approximately 495 km2. The River has a mean annual flow of about 3.50 m3/sec. Big Platte Lake is the largest lake in the system with a mean depth of 8.2 m and hydraulic retention time of 0.75 years. The lake is the receiving water body for roughly 95% of the drainage area before emptying into Lake Michigan at the Sleeping Bear Dunes National Lakeshore.The Platte River and Big Platte Lake are under pressure from both point and non-point nutrient sources. The only point source in the watershed is a Coho and Chinook salmon hatchery operated by the Michigan Department of Natural Resources (MDNR). This source is highly regulated and currently represents a relatively small fraction of the total phosphorus loading to the lake. On the other hand, internal and non-point nutrient loads are expected to increase throughout the watershed because the area is one of the fastest growing regions in the State.Several years ago the water quality of Big Platte Lake noticeably declined with an expansion of fish production and phosphorus loads from the hatchery. After a lengthy court case, the MDNR and the Platte Lake Improvement Association (PLIA) agreed on a program to reduce the hatchery phosphorus discharge. In addition, a phosphorus standard of 8 mg/m3 was established for the lake. Consequently, the hatchery loading has declined and further reductions are expected with the completion of ongoing major renovations. These reductions have resulted in improvements in the water quality of Big Platte Lake. However, additional watershed scale efforts are needed to maintain the water quality of the lake over the long-term. Thus, the MDNR, PLIA, and the Benzie County Conservation District are working together to reduce non-point phosphorus loading to the lake through comprehensive watershed management.This partnership currently conducts a comprehensive water quality monitoring program for the hatchery, lake, and several tributaries. The data generated by the program are critical to the development and preliminary validation of two system models, a watershed loading model (BASINS) and a lake water quality model. The reliability of the lake water quality model is being enhanced by conducting special laboratory and field studies that can be used to quantify the value of several model coefficients. The two models will be used to assess seasonal and long-term improvements in lake water quality that result from reductions of point and non-point total phosphorus loading. In this manner, the models will facilitate local water resource protection efforts by providing quantitative tools for evaluating the impact of future changes in land-use, the effectiveness of local ordinances, and public outreach activities.