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 ab undance were conducted over the subsequent 30 mo. Charophyte biomass d eclined progressively from 1994 through 1996; regardless of year, char ophyte presence was observed only during the mid to late summer and fa ll months. Ash free dry mass (AFDM) levels ranged from non-existent to >20 g/m(2). Abiotic factors influencing charophyte populations were i nvestigated. Regression analysis showed that charophyte biomass was in versely 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 photosynthesi s-irradiance curves confirmed this hypothesis, as I-k (irradiance at w hich 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 a mbient light conditions. Inorganic phosphorus concentration was not si gnificantly related to charophyte biomass. Nutrient kinetics favored p hytoplankton growth over Chara, assuming phosphorus limited conditions . Although nutrient concentration may influence Chara phenology and ab undance, light appears to be a stronger regulator in this subtropical lentic ecosystem.