Phosphorus Retention in West Michigan Two-stage Agricultural Ditches

Input of excess nutrients into a water body can negatively impact ecological structure and function, as well as the economic vitality of surrounding communities, by contributing to eutrophication. For example, phosphorus (P) and sediment inputs from agricultural drainage have facilitated the development of hypereutrophic conditions in Lake Macatawa, a drowned river mouth lake located in Holland, Michigan. Two-stage ditches, an agricultural best management practice (BMP), are used in some areas of the Midwest to reduce N export downstream via denitrification. This BMP simulates a mini-floodplain by replacing a traditional, trapezoidal ditch with a channel that has excavated benches on each side to help capture nutrients and sediment when the ditches flood. However, less is known about the ability of two-stage reaches to reduce P export. This project assesses the effectiveness of two- stage ditches within the Macatawa watershed at retaining P. Both biotic and abiotic factors were analyzed as potential P sinks. Initial, baseline results showed that total P varied between 0.1 to 1 mg P/g dry sediment and tended to be higher in the upstream, traditional reach compared to the downstream, two-stage reach. Equilibrium P concentration values suggest retention of P within the two-stage. P was bound within stable fractions in both two-stage and traditional reaches. Sediment held over 96% of TP within each reach compared to \u3c 4% in bench vegetation and algae combined. Turbidity but not P was reduced in one study ditch while P but not turbidity was reduced in the other study ditch. Ability to retain P appears to be impacted by physical as well as biogeochemical characteristics. Results will be used to inform management decisions within the watershed