Connectivity of the Apalachicola River flow variability and the physical and bio-optical oceanic properties of the northern West Florida Shelf
Introduction
The Apalachicola River is an important source of freshwater and nutrients to the northern West Florida Shelf (WFS) in the northeastern Gulf of Mexico (GoM) (Fig. 1). The watershed, which includes the Chattahoochee and Flint Rivers, drains approximately 50,000 km2 of southern Georgia, eastern Alabama and north Florida. The river empties onto the northern WFS through Apalachicola Bay, an estuary with great ecological diversity that supports finfish and shellfish populations of significant economic value. The annual mean discharge of the Apalachicola River (estimated from the flow rate at Sumatra, FL, about 42 km from the mouth) is 736 m3/s, making it the largest source of freshwater to the GoM east of Mobile Bay. The river flow rate typically peaks in March with an average monthly discharge of approximately 50% above the annual mean and has a minimum in October of roughly half the annual mean. The historical river flow rate varies dramatically from its annual cycle. The daily-averaged flows measured at Sumatra, FL, from 1977 to 2000 range from a minimum of roughly 140 m3/s to a maximum of over 5000 m3/s.
Previous studies have linked precipitation variability over the southeastern United States, which encompasses the Apalachicola-Chattahoochee-Flint (ACF) river system watershed, with multiple climate modes (e.g., Gershunov and Barnett, 1998). A significant precipitation signal has been shown to be connected with the El Niño/Southern Oscillation (ENSO), which typically has higher rainfall during the warm (El Niño) phase (Ropelewski and Halpert, 1986) and reduced precipitation over the region during the cold (La Niña) phase (Smith et al., 1998). Enfield et al. (2001) also examined the interdecadal modulation of the ENSO teleconnections suggesting a greater correlation between rainfall over the southeastern United States and the Southern Oscillation during the cold phase of the Atlantic Multi-Decadal Oscillation (AMO).
The Apalachicola River is a major nutrient source for the northeastern GoM. It provides 92% of the total dissolved inorganic nitrogen to the Apalachicola Bay estuary and exports over 80 mg N/m2 per day to the GoM in winter (Mortazavi et al., 2000). This nitrogen input enhances primary productivity in the near-shore waters. Offshore of Apalachicola Bay are habitats for commercially important reef fishes, including the Middle Grounds to the southeast and marine protected areas to the south and southwest. For example, commercially important gag grouper (Mycteroperca microlepis) typically spawn at locations concentrated roughly along a hard bottom region between the 70 and 90 m isobaths south of Apalachicola Bay in February and March (Hood and Schlieder, 1992), with the larvae settling in the sea grass beds of the coastal zone in March through May (Fitzhugh et al., 2005). Impacts of the Apalachicola River nutrient and freshwater inputs on the marine ecosystems supporting fish populations over the inner, middle, and outer shelf regions remain unclear.
The large variability of the Apalachicola River flow rate manifests in variability of the freshwater and nutrient budgets for the northeastern GoM. Variations in bio-optical properties, particularly satellite-derived chlorophyll concentrations indicative of phytoplankton abundance, offshore of the Apalachicola Bay and the precipitation anomalies inland over western Georgia appear correlated at interannual time scales, particularly during the spring months. This study seeks to explain the connectivity between precipitation variability over the watershed and variability of oceanic properties offshore. The time series of the Apalachicola River flow rate recorded over a 78-year period is examined. Analyses of satellite-observed ocean color data and numerical model experiments show the response of the ocean salinity field and chlorophyll concentration estimated from the ocean color data to variability in the Apalachicola River discharge, and are used to estimate the spatial extent of the river's influence in the northeastern Gulf. Numerical model simulations predict a region of low and high salinity anomalies extending out over the northern WFS during anomalous wet and dry years in the southeastern United States. The spatial patterns of chlorophyll concentration anomalies derived from satellite data are in good agreement with the model-predicted salinity anomaly patterns over the northern WFS induced by variations in the Apalachicola River discharge. Model experiments demonstrate that wind-driven transport of the buoyant low-salinity water by synoptic scale atmospheric forcing is critical for determining the spatial extent of the river's influence over the shelf, and providing the mechanism for connecting the variability of the river discharge with the variability in oceanic properties offshore.
Section snippets
Variability of the Apalachicola River
The Apalachicola River begins at Lake Seminole near the Alabama-Georgia-Florida border, formed by the construction of the Jim Woodruff Dam in 1957 at the confluence of the Chattahoochee and Flint Rivers (Fig. 1). Along the Chattahoochee River is a series of lakes formed by the construction of several dams, the last of which was completed in 1975. The 560 km long Flint River has two major impoundments upstream from Lake Seminole built in the 1920s and 1930s. Although some surface water from these
The influence of the Apalachicola River on ocean color
Monthly climatology maps are constructed from the Sea Viewing Wide Field-of-View Sensor (SeaWiFS) satellite chlorophyll a concentration standard mapped images (SMI) from 1997 through 2006. This 9 km resolution chlorophyll a SMI product is made available from the NASA Goddard Space Flight Center Distributed Active Archive Center and is computed from the Level 2 chlorophyll a.product. Monthly anomalies from this climatology are then computed from monthly SMI chlorophyll a maps. This satellite
Model configuration
A set of numerical model experiments is performed to better understand the scope of influence of the Apalachicola River's variability in the northeastern GoM, and to explain the mechanisms responsible for linking the terrestrial climate variability with the regional oceanic variability. The Navy Coastal Ocean Model (NCOM) (Martin, 2000) has been applied to the GoM and northwestern Caribbean Sea by Morey et al., 2003a, Morey et al., 2003b at 1/20° horizontal resolution to study the transport of
Discussion and conclusions
In this paper, historical data have been analyzed to describe the variability of the freshwater discharge to the West Florida Shelf by the Apalachicola River at time scales from seasonal to interdecadal. This river is of particular regional importance for a number of reasons. First, it feeds into one of the historically least polluted estuaries in the southeastern United States (Livingston and Kitchens, 1984). Second, it is the largest point source of freshwater along the Florida coastline and
Acknowledgments
The authors would like to thank David Zierden and Melissa Griffin of the Florida State Climate Office and Dr. James O’Brien for their suggestions regarding this work. The ENSO classification used for this work was developed by the Florida State Climate Office. Gary Watry provided invaluable assistance with the figures. The NCOM development by Drs. Alan Wallcraft and Paul Martin at the Navy Research Laboratory is appreciated. This work was supported by a NASA grant as part of the Ocean Vector
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