Does Phosphorus from Agricultural Tile Drains Fuel Algal Blooms?

PURPOSE: An essential nutrient for plant growth, phosphorus (P) is often implicated as a contributing factor to algal blooms. Lake Macatawa is a hypereutrophic lake located in west Michigan, and the watershed is dominated by row crop agriculture. The objectives of this study were to assess agricultural tile drains as a source of P in the Macatawa Watershed. SUBJECTS: Nine tile drain outlets in the Macatawa Watershed from March 2015 – February 2016. METHODS: 1) Conduct a tile drain effluent survey by measuring soluble reactive phosphorus (SRP) and total phosphorus (TP) once per month; 2) investigate the change in tile drain P concentrations temporally over a one-year period; and 3) use seasonal algal bioassays to measure the bioavailability of P found within the tile drains. ANALYSIS: Kruskal-Wallis tests with a Holm’s adjustment compared P concentrations between sample sites and between sample dates. Linear regression was used to relate SRP concentration to algal growth after the bioassays. RESULTS: Tile drain outlet SRP and TP concentrations varied temporally and spatially. The SRP:TP ratio measured at the tile drain outlets had a significant, 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. The highest P loads, corresponding to high flow, occurred during winter sample dates. CONCLUSIONS: P loss to tile drains should be limited in the Macatawa Watershed with best management practices targeting locations and seasons with the highest P load