The influence of environmental variables and a managed water recession on the growth of charophytes in a large, subtropical lake

Two separate studies measured the biomass of benthic charophytes in Lake Okeechobee, a large subtropical lake located in south-central Florida, USA. In one study, five stations in the southern region of the lake were sampled monthly or bimonthly over a 6 year period (1994-1999). A distinct seasonal pattern was observed, with maximum biomass occurring in summer and early fall. Charophyte abundance was positively related to irradiance and negatively related to water depth. A second study took place in 1999-2000, before and after a managed lake drawdown in April, 2000. This whole-ecosystem experiment was designed to lower lake stage by similar to12.5% (0.6 m). We compared charophyte biomass at 42 stations throughout the lake prior to and after this managed recession and evaluated the influence of various environmental variables on growth. This analysis indicated that the recession resulted in increased transparency/bottom irradiance and greater charophyte biomass at nearshore stations. The data are consistent with prior studies, both in Lake Okeechobee and other lakes around the world, which indicate charophyte growth and abundance are strongly influenced by irradiance. We hypothesize that a positive feedback loop resulted in Lake Okeechobee when water levels were lowered, whereby shallower water allowed greater light penetration to the bottom, resulting in increased growth of submerged charophytes. Chara, in turn, probably stabilized sediments, leading to greater transparency and a further improvement of environmental conditions for plant recruitment and growth. (C) 2002 Elsevier Science B.V. All rights reserved.