A study of effects of reduction of submarine groundwater discharge on thermal habitats for manatee in a spring-fed estuary using a laterally averaged hydrodynamic model
Introduction
The Homosassa River is a spring-fed estuary located in Citrus County, on the Gulf coast of central Florida (Fig. 1). The river is about 13 km long from its headspring to the Gulf of Mexico and has a width varying from about 60 m in the upstream reach to about 305 m near the mouth (Yobbi and Knochemus, 1989). The Homosassa River is a shallow waterbody, with a mean water depth of approximately 1.2 m and the riverbed less than one meter deep from the mean tide level for most of the area of the estuary. The estuarine system includes several tributaries and distributaries, among which the Southeast Folk and the Hall River flow to the upstream area of the Homosassa River, while the Salt River and Mason Creek branch off from the main stem of the Homosassa River to meet the Gulf of Mexico (GOM). The estuary also has some other smaller tributaries, including a channel system in the town of Homosassa and the Hidden River.
The Homosassa River flows through a Coastal Swamps region, where poorly drained and saturated organic soils overlie limestone rocks of the Upper Florida Aquifer (UFA). As most of the wetland is self-percolating and barely contributes any quantifiable surface water runoff, the estuary receives an insignificant amount of freshwater runoff from its 145 km2watershed. Most of the hydrologic loading to the Homosassa River comes from the UFA in the form of submarine groundwater discharge (SGD) out of numerous spring vents located in the headspring area and in several of its tributaries, mainly the Southeast Fork and the Halls River. Locations of the springs that discharge groundwater flows to the Homosassa River are shown as solid dots in Fig. 1. The headwaters of the river are the Homosassa Springs and several other springs. The contributing groundwater field for the spring flow discharge is subject to the shape of potentiometric surface in the region and thus varies with time, depending on the precipitation pattern in the region. In a similar way that the watershed for surface water runoff is delineated, the contributing groundwater field, or springshed, is delineated based on the potentiometric contours. For the spring flows entering the Homosassa River, the area of the springshed varies around 700 km2, with a slight year-to-year variation.
Like some other estuaries along the Gulf coast of Florida, the Homosassa River estuary is under the action of microtidal forces, which are primarily semidiurnal. As the estuary is relatively shallow, it is generally well or partially mixed. The system is ecologically important for many marine species, including Florida manatees (Trichechus manatus latirostris) and Common Snook (Centropomus undecimalis), which use the spring-fed estuaries as thermal refuges in winters because a large amount of spring water with a relatively constant temperature of about 23.0 °C or higher flows to the river. These marine species can have a huge economic impact on local communities in Florida, as they can attract millions of visitors to the sunshine state. Citrus County is especially famous for manatee-related tourism in Florida. For example, the headwater area of the Homosassa River (Homosassa Springs) is a great destination for many tourists coming to Florida to visit, where one can view manatees and enjoy numerous other recreational activities. Snorkeling in the coastal springs and swimming with Florida manatees in the Homosassa River are an incredible experience, which many visitors will forever cherish.
Florida manatee is a subspecies of the West Indian manatee, which has been included on the US Endangered Species List since the 1970s. To prevent the population decline, the West Indian manatee is protected by different federal and state laws, including the Marine Mammal Protection Act of 1972, the Endangered Species Act of 1973, and the Florida Manatee Sanctuary Act of 1978 (https://myfwc.com/conservation/you-conserve/wildlife/manatee/). As a result, the Wet Indian manatee had a population increase in recent years and was reclassified from endangered to threatened in 2018 (The Law Library, 2018) under the authority of the Endangered Species Act of 1973. Nevertheless, all federal protections for the West Indian manatee under the Endangered Species Act remain in place. Manatee protection has not only provided a great ecological value to the estuary, but also benefited local communities in Florida. A study by Solomon et al. (2004) strongly supported the argument that manatee protection provides Citrus County with a significant net benefit. Their study estimated that the net benefits of manatee protection in Citrus County was approximately $8.2 – $9 million, primarily gained through eco-tourism.
There are only limited previous studies on effect of SGDs on thermal habitats in the Homosassa River estuary. HSW Engineering, Inc. (2011) applied the Environmental Fluid Dynamics Code (EFDC), originally developed by Hamrick (1992), to simulate circulations, salinity transport processes, and thermal dynamics in the Homosassa River. They had to expand and deepen the downstream area of the river to form a large funnel to allow estuarine water to be transported upstream (HSW Engineering, Inc., 2011). Clearly, this artificial alteration of the topography and bathymetry dramatically changed the shape of the estuary that was simulated and should not be made in any of hydrodynamic modeling studies. Model results from an estuary that has a dramatically different shape from the real estuary are meaningless.
As mentioned in Chen (2012), most previous coastal and estuarine hydrodynamic modeling studies did not consider effects of SGDs (Johnson et al., 1991; Blumberg and Kim, 2000), partly because SGDs are much lower than river flows and precipitations for these estuaries and partly because SGDs are very difficult to quantify in many cases. There are only very limited estuarine and coastal hydrodynamic modeling studies which considered SGDs. Ganju et al. (2012) applied the 3D model ROMS (Warner et al., 2008) to West Falmouth Harbor, Massachusetts to verify their tidal and groundwater flux estimates to the estuary based on velocity and salinity measurements. estimated and considered SGDs in the hydrodynamic simulations for Crystal River/Kings Bay using the 3D model UnLESS3D to analyze thermal habitats for manatee in the estuary. He found that SGDs in Crystal River/Kings is not only negatively proportional to the tides but also positively proportional to the first derivative of tides with respect to time.
As almost all the hydrologic loading to the Homosassa River estuary comes from SGDs from numerous spring vents, the term flow and the term SGD are exchangeable in this paper. In the following sections, the laterally averaged model LAMFE is briefly introduced, followed by a short description of model calibration and verification against measured real-time water levels, salinities, and temperatures at five fixed stations in the estuary during a 34-month period. Model simulations for a series of the SGD reduction scenarios are presented, with simulated thermal habitats during a 125-month period being analyzed. Effects of the SGD reduction on chronic and acute thermal habitats for manatee were evaluated and discussed. Conclusions of the modeling study are provided at the end of the paper.
Section snippets
Laterally averaged hydrodynamic model
Because the Homosassa River is a narrow and meandering estuary, its circulation pattern and salinity and temperature distributions vary primarily in the vertical and longitudinal directions. Vertically two-dimensional variations are typical for narrow estuaries (Prandle, 1985; Jay and Smith, 1990; Chen, 2004a), for which the most suitable simulation tool is a laterally averaged model. This study used the Laterally Averaged Model for Estuaries (LAMFE), developed by the author, for the simulation
Model calibrations and verification
For the LAMFE model application, the Homosassa River estuary was discretized with 406 grids along the river main stem and its 21 branches to ensure that the main physical characteristics of the system can be properly resolved by the model. The horizontal spacing of the grids varies between 29 and 277 m. Fig. 2 shows cross sections that form the 406 LAMFE grids along the main stem of the Homosassa River and its branches. In the vertical direction, 15 layers were used to discretize the water
Simulated thermal habitats for SGD reduction scenarios
To study effects of the SGD reduction on thermal habitats, a series of model simulation were conducted, including the existing flow condition, a baseline flow condition (BFC), and 12 flow reduction scenarios, ranging from 2.5% to 30% reduction with a constant increment of 2.5%. The BFC is an imaginary flow condition that would exist if no ground water were withdrawn in the springshed. It is estimated that the existing withdrawal causes about 1.85% reduction of SGDs entering the Homosassa River
Effects of SGD on thermal habitats for manatee
Because of their inability to survive in cold environment, manatees that are exposure to water below 20 °C for longer than 4 – 7 days could suffer disastrous losses (Rouhani et al., 2007). As a result, a time scale shorter than 96 h should be used in analyzing effects of SGD reduction on ≥ 20°C water volume and surface area in the Homosassa River. To be conservative, this study followed previous studies by Rouhani et al., 2007), JEI and ATM (2007), , and Herrick et al. (2017) and chose a time
Conclusions
Effects of the SGD reduction on thermal habitats for manatee in the Homosassa River, a spring-fed estuary located on the Gulf coast of Florida, were studied using a laterally averaged hydrodynamic model. After the model was calibrated and verified against real-time data of water level, salinity, and temperature measured at five fixed stations during a 34-month period between 11/4/2014 and 8/31/2017, it was used to conduct a series of simulations for 125 months, from October 2007 to February
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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