Meteorological factors affecting estuarine conditions within Lake George in the St. Johns River, Florida

Proceedings of the Seventh International Conference on Hydroscience and Engineering, Philadelphia, PA, September 2006. http://hdl.handle.net/1860/732Lake George is a flow-through lake located in the St Johns River (SJR), an elongated shallow river estuary. Tide propagates upstream as far as the lake (190 km) where it is dampened out. The filling and draining of Lake George is dominated by subtidal variability of Atlantic Ocean waterlevel (Morris, 1995). Summer cyanobacteria concentrations are often high with chlorophyll-a levels regularly exceeding 100 ug L-1. Such high levels of cyanobacteria cause undesirable shifts in higher tropic levels. The cyanobacteria add approximately 1400 MT yr-1 of nitrogen by N-fixation, further contributing to eutrophication of the downstream marine portion of the estuary. Because of the importance of mixing and circulation processes in Lake George to understanding phytoplankton dynamics, a 3-D hydrodynamic model (EFDC) was applied to the lake. Salinity (chloride) entering the lake through springs along the western shore was used as a conservative tracer for verification of the model’s mixing and transport processes. Simulated dye tracers were used to determine flushing rates and delineate volume sources under varying meteorological conditions. Average turnover rate of the lake was 84 days, but during low-flow periods turnover rate ranged to 180 days. The estimated GPP of the lake was 800 gC m-2yr-1. These two results indicate that peak algal biomass is partially controlled by flushing. Under these conditions model tests show that water quality is significantly influenced by local groundwater sources entering by springs. The Ocklawaha River enters the St Johns 8 km downstream from Lake George. Reverse flows can push water entering the SJR from the Ocklawaha up into lake George. Under low flow conditions, this may account for up to 6% of the total volume entering lake. This study is a continuation of similar work underway downstream (Sucsy, 2002). This paper will summarize an evaluation of the subsurface and meteorological processes that influence the water quality and estuarine character of the lake