Investigating sources and transformations of nitrogen using dual stable isotopes for Lake Okeechobee restoration in Florida

Abstract

An analysis of nitrate concentrations and isotopic compositions was undertaken on Florida's large (1730 km2) and shallow (mean depth 2.7 m) Lake Okeechobee to determine possible connectivity between the lake's many inflows and outflows and to illustrate how stable isotopes can assist in the restoration of a very large subtropical lake. The Kissimmee River, as expected, dominated inflows to Lake Okeechobee (58.4 ± 2.11 m3/s) much higher than the second most important inflow, Fisheating Creek, (9.2 ± 0.27 m3/s).The primary outflows (which are sometimes inflows) are the Caloosahatchee (27.7 ± 0.63 m3/s), which flows westward to the Gulf of Mexico and the St. Lucie River (6.98 ± 0.30 m3/s) that flows eastward to the Atlantic Ocean. Water samples of inflows and outflows were collected at 14 locations around the lake in both the dry and wet seasons in 2018. Total nitrogen and total phosphorus averaged 1.56 ± 0.42 mg L−1 and 0.11 ± 0.06 mg L−1 respectively with no significant seasonal variations. Dissolved organic nitrogen (DON = TKN-NH4+) was the dominant form of nitrogen at all sampling sites, with an average value of 1.18 ± 0.32 mg L−1. The TN:TP ratios were mostly less than 22:1.Isotope δ15N-NO3− ranged from −8.75 to 9.14‰, and δ18O-NO3− from −0.26 to 11.97‰. Inflow δ15N-NO3− and δ18O-NO3− measurements ranged from −3.92 to 9.14‰ and 0.50 to 11.97‰. A Bayesian mixing model output revealed that non-point sources from NH4+ fertilizers and soil nitrogen were the main nitrate sources in Lake Okeechobee. NH4+ fertilizer contributes 36.7% of nitrate in the dry season and 54.9% in the wet season, while soil N contributes 31.4% in the dry season and 25.1% in the wet season. Although pasture is the dominant land use in the watershed of Lake Okeechobee, manure was not the dominant source of nitrate; it contributes only 12.0% of the nitrate in the wet season and 25.4% in the dry season. Isotopes of NO3− suggested that denitrification was strong, and nitrification was weak in both the dry and wet seasons. High levels of NH4+ during the wet season, partly caused by weakness of nitrification, promoted uptake of NH4+ that affects microbial food web recycling processes in the lake. These results indicate that monitoring and regulatory strategies for the lake restoration should consider the control of nitrogen pollution sources from agriculture to complement the control of phosphorus inflows that are thought to be the main drivers for harmful algal blooms in the lake and downstream of the lake.