LAUSR

Abstract Phosphorus (P) loading from nonpoint sources is often implicated as a contributing factor to the proliferation of algal blooms in freshwater ecosystems. However, the influence of subsurface tile drains as a source of P, especially in agricultural areas, has received limited attention. We examined the importance of tile drain effluent in the Macatawa Watershed; this watershed is dominated by row crop agriculture and drains into hypereutrophic Lake Macatawa, which connects to Lake Michigan. Our objectives were twofold: 1) assess the importance of tile drain effluent as a source of P in the Macatawa Watershed by measuring tile drain P concentrations spatially and temporally over a one-year period; and 2) assess the ability of tile drain effluent to stimulate algal blooms using bioassays with natural phytoplankton communities. During March 2015–February 2016, P concentrations varied significantly among sample sites (SRP: