Proceedings of the 7th International Conference on HydroScience and Engineering Philadelphia, USA September 10-13, 2006 (ICHE 2006) ISBN: 0977447405 Drexel University College of Engineering Drexel E-Repository and Archive (iDEA) http://idea.library.drexel.edu/ Drexel University Libraries www.library.drexel.edu The following item is made available as a courtesy to scholars by the author(s) and Drexel University Library and may contain materials and content, including computer code and tags, artwork, text, graphics, images, and illustrations (Material) which may be protected by copyright law. Unless otherwise noted, the Material is made available for non profit and educational purposes, such as research, teaching and private study. For these limited purposes, you may reproduce (print, download or make copies) the Material without prior permission. All copies must include any copyright notice originally included with the Material. You must seek permission from the authors or copyright owners for all uses that are not allowed by fair use and other provisions of the U.S. Copyright Law. The responsibility for making an independent legal assessment and securing any necessary permission rests with persons desiring to reproduce or use the Material. Please direct questions to [email protected] The 7th Int. Conf. on Hydroscience and Engineering (ICHE-2006), Sep10 –Sep13, Philadelphia, USA METEOROLOGICAL AND SUBSURFACE FACTORS AFFECTING ESTUARINE CONDITIONS WITHIN LAKE GEORGE IN THE ST JOHNS RIVER, FLORIDA. Joseph Stewart1, Peter Sucsy2, and John Hendrickson3 ABSTRACT Lake 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 μg 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. For the period studied, this accounted for 5% 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. 1. STUDY AREA DESCRIPTION and RESOURCE ISSUES 1.1 Study Area Description ---------------------------------------------- 1 Engineer Scientist, Saint Johns River Water Management District, Palatka, FL 32178, USA ([email protected]) 2 Supervising Engineer Scientist, Saint Johns River Water Management District, Palatka, FL 32178, USA ([email protected]) 3 Environmental Scientist V, Saint Johns River Water Management District, Palatka, FL 32178, USA ([email protected]) Centered about 190 km upstream from the Atlantic Ocean, Lake George is Florida's 2nd largest lake, with an area of 189 km2 and a volume of 0.493 km3. It is approximately 10 km wide by 15 km in length. The mean depth of the lake is 2.5 m. To investigate Lake George, boundaries were set on the SJR to the south at Astor (SR 40) and to the north at Buffalo Bluff Rodman (Figure 1). A record of daily discharge and waterlevel Dam existed at these locations for the study period (1995 - 2005). Temperature and specific conductivity are Black collected continuously at Buffalo Bluff. Three features Point that are referenced in the study: Drayton Island (big) and Hog Island (small), on the north end of the lake; and Black Point, north of Drayton Island where the river returns to a single channel course. The largest tributary of the SJR, the Ocklawaha River, enters the system 8 km downstream of Lake George. A range of water quality parameters are regularly collected at both boundary locations, within the lake, and from within the Ocklawaha. Lake George is unaffected by marine salinity (the greatest upstream encroachment of marine salinity is 112 km) (Morris, 1995), but can reach appreciable levels of dissolved solids due to the influence of in-lake and upstream brackish springs. This imparts marine characteristics into the lake. Tides propagate upstream as far as the lake where they are dampened out. Figure 1: Lake George Study Area Large-scale processes in the Atlantic can translate upstream well beyond lake George, where the presence of local source of salt allows a large assemblage of marine species to reside in the lake. The continuous supply of salt may have had a similar role in the migration of marine species well upstream of Lake George for thousands of years (Odum, 1953). 97 species of fish are listed for the SJR around the lake, with approximately 41 marine species, including stripped bass and mullet (McLane, 1955). It was one of the most productive fresh water fisheries in the late 19th century, but began to decline in the mid 1900s. Shrimp migrate into the area under ideal conditions, and the lake still supports a local blue crab industry. H.T. Odum’s method for estimating primary production was developed using research that included local spring runs (Odum, 1956). Florida owes is geologic structure to the variability of sea level over millions of years. During the Tertiary Period, reefs that grew when the entire region was part of a shallow sea were then covered with sand as the state emerged from the water (White, 1970). This pattern repeated itself several times, creating shorelines that are echoed in the terraces of Florida today. The springs of the SJR in the region around Lake George also owe their existence to this process. Water flows through underground passages of relic limestone reefs. The presence of this constant artesian source has also helped to preserve the present course of the SJR (Pirkle, 1971). 1.2 Resource Issues Pollution Load Reduction Goals (PLRGs) and Total Maximum Daily Loads (TMDLs) being developed in the Lower St John River (LSJ) basin downstream, and the Middle (MSJ) and Upper (USJ) St Johns River basins upstream. The upstream boundary for the study area was set at Astor, Florida. The drainage basin upstream of Astor is 8624 km2. A large portion of the USJ flood plain was converted to farming during the 20th century. Agricultural practices continue, but most of the flood plain has been bought back by the state of Florida and managed for flood control, to improve water quality, and environmental benefits. The MSJ, including the Orlando metropolitan area, is undergoing rapid urbanization. Nutrient loading from urban runoff finds its way into the SJR through surface runoff, and other sources such as septic systems and wastewater treatment plants. There is also an increase in nutrient concentration coming into the system from springs. The low gradient over the last 200 km of the river increases the residence time considerably in comparison to the upstream 300 km of the St Johns River. Historically, vast expanses of aquatic vegetation, primarily water lettuce, covered areas of the St Johns River. In the late 1800s, water hyacinth was introduced to the system. By 1900 it had out competed water lettuce. Whole sections of the River would be occasionally blocked to navigation. (Figure 2). Considered an impediment to navigation and commercial use of the river, the Army Corp of Engineers sought ways to eradicate it. For the first half of the 20th century, mechanical means were used with minimal effect. Intensive spraying of herbicides that started in the late 1940s was successful in bringing floating vegetation under control (Simberloff, 97). Figure 2: Water hyacinth on the SJR, pre 1900 However, removing the vegetation had negative consequences. The abundant vegetation in the river shaded the water column from direct sunlight, without it, more light and heat became available to algae production. The growth of aquatic vegetation removes nutrients from the water column. The application of herbicide results in the release of nutrients back into the water column as a plant decays (Moody, 1970). By one estimate, up to 600 tons of N are fixed in Lake George per year, as estimated from work in 1999 (Hendrickson, pers comm.). However, phosphorus is the limiting nutrient in the system. While a portion of the phosphorus that is loaded to the system is anthropogenic in origin, groundwater in Florida can have high natural concentrations of dissolved phosphorus due to large deposits of phosphate rock around the state (Odum, 1952). Phosphorus-laden discharge entering the Lake George from the middle and upper basin has time to attenuate due to long residence times under low-flow conditions. Management scenarios are currently being Figure 3: Algae surface scum near Lake George evaluated to determine the best course to take in reducing the likelihood of harmful algae blooms in Lake George and downstream.