Internal Nutrient Loads from Sediments in a Shallow, Subtropical Lake

Fluxes of dissolved inorganic nitrogen, DIN, (as ammonia, NH4-N) and phosphorus (as dissolved reactive phosphorus, DRP) from the sediments to the water column of Lake Okeechobee were determined from two separate techniques: increases in nutrient concentration in the water column above intact cores and concentration gradients determined with pore water equilibrators. These fluxes were estimated from different sediment types within the lake (sand, peat, mud) and at two major inflows (Kissimmee River and Taylor Creek). DRP release from peat sediments was highest. Measurements in other sediments were not significantly different from each other. DRP flux to the lake was estimated as 326 Mt·yr−1 in 1989 and had increased to 472 Mt·yr−1 in 1999. Because of measurement variation, this increase was not statistically different. These estimates of internal DRP loads are greater than estimated external surface DRP loads that averaged 316 Mt·yr−1 from 1979–1988 and 258 Mt·yr−1 from 1989−1999. DIN flux was highest near Taylor Creek. There was no consistent pattern between sediment type and DIN flux. Internal loads of DIN were estimated in 1999 as 4,500 Mt·yr−1, which is greater than the external surface loads of DIN estimated for the period of 1989–1998, of 896 Mt·yr−1. Sediment oxygen demand measured in the cores was strongly correlated to DRP and DIN flux, indicating that these fluxes are largely a result of mineralization of organic material. This study indicates that sediment diffusive fluxes are a significant source of DRP and DIN to the lake water column.