Thermal Stratification and the Potential for Enhanced Phosphorus Release from the Sediments in Lake Okeechobee, USA

Abstract

We inferred the potential importance of enhanced phosphorus (P) release from the sediments during periods of thermal stratification in Lake Okeechobee USA, a large, shallow, eutrophic lake. This was accomplished by determining the frequency and duration of thermal stratification, using monitoring data collected during 1997 and 1999, with multiparameter sondes, which were positioned at a weather platform in the center of the lake. We also monitored dissolved oxygen, pH, specific conductance, turbidity and redox potential, to assess if any relationships existed between these variables and thermal stratification. Thermal stratification was infrequent, documented for 42 and 74 total hours, and occurred for an extrapolated period of at least one hour on approximately 18 days or 5% of both years. Thermal stratification occurred almost exclusively during the summer (May – September), and was brief, typically lasting for one to six hours per event, although there was one event during 1999, where 49 continuous hours of thermal stratification were documented. Dissolved oxygen (DO) concentrations and redox potential (ORP) measurements collected 0.5 m above the sediment surface suggested that the top of the sediment may not have ever become anoxic and was rarely reducing. Chemical characteristics during periods of thermal stratification, compared to those considered favorable for P release based on previous Lake Okeechobee sediment studies, indicate that thermal stratification is usually insufficiently long for chemical conditions (anoxic or low DO and ORP) to develop above the sediment surface that would enable enhanced P release from the sediments.