James S. Cherry II and James D. Hagy III
US Environmental Protection Agency, Gulf Ecology Division, 1 Sabine Island Dr., Gulf Breeze, Florida 32561
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
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,
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
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
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 ?Nobs ?Nexp, where DNobs is DDIN and ?Nexp 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.
We thank J. Caffrey, Center for Environmental
Diagnostics and Bioremediation,
Figure 2. Survey
station locations and the names of major regions of the
Figure 3. Salt and water
budget for the
Figure 4. DIP budget for the Pensacola Bay System for 2002-2004. DIP flux units are 103 mol d-1.
Figure 5. DIN budget for the Pensacola Bay System 2002-2004. DIN flux units are 103 mol d-1.
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