NP Budget of Chesapeake Bay, USA

S. V. Smith and K. L. Webb

14 February 1999

Chesapeake Bay  is the largest estuary in the United States and perhaps the most intensively studied estuary in the world.  Among the wealth of information about the system are several budgets of nitrogen and phosphorus fluxes, as well as many component studies. Of specific use for comparison in the present analysis are the detailed analysis of total N and P budgets presented by Boynton et al. (1995) and the assessment of gross and net ecosystem metabolism by Kemp et al (1997). Much of the material presented here--both the budgetary information presented and literature citations--are taken with little or no modification from that publication. Another major data source has been the U. S. Environmental Protection Agency WWW home page for their Chesapeake Bay Program (http://www.epa.gov/r3chespk). Another important research program dealing with the bay is the National Science Foundation Land Margin Ecosystems Research (LMER; http://www.mbl.edu/html/ECOSYSTEMS/lmer/lmer.html) program dealing with Chesapeake Bay (http://www.chesapeake.org/ties/ties.html). Because of the rich data availability and specifically the earlier budgeting activities, the system is therefore ideally suited for comparison between the LOICZ budgeting approach (Gordon et al., 1996; also see the Guidelines described in these web pages) and this other relevant information.

Chesapeake Bay (37-39° N; 76° W) covers an area of approximately 11,000 km² on the Atlantic coast of the U. S. The average depth of the system is about 7 meters, and the system volume is about 78,000 x 10⁶ m³. The watershed of Chesapeake Bay occupies approximately 160,000 km² and has a population of approximately 15 million persons. About half of the watershed area is forested and a quarter is in cropland or pasture land. The watershed includes 3 major urban areas immediately adjacent to the bay: Baltimore, Washington, and the so-called Virginia Tidewater region (including the cities of Norfolk, Newport News, Hampton, and Portsmouth. Each of these metropolitan areas has a population of about 1 million persons.

Total river flow into the bay is approximately 60 x 10⁶ m³/year, with the Susquehanna (36 x 10⁶), Potomac (10 x 10⁶), and James (8 x 10⁶) dominating the flow. Direct precipitation averages about 1,000 mm/year and evaporation over the bay averages 500 mm/year. This difference (500 mm/year) is equivalent to a net freshwater input volume of about 10% of the river flow and is not included in this budget.

The salinity of the system has strong horizontal gradation, from freshwater conditions at the heads of the subestuaries to salinities of approximately 25 psu in the outermost portion of the bay. Salinity immediately outside the bay mouth averages about 29 psu.

Biotically, the system is diverse. Primary production is dominated by phytoplankton, but seagrasses and salt marshes are also important primary producers in the system. Primary production averages about 660 g C m^-2 year^-1 along the mainstem, although with low production (~ 100 g m^-2 year^-1) near the head of the bay (Kemp et al., 1997). Within the mainstem, plankton account for about 2/3 of the primary production. The bay has traditionally been important for fisheries, with oysters, crabs, and fin fishes, having been historically important.

In order to account for strong gradients in water composition for budgeting purposes, the bay was divided into 6 "boxes" in series: (1) north of 39.2; (2) 38.7-39.2; (3) 38.2-38.7; (4) 37.7-38.2; (5) 37.2-37.7; (6) south of 37.2 and east to the line between Cape Charles and Cape Henry, at the bay mouth. Oceanic data were taken from stations immediately beyond the oceanic boundary (Figure 1).

The areas and volumes of each box are shown in Table 1.

Table 1. Dimensions of budget boxes; river inflow into each box.

Roughly, Box 1 is dominated by the Susquehanna River inflow into the mainstem of the bay; Box 2 has little freshwater inflow but receives waste discharge (including nutrients) from Baltimore; Box 3 receives inflow from the Choptank River. These three boxes comprise the "upper mainstem estuary." Box 4 is dominated by the Potomac River, which receives waste inputs from the Washington metropolitan area. These 4 boxes approximate the Maryland portion of the bay, while the remaining two boxes approximate the Virginia portion. Box 5 receives discharge from the Rappahannock River, and the York River enters the bay at the boundary between Boxes 5 and 6. Box 6 receives discharge from the James River. Boxes 5 and particularly 6 receive wastes from the cities of the Virginia Tidewater region. Each of the rivers listed above and several smaller rivers are considered to be "subestuaries" draining into the main estuary of Chesapeake Bay.

The decision was made to develop the model at steady state, using annual, vertically-averaged data within each box. Data used were the "bay mainstem data," downloaded from the US EPA Chesapeake Bay Program. The annual average between 1985 and 1995 was compiled for each variable, then the averages of these annual data were calculated. Variables budgeted were salinity, DIP, DOP, DIN (NO₃ + NO₂ + NH₄), and DON. Means were calculated for PP and PN as well, although these data were not budgeted directly.

Loading data were compiled as follows: Total P and total N loading into the Maryland mainstem portion of the bay, and the Choptank, Patuxent, and Potomac subestuaries were taken directly from Boynton et al. (1995), and assigned to the appropriate boxes according to the following considerations. Boynton et al. divide loading terms into "above fall line" and "below fall line," for both point sources and diffuse sources; and atmospheric deposition (wet deposition only, so an underestimate of total atmospheric deposition). Point-source discharge was assumed to consist entirely of dissolved inorganic nutrients. As estimated from data in Boynton et al., diffuse discharge was apportioned 0.2/0.1/0.7, and 0.7/0.2/0.1 among dissolved inorganic, dissolved organic, and particulate forms of P and N, respectively. Atmospheric deposition was apportioned 0.1/0.0/0.9 and 0.6/0.3/0.1 for P and N, respectively. Other considerations with respect to these loadings were: all point-source discharge into the Maryland mainstem were attributed to Box 2 (on the assumption that Baltimore discharge, which enters the mainstem via the Patapsco subestuary, is the dominant mainstem point-source discharge); all diffuse discharge to the Maryland mainstem was assigned to Box 1 (on the assumption that this discharge is dominated by the Susquehanna River. Point source and diffuse discharge data for Virginia (other than that already accounted for by the Potomac) were collected from various sources. Assumptions about apportionment among the forms of P and N were the same as described above, and atmospheric deposition were extrapolated on an areal basis from the Maryland data in Boynton et al. The decisions about assignments of flow by box would affect details about the biogeochemical function of these individual boxes but do not affect interpretation of the overall bay.

Using these criteria for box assignments, Table 2 summarizes the water, dissolved inorganic and organic, and particulate N and P (DIN, DON, PN, DIP, DOP, PP) loading for Chesapeake Bay.

Table 2. Loadings of fresh water, phosphorus, and nutrients into Chesapeake Bay.

WATER

106 m3/yr

DIP

106 mol/yr

DOP

 106 mol/yr

PP

106 mol/yr

TP

106 mol/yr

DIN

106 mol/yr

DON

106 mol/yr

PN

106 mol/yr

TN

106 mol/yr

Figure 2. Combined water and salt budgets for Chesapeake Bay.

These data are used to calculate the water, salt, and dissolved nutrient budgets calculated according to the LOICZ Guidelines. Because these calculations rely so heavily on the Boynton et al. (1995) data set, it is useful at this point emphasizing the major methodological differences between these two budgeting approaches. First, the Boynton et al. budgets are for total N and P, without direct partitioning among the forms of N and P (although some inferences about these forms are drawn); by contrast, the budgets presented here are primarily for dissolved N and P, although some inferences are drawn about particle flux. Second, the Boynton et al. budget depends on these input fluxes and component outputs of sedimentation and denitrification to estimate the hydrographic fluxes of TP and TN between the bay and ocean or between the subsystem of interest and the mainstem of the estuary. By contrast, the approach employed here according to the LOICZ Guidelines uses salt and water budgets to derive the hydrographic fluxes of dissolved components, uses mismatches between the terrigenous + atmospheric inputs and the hydrographic outputs of these dissolved materials to calculate the summed "internal sources-sinks" of these materials, and then uses simple stoichiometric assumptions to partition P and N fluxes among primary production - respiration (here designated as [p-r]; also known as net ecosystem metabolism) and nitrogen fixation - denitrification (nfix-denit).

Figure 2 summarizes the steady state water and salt budgets, with the bay divided into the 6 boxes. River water driving the residual flow primarily enters in Box 1 (Susquehanna River), although Boxes 4 (Potomac) and 6 (James) also receive relatively high flow. Precipitation, evaporation, groundwater and sewage are ignored in this water budget.

Net residual flow at the bay mouth is about 58 x 10⁹ m³ year^-1. Mixing between Boxes 1 and 2 totals about 33 x 10⁹ m³ year^-1--about the same as residual flow between those boxes. Mixing rises rapidly down the axis of the bay and varies between 200 and 370 x 10⁹ m³ year^-1 along the remainder of the system. Clearly mixing dominates the water exchange between the bay and ocean. As pointed out by Boynton et al. (1995), the hydraulic residence time (the time for river water to fill the basin) is about 1.75 year. However, the bay exchange time (t , including mixing) is V_bay/(|V_R6|+V_X56) = 0.19 year.

Figure 3. Chesapeake Bay DIP budget.

Figures 3 and 4 summarize the dissolved inorganic and organic P (DIP, DOP) budgets for the 6 boxes. Note that particulate P (PP) is not budgeted directly, although this constitutes about half the terrigenous + atmospheric P input to the bay (Table 2). It is assumed that this PP input either reacts within the system (entering the dissolved pool) or sediments there. DIP is the other large input of P to the system. This assumption is consistent with the discussion in Boynton et al., in which it is concluded that the subestuaries appear to be net sedimentation systems. The boxes tend to be net DIP sinks, with the total system sink (D DIP) being about -130 x 10⁶ mol year^-1. This represents about 85% of the terrigenous DIP discharge into the system. DOP input to the system is relatively small, and D DOP totals only about -10 x 10⁶ mol year^-1 (~40% of the input).

Figures 5 and 6 summarize the dissolved inorganic and organic N (DIN, DON) budgets. In contrast with phosphorus, particulate nitrogen (PN) is not a large input; PN constitutes only about 7% of the terrigenous + atmospheric input (Table 2). The very low PN:PP ratio of the input (4:1) contrasts with the higher ratio of land plants (~ 20:1; Vitousek et al., 1988) and soil (~7:1; Brady; 1990) and is indirect evidence that the PP input is dominated by inorganic, rather than organic, material. With respect to the N inputs, a further important point to note is that atmospheric deposition constitutes about 10% of the terrigenous + atmospheric loading. An important aspect of this observation is the fact that the atmospheric input estimate is based on wet deposition only. One might anticipate that dry fallout could be of a similar magnitude.

DIN constitutes almost 80% of the terrigenous + atmospheric N loading, and almost all of that input is taken up within the bay (D DIN ~ -7,600 x 10⁶ mol year^-1). DON is most of the remainder of the input, but according to the budget, DON production in the bay is and about 75% as large as this input (D DON ~ +1,100 x 10⁶ mol year^-1). Because these nonconservative fluxes are based on the difference between dissolved material input from land and atmosphere and hydrographic exchange with the ocean, a higher input would imply that the actual nonconservative flux of DIN and DON would differ somewhat from the fluxes calculated according to this procedure.