The response of a freshwater wetland to long-term 'low level' nutrient loads: nutrients and water budget

Boney Marsh is a small constructed freshwater wetland located along the floodplain of the Kissimmee River in south Florida, USA. River water, with average Tot-P concentrations of 0.052 mg l(-1), Tot-N of 1.70 mg l(-1), and Cl- of 15.95 mg l(-1), was diverted through the marsh to quantify mass retention and fate. Comprehensive mass balance budgets for Tot-P, Tot-N, and Cl- were developed based on input (inflow, precipitation) and output (outflow, evapotranspiration, seepage). Cl-, as well as Na+, budgets indicated that groundwater accounted for approximately 7% of the total water budget. Annual mass loadings to Boney Marsh were 0.5, 15.7, and 147.9 g m(-2) year(-1) for Tot-P, Tot-N, and Cl-, respectively. Mean annual nutrient removal was estimated at 72% for Tot-P and 34% for Tot-N, and P-assimilation capacity remained high and unchanged for the period of record. The subtropical marsh system accumulated Tot-P at a much higher rate than Tot-N, with averaged net sedimentation rates of 20.4 and 8.3 year(-1), respectively. Boney Marsh net sedimentation coefficients were higher than lakes with similar depths. The N:P mass ratio in the wetland water column increased during the period of record, and was primarily due to a high P-sedimentation rate and a declining N-sedimentation rate.