Nutrient Accumulation, Decomposition and sedimentation of aquatic macrophytes in southern Minnesota

Five species of aquatic macrophytes from an agricultural drainage ditch and a second-order stream were analyzed for nitrogen, phosphorus and ash throughout the 1976 growing season. These macrophytes were observed to accumulate large amounts of these nutrients from the sediment and water. Determination of correlation coefficients showed that there was little or no significant correlation between nutrients in the water and in the plant tissue. Test for equal correlation coefficients revealed little difference between the different plant species within the same site or for the same plant species at different sites. The decomposition of two species of aquatic macrophytes proceeded rapidly with complete processing taking 50 to 64 days. The decomposition involved two phases: Phase I was a rapid weight loss due to leaching while Phase II was a gradual decline in dry weight of the remaining macrophyic matter. Rate coefficients for weight losses and nutrient regeneration were determined and found to be greater than those reported in the literature. C:N of the decomposing macrophytes were less than 17:1 throughout the study. The potential contribution of decaying macrophytes to the sediments was determined for the drainage ditch. Sediment analyses revealed that nutrient concentrations and organic matter were higher for the drainage ditch than for the river. The top 4 cm of the ditch sediments contained 5 to 13 times more nutrients than the actively growing macrophytic biomass within the ditch during the 1976 growing season.