Nutrient release and detritus production by herbicide-treated freely floating aquatic vegetation in a large, shallow subtropical lake and river

Abstract

The areal extent of freely floating aquatic vegetation (FFAV), treated annually with herbicides in Lake Okeechobee and the Kissimmee River, was compiled, and preliminary estimates were made of the amounts of detritus produced and total nitrogen (N) and total phosphorus (P) released. On average, over the 24-yr period of record for Lake Okeechobee from 1975 through 1998, 14,281 ± 1,544 metric tons (m.t.) of detritus was produced, and 285 ± 31 m.t. of N and 74 ± 8 m.t. of P were returned to the water column. Nutrients released in the lake by treated FFAV were estimated to have a molar N to P ratio of 9:1, and to have ranged minimally from 1 to 17 percent and from 4 to 49 percent of its external annual N and P loads, respectively. In Lake Okeechobee, treated FFAV was heterogeneously distributed, and occurred primarily in the southern and western near-shore regions. Fisheating Bay received the largest amount of this treated material. Treatable FFAV was positively correlated with lake stage. In the Kissimmee River, Lake Okeechobee's main inflow, on average, over its 16-yr period of record from 1983 through 1998, 4,427 ± 593 m.t. of detritus was produced, and 88 ± 12 m.t. of N and 23 ± 3 m.t. of P were returned to the water column. Some of these nutrients and detritus likely reach Lake Okeechobee and contribute to its external N, P and detrital loads. Treated FFAV has substantially increased the amount of, and temporally and spatially altered, both detritus production in, and the internal N and P loading to, both Lake Okeechobee and the Kissimmee River. These nutrient and detrital additions only further exacerbate the already ongoing processes of cultural eutrophication within these waterbodies.