Does Phosphorus from Agricultural Tile Drains Fuel Algal Blooms?

Phosphorus (P) is often implicated as a contributing factor to algal blooms. Attention has been focused on P in surface runoff, but agricultural tile drains also can be a source. Lake Macatawa is a hypereutrophic lake located in west Michigan, and the watershed is dominated by row crop agriculture. Further research is needed to understand the influence of bioavailable P originating from tile drains on water quality in Lake Macatawa. The objectives of this study were to 1) conduct a tile drain effluent sampling survey to assess their importance as a source of P in the Macatawa Watershed; 2) investigate the change in tile drain P concentrations spatially and temporally over a one-year period; and 3) use growth chamber algal bioassays and the ratio of soluble reactive phosphorus to total phosphorus (SRP:TP) to assess tile drain P bioavailability. During March 2015 – February 2016, P concentrations varied significantly among sample sites, and the highest P loads occurred during the non-growing season. The SRP:TP ratio measured at the tile drain outlets had a positive correlation with acreage drained by the tile system. Four of six bioassays resulted in a positive relationship between SRP and algal growth, but results from only one bioassay were statistically significant. There was a clear change in the algal community structure when incubated in tile drain water, and dominance was by diatoms, not cyanobacteria as expected. Based on these results, there is a need to quantify the tile drained area in the Macatawa watershed and manage for high P loads during the non-growing season