Nitrogen dynamics in Lake Okeechobee: forms, functions, and changes

Total nitrogen (TN) in Lake Okeechobee, a large, shallow, turbid lake in south Florida, has averaged between 90 and 150 μM on an annual basis since 1983. No TN trends are evident, despite major storm events, droughts, and nutrient management changes in the watershed. To understand the relative stability of TN, this study evaluates nitrogen (N) dynamics at three temporal/spatial levels: (1) annual whole lake N budgets, (2) monthly in-lake water quality measurements in offshore and nearshore areas, and (3) isotope addition experiments lasting 3 days and using 15N-ammonium (15NH4+) and 15N-nitrate (15NO3−) at two offshore locations. Budgets indicate that the lake is a net sink for N. TN concentrations were less variable than net N loads, suggesting that in-lake processes moderate these net loads. Monthly NO3− concentrations were higher in the offshore area and higher in winter for both offshore and nearshore areas. Negative relationships between the percentage of samples classified as algal blooms (defined as chlorophyll a > 40 μg l−1) and inorganic N concentrations suggest N-limitation. Continuous-flow experiments over intact sediment cores measured net fluxes (μmol N m−2 h−1) between 0 and 25 released from sediments for NH4+, 0–60 removed by sediments for NO3−, and 63–68 transformed by denitrification. Uptake rates in the water column (μmol N m−2 h−1) determined by isotope dilution experiments and normalized for water depth were 1,090–1,970 for NH4+ and 59–119 for NO3−. These fluxes are similar to previously reported results. Our work suggests that external N inputs are balanced in Lake Okeechobee by denitrification.