PHOSPHORUS RETENTION IN CONSTRUCTED FRESH-WATER RIPARIAN MARSHES

Four constructed freshwater riparian marshes in northeastern Illinois, USA, each experimentally subjected to either high or low loadings of turbid river water over a 3-yr period, were investigated for phosphorus retention and fate. Average phosphorus concentrations decreased by 64-92% in low-flow wetlands (to 11-40 mu g P/L) and by 53-90% (to 12-57 mu g P/L) in high-flow wetlands. Intensive sampling at 4-h intervals during spring and summer of the final year showed phosphorus mass decreases of 81-74% for low- and high-flow wetlands, respectively. During this frequent sampling, inflow averaged 176 mu g P/L, while outflows were 34 mu g P/L for the low-flow wetland and 45 mu g P/L for the high-flow wetland. Spatial patterns of isopleths showed clear gradients in high-flow wetlands and near-homogeneous concentrations in low-flow wetlands. Annual budgets illustrated that most inflowing phosphorus was retained through sedimentation with some capacity for phosphorus retention by macrophytes and a lesser amount by microbial (periphyton acid planktonic) communities. A simple Vollenweider-type model predicted well the retention of phosphorus in these wetlands for low- and high-flow wetlands with 3 yr of data, i.e., the model was calibrated and verified with six independent data sets. A mean deviation of only 9.3% between measured and simulated retention was found when the retention coefficient was k = 6.51 wk(-1). Adding a factor for seasonal temperature fluctuation did not improve the fit, whereas using a model where the retention is a function of flow provided the best overall fit to the data (mean deviation of 6.3% between data and predictions). These constructed wetlands retained about the same amount of phosphorus per unit area (0.5-3 g P . m(-2). yr(-1)) as have several other natural and constructed wetlands receiving similar concentrations of phosphorus.