The Pensacola Bay System, Florida, USA

James S. Cherry II and James D. Hagy III

 

US Environmental Protection Agency, Gulf Ecology Division, 1 Sabine Island Dr., Gulf Breeze, Florida 32561

 

E-mail: cherry.james@epa.gov, hagy.jim@epa.gov

 

i. Study Area Description The Pensacola Bay system (30º 27'N 87º 05'W), located in northwest Florida, USA, is a branched complex of shallow estuaries that includes Escambia Bay, Blackwater Bay, East Bay, and Pensacola Bay.  The combined system is medium sized (370 km2), has a mean depth of 3.3 meters and has a volume of 1202 x 106 m3. It has been characterized as a partially-stratified, drowned river valley estuary. The Pensacola Bay System (Fig. 2) is micro-tidal with diurnal tidal amplitudes ranging from 15 to 50 cm.  The system receives freshwater inputs from the mostly forested watersheds of the Escambia, Yellow and Blackwater Rivers.  The Escambia River discharges into Escambia Bay, whereas the Blackwater River and Yellow River discharge into Blackwater Bay. Slightly more than 60% of the freshwater input is from the Escambia River.  Total drainage area, excluding the estuary itself, is 18,318 km2.  Blackwater Bay enters East Bay, which joins Escambia Bay at Garcon Point to form Pensacola Bay proper.  Pensacola connects to the Gulf of Mexico through a narrow (~800 m wide) pass.  Flushing time (t) for the system at average freshwater inflow is ~10 days. The system is mesotrophic and regularly develops hypoxia below the pycnocline.  However, anoxic conditions rarely develop (USEPA, unpublished data).

Figure 1. Santa Rosa Sound, a part of the Pensacola Bay System, as seen looking east from US EPA's Gulf Ecology Division facilities.

 

ii. Salt and Water Balance

All computations pertain to a study period extending from April 2002 through November 2004.  Annual means were computed from seasonal means.  Three major river basins, Escambia River, Blackwater River, and Yellow River contribute freshwater to Pensacola Bay.  Data from 4 flow gauges on the three major rivers account for flow from 74% of the watershed area.  Flow from non-gauged portions of the watershed was computed from a run-off rate calculated as the discharge/watershed area from Pond Creek, a small (152 km²) gauged basin near the non-gauged portions of the watershed.  Estimates of direct precipitation and evaporation from the Bay surface were obtained from weather stations at Pensacola Regional Airport and Milton, FL.  Total freshwater inputs from gauged and non-gauged portions of the watershed averaged 314 × 10⁵ m³ d^-1(Fig. 3).  Direct precipitation averaged 14 × 10⁵ m³ d^-1 (3.9 mm d^-1) whereas precipitation minus evaporation was 4.1 × 10⁵ m³ d^-1 (Fig 3).

 

Salinity and nutrient data were monitored on 29 surveys conducted approximately monthly from April 2002 through December 2003 and from May 2004 through November 2004.  The surveys included 15 sites oriented along two transects of Pensacola Bay (Fig. 2).  Vertical profiles of salinity were obtained using a SeaBird CTD.

 

iii. DIP Balance

 

Dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) concentrations for freshwater inputs were computed as a flow-weighted mean of concentrations in the Escambia River (P01) and Blackwater River (P10) (Fig. 2).  Non-gauged freshwater inflows were assumed to have the same concentration.  Nutrient (NO₃^-, NO₂^-, NH₄⁺, PO₄³-) samples were obtained from surface and bottom waters using either a Van Dorn bottle or a submersible pump and were analyzed in the laboratory using standard automated colorimetric methods.

 

DIP concentrations averaged 0.15 µM in the freshwater inputs, diminishing slightly to 0.14 µM within the estuary, and decreased to 0.10 µM at the seaward end member.  ?DIP was 2.7 10³ mol d^-1.  Positive DIP flux indicates a net internal release of DIP into the estuary (Fig. 4).

 

iv. DIN Balance

 

DIN concentrations in water flowing into the system from riverine sources averaged 13.9 µM, whereas concentrations in precipitation averaged 15.4 µM.  DIN concentration diminished within the estuary to 4.3 µM, and further decreased to 1.19 µM at the seaward end member. DIN concentration in rainfall was computed as the average of the mean from three sites at the northern end of the Escambia Bay (data from J. Caffrey, unpublished) and a single site near the lower bay (data from J. Lehrter, unpublished). Dry nitrogen deposition into Apalachicola Bay, east of Pensacola Bay, has been previously determined to be 52% of wet deposition. Likewise, dry nitrogen deposition into Mobile Bay, west of Pensacola Bay, was determined to be 58% of wet nitrogen deposition (Myers et al 2001).  Based on these data we estimated dry nitrogen deposition into the Pensacola Bay system to be 55% of wet deposition.  ?DIN for the estuary was -108 10³ mol d^-1 indicating that internal DIN losses exceeded internal DIN sources (Fig 5).

 

v. Stoichiometric Calculations

 

Estimates for biogeochemical processes were made assuming C:N:P =106:16:1. Therefore, net ecosystem metabolism (NEM), defined as the difference between carbon production and respiration (p-r), was estimated as -?DIP×(C:P).  NEM was computed to be -2.8 10⁵ mol C d^-1 (-0.76 mmol C m^-2 d^-1), indicating that the estuary was net heterotrophic.

 

Net denitrification (nfix-denit) was estimated as ?N_obs – ?N_exp, where DN_obs is DDIN and ?N_exp is DDIP×(N:P).  Accordingly, (nfix-denit ) = -1.5 10⁵ mol d^-1 (-0.41 mmol N m^-2 d^-1­), indicating that denitrification exceeded nitrogen fixation.

 

vi.  Acknowledgements

 

We thank J. Caffrey,  Center for Environmental Diagnostics and Bioremediation, University of West Florida, Pensacola, FL., and J. Lehrter,  US Environmental Protection Agency, National Health and Environmental Effects Research Laboratory, Gulf Breeze, FL, for sharing unpublished data.

  

 

 

 

Figure 2. Survey station locations and the names of major regions of the Pensacola Bay system.  The inset shows the location of Pensacola Bay (indicated by arrow) and its watershed (shaded grey) within the Gulf of Mexico.