Localized Changes in Transparency Linked to Mud Sediment Expansion in Lake Okeechobee, Florida: Ecological and Management Implications

During the last decade, water quality monitoring data have indicated declines in Secchi transparency in the southwestern pelagic region of Lake Okeechobee. Sampling stations that previously displayed intra-annual variation in Secchi depths from 0.2 to 2.7 m now display Secchi depths that never exceed 1.0 m. The historical changes in transparency occurred abruptly, and the recent low transparencies have been very stable. Ratios of Secchi/total depth indicate that light often may have reached the lake bottom (ratios > 0.5, and as high as 1.0) prior to the new stable state, but never reached the bottom afterwards (ratios <0.3). Secchi depth vs. chorophyll a relationships also have changed in the southwestern pelagic region. Stations previously displayed significant inverse log relationships between the two variables, similar to what is observed today at near-shore, sand-bottomed sites in Lake Okeechobee. In recent years, there has been no relationship between Secchi depth and chlorophyll a. This is similar to the mud-bottomed central pelagic region, where abiotic turbidity controls light attenuation. One explanation for the sudden changes in Secchi transparency is migration of mud sediments from mid-lake towards the southwestern shore. A comparison of sediment maps constructed in 1975 and 1988 indicates that the area of lake bottom covered by mud sediment increased from approximately 30 to 44% over that period. The major direction of expansion was to the southwest, and the postulated movement coincided closely with dates when Secchi declines were observed at water quality monitoring stations. The declines in transparency that have occurred in the southwest near-shore region of Lake Okeechobee may have important ecological and societal implications. This near-shore region is the primary foraging and nesting habitat for commercial and sport fish, wading birds, and migratory water fowl. Losses of submerged plant communities due to reduced light penetration may adversely affect these communities. In addition to a present goal of external P load reduction, lake managers should give consideration to in-lake methods that might prevent further water quality degradation in near-shore regions. Because water depth can act synergistically with turbidity to bring about the changes described above, one option is a substantial draw-down of the lake, followed by actions to prevent the prolonged high lake stages that recently have led to submerged plant declines. Selective dredging of sediments from the southwest region of the lake might also be considered, although this would likely be only a short-term solution to the water quality problems, given the remaining large source of mud at the center of the lake.