Flows, Salinity, and Some Implications for Larval Transport in South Biscayne Bay, Florida

A numerical model was developed to describe the currents, residence times, salinity patterns, and larval transport in a South Florida lagoonal system. Model forcing included tides, wind, and freshwater inflows prescribed from 4 yrs of recorded observations. Physical and biological performance tests were applied to two hindcasts. Computed tidal currents agreed with observations to within 0.01 m s. Transports in ocean inlets explained 87% of observed variance over a 45-d period. The average difference of computed salinities, expressing the effects of model advective fields, and historic salinity data from 54 locations was less than 2 psu. Computed residence times varied widely from several months in the more enclosed Barnes Sound, to on the order of a month in the western parts of South Biscayne Bay, and to near zero in the vicinity of the ocean inlets. The hydrodynamic model was also coupled to one for the Lagrangian drift dynamics and recruitment of post-larval pink shrimp to study both passive-particle and behaviorallymediated drift of population cohorts. Simulations of drift trajectories of pink shrimp larvae and shrimp spatial abundance distributions were in agreement with settlement patterns of live shrimp larvae from synoptic sampling surveys carried out over the last 25 yrs. The coupled models could be used to evaluate ecosystem effects and risks to coastal marine resources from a regional redistribution of surface and groundwater. South Biscayne Bay, Card Sound, and Barnes Sound make up a system of connected shallow barrier island lagoons on the southeastern coast of Florida. This lagoonal system is connected to the Atlantic Ocean to the east and is bordered by mangrove shorelines and the city of Miami to the west (Fig. 1). Located at the northernmost end of the Florida Keys coral reef tract, Biscayne Bay is a unique tropical marine environment of national significance, renowned for its productive ecosystem, diverse and abundant natural resources, and spectacular scenic beauty. The Bay plays a critical role in the function and dynamics of the larger Florida Keys coral reef ecosystem as a downstream receptor of larvae and juveniles from offshore spawning adults, and a source point for adults that migrate to the reef tract (Ault et al., 1999a,b, 2001). The Florida Keys currently supports a multibillion-dollar tourist and fishing economy. Rapidly growing human populations, overfishing, habitat degradation, pollution and anticipated changes in regional water quality from Everglades ‘restoration’ makes the south Florida coastal region an ‘ecosystem-atrisk’ as one of the nation’s most significant, yet most stressed, marine resource regions under management of NOAA, the State of Florida, and the National Park Service (Bohnsack et al., 1994, Ault et al., 1997, 1998, 2001; Schmidt et al., 1999). A network of drainage canals completed during the second half of the 20th century has greatly altered the distribution of freshwater within the watershed, and therefore also the quantity, quality, and timing of freshwater discharges to Biscayne Bay (Larsen et al., 1995). The canal system was originally put in place to provide drainage, but was subsequently enhanced to serve the additional functions of flood and salinity-intrusion control. Because of the naturally flat topography of adjacent wetlands and the shallow phreatic (free surface) aquifer, the management of the hydrologic system was constrained to a very narrow water table range and a small soil water storage capacity. These constraints 696 BULLETIN OF MARINE SCIENCE, VOL. 72, NO. 3, 2003 necessitated alterations in the quantity, quality, and temporal distribution of freshwater runoff to the Bay, which became more pulsed with larger peak discharges in the wet season. During the dry season less freshwater reached Biscayne Bay because of the reduced terrestrial storage and lowered groundwater levels (Larsen et al., 1995). Restoration of freshwater flows to the Bay has become a regional water supply allocation issue Figure 1. Location map of Biscayne Bay study area with landmarks. The inset shows the larger South Florida region. White lines on land show the principal canal network. 697 WANG ET AL.: FLOWS, SALINITY AND LARVAL TRANSPORT IN BISCAYNE BAY, FLORIDA closely linked to similar issues facing the major coastal environments of the modern day modified drainage basin, which includes metropolitan Miami and the Everglades National Park (ENP). In the present system, salinity variations in Biscayne Bay result primarily from canal discharges through gated control structures managed to meet the municipal water supply, agricultural, and flood control objectives. Additional, but smaller freshwater exchanges in the Bay are driven by overland runoff, rainfall, and evaporation. Along with the creation of the canals, profound changes in groundwater seepage to the Bay occurred over the past several decades. Historically, a coastal ridge acted as a groundwater divide with water west of the ridge flowing predominantly in a southwesterly direction away from Biscayne Bay and toward Florida Bay, while east of the ridge seepage was mainly toward Biscayne Bay (Fennema et al., 1994). In the present extensively managed system, canals have punctured massive holes through the ridge, voiding its flow-dividing role, and hydraulic gradients of the phreatic aquifer slope towards Biscayne Bay even far west of the ridge. As a result, the Bay now not only receives the rainfall accumulation in the local watershed, but also large amounts of groundwater from west of the ridge, including from the ENP. This groundwater seeps into the canals and is released to tide to avoid flooding. The quantity of direct groundwater seepage out of Bay bottoms is now but a small fraction (less than 5%) of these canal flows (C. Langevin, USGS, pers. comm.). This quantity is a significant reduction from predrainage seepage because the groundwater table has been lowered resulting in smaller hydraulic gradients toward the coast. Models of the South Florida Water Management District (SFWMD) estimated that an additional 1200 ¥ 10 m of freshwater is discharged to the Bay per year (Larsen et al., 1998) as a result of the anthropogenic modifications to the hydrologic system. Not all of this water is drained from the ENP; some of it is due to reduced evapotranspiration from the changes in land use, urbanization, and drainage. The salinity variations and the hydrodynamic regime in part established by the freshwater runoff have been important controls on the type and health of biota and flora found in the Bay (e.g., Berkeley and Campos 1984, Serafy et al., 1997, Ault et al., 1999a,b). Over the last several decades, the Florida Keys and the south Florida coastal bay systems have undergone dramatic environmental changes which have led to an intensive effort to restore the South Florida ecosystem by altering the hydrology to a more natural condition and taking other management actions. The Everglades restoration program includes a comprehensive effort to understand and model the physical and biological processes of Florida and Biscayne Bays and their connectivity to the coral reef tract. A quantitative understanding of the role of cross-shelf ontogeny, essential habitats, and animal survivorship in the community dynamics of the regional ecosystem is critical to the sustainability and conservation of key natural resources (Bohnsack and Ault 1996, Lindeman et al., 1999, 2000). In this study, we describe the development of a hydrodynamic transport and salinity prediction model to help quantify the effects of water management options. The model is intended as a tool for evaluating the effects of regional redistribution of surface and groundwater on the salinity and biophysical regime of shallow lagoonal systems. As a first step, the hydrodynamic and mass transport has been integrated into a coupled biophysical 698 BULLETIN OF MARINE SCIENCE, VOL. 72, NO. 3, 2003 prey-predator model to assess the impacts of water management strategies on important fish and macroinvertebrate resources.