Ecological properties of charophytes in a large subtropical lake

Benthic charophytes were observed during Fly 1994 in the southern end of Lake Okeechobee, Florida. Surveys of charophyte distribution and abundance were conducted over the subsequent 30 mo. Charophyte biomass declined progressively from 1994 through 1996; regardless of year, charophyte presence was observed only during the mid to late summer and fall months. Ash free dry mass (AFDM) levels ranged from non-existent to >20 g/m(2). Abiotic factors influencing charophyte populations were investigated. Regression analysis showed that charophyte biomass was inversely related to water depth and positively related to Secchi depth suggesting that irradiance strongly influences charophyte distribution in this lake. Data from photosynthetic measurements and photosynthesis-irradiance curves confirmed this hypothesis, as I-k (irradiance at which photosynthesis is initially saturated) was an order of magnitude greater than the ambient light reaching the charophyte populations. In addition, autotrophic respiration exceeded net photosynthesis under ambient light conditions. Inorganic phosphorus concentration was not significantly related to charophyte biomass. Nutrient kinetics favored phytoplankton growth over Chara, assuming phosphorus limited conditions. Although nutrient concentration may influence Chara phenology and abundance, light appears to be a stronger regulator in this subtropical lentic ecosystem.