NP Budget for the Río de la Plata Estuary, Argentina/Uruguay

S. V. Smith

7 April 1997

Figure 1. (A) Drainage basin for the Paraná, Uruguay, and smaller rivers which enter the Rio de la Plata Estuary.

(B) Map of the area immediately surrounding the Rio de la Plata estuary, at the boundary between Argentina and Uruguay.

(C) Schematic representation of the river input region (Zone III of Pizarro and Orlando, 1984) and the area budgeted here.

The Río de la Plata Estuary (Figure 1; 34-36° S, 55-58° W), between Argentina and Uruguay, is one of the large estuaries of the world. The estuary receives drainage from the confluence of the Paraná and Uruguay Rivers, with a drainage basin of about 3 x 10⁶ km² (second largest in South America, covering parts of the countries of Argentina, Uruguay, Paraguay, Brazil, Bolivia, and Peru) and an average flow (V_Q) of approximately 7 x 10¹¹ m³/yr (third largest in South America). Over 12,000,000 people live adjacent to the tidal freshwater and estuarine portions of the estuary. The total estuary area is approximately 35,000 km². Of this area, approximately the outer 22,000 km² represents the estuarine transition from freshwater to marine conditions. This is the portion of the system budgeted here. This portion is estimated to have an average depth of 10 m.

Although we do not have a great deal of information on the biotic community composition of this system, we assume that it is a plankton-dominated system. The water is highly turbid, so it is unlikely that benthic plants are significant primary producers throughout most of the estuary. From data in Carreto et al. (1986), chlorophyll in the outer part of the estuary is about 5 mg/m³. A typical C:chlorophyll ratio for plankton is about 50 mg C per mg chlorophyll, implying a phytoplankton standing crop of 250 mg C/m³, or about 2.5 g C/m² through the 10 m water column. If we assume about a 3-day phytoplankton turnover time, this gives primary production to be about 300 g C m^-2 yr^-1. An alternative approach to estimate primary production is to use the functional relationship developed by Nixon et al. (1996) to describe primary production as a function of DIN loading. This relationship gives primary production to be about 200 mg m^-2 yr^-1. This range seems a reasonable estimate of primary production in this system.

Salinity and dissolved inorganic N and P data are available for this system in Bazán and Arraga (1993), Carreto et al. (1986), and especially the comprehensive analysis by Pizarro and Orlando (1984); the data are summarized in Table 1.

Table 1. Water composition data used for the budget calculations. Precipitation, groundwater, and other water sources are ignored for this system.

The budgetary calculations do not need to consider river, sewage, and other nutrient discharges directly, because this material largely enters the box which is here labeled "inflow river water" (Zone III of Pizarro and Orlando, 1984; the outermost freshwater portion of the system). Nevertheless, some comments are useful. Pizarro and Orlando provided considerable detail on estimated sources and fates of phosphorus and nitrogen in this system. They estimated that the total (dissolved + particulate) phosphorus loading into the Río de la Plata is approximately 6 x 10⁸ mol/yr (75% river discharge; 25% local sewage) and the total nitrogen discharge is 18 x 10⁹ mol/yr (77% river; 23% sewage). By contrast, the Table 1 river inflow (i.e., Zone III, which should apparently include most direct waste inputs immediately upstream of the estuarine zone) concentrations multiplied by average river flow yield land-derived delivery to the estuary of 11 x 10⁸ mol PO₄ and 27 x 10⁹ mol NO₃ + NH₄ per year. These discrepancies seem large, especially since there are particulate and dissolved organic N and P inputs not included in the inventory calculated from river flow and inorganic nutrient concentrations. One plausible explanation is that the river fluxes have been adjusted downward for uptake by vegetation along the river banks. This downward adjustment seems large. Alternatively, Pizarro and Orlando may have missed some substantial loading terms. It would be useful to understand these biogeochemical processes in the river better, but this resolution does not affect the estuarine water and salt budgets.

At steady state, a simple box model describes salt flux (Figure 2). V_X represents mixing between the estuary and coastal ocean. Freshwater inflow (V_Q) is given above to be approximately 7 x 10¹¹ m³/yr. V_R is the residual flow to maintain a constant volume, and in this system is assumed to be -V_Q, with a salinity intermediate between the estuary and ocean (S_R = 22 psu). We solve for V_X as the unknown: 11 x 10¹¹ m³/yr.

Figure 2. Steady state hydrographic box model for the Rio de la Plata, based on water and salt budgets.

V_Q and V_X can be put into the perspective of water exchange time in the budgeted box. That box has a volume of 2.2 x 10¹¹ m³. This volume divided by the sum of V_Q + V_X = 0.12 years, or about 44 days. This is an estimate of water residence time (t ) in the estuarine portion of this estuary.

The concentration data for PO₄, NO₃, and NH₄  in Table 1 and the water flux data in Figure 2 are used to calculate the hydrographic fluxes for these dissolved nutrients (Table 2). It should be emphasized that the budget developed here is based strictly on inorganic nutrients. The values reported as DY's are the nonconservative fluxes required to balance the hydrographic fluxes. There are two important points to note about the DY values.

The first observation is that the estuarine portion of this system appears to be a sink for up to about 25% of the river PO₄ influx. However, based on the discrepancy noted above between direct river-flow calculations of terrestrial PO₄ influx and estimates which supposedly are for total (dissolved + particulate) P loading, the apparent sink for DIP might simply represent an overestimate of terrestrial PO₄ inputs. We conclude therefore that the PO₄ input from land and output to the ocean are, within the limits of resolution of this budget, in balance.

Table 2. Hydrographic fluxes of dissolved inorganic nutrients (Y's) in the Rio de la Plata Estuary, and calculated nonconservative fluxes (DY) to balance the budgets.

The second important observation about nonconservative fluxes is that at least 50% of the river dissolved inorganic nitrogen (DIN) input is not accounted for by the hydrographic flux at the mouth of the estuary. There appears to be a DIN sink of about 15 x 10⁹ mol/yr. Part (9 x 10⁹ mol/yr) of this discrepancy might be explained by an overestimate of the river DIN loading, as discussed above. Because that loading does not take into account the possibility of significant atmospheric wet and dry deposition of nitrogen, it seems clear that the estuary is a sink for at least 6 and perhaps more than 15 x 10⁹ mol/yr of inorganic N. If the small PO₄ loss of 0.3 x 10⁹ mol/yr is both real and due to organic production, then stoichiometric calculations suggest that 16 times that amount of DIN (or about 5 x 10⁹ mol/yr) might be sequestered in organic material.

The remainder of the DIN sink must have some other explanation. A net DIN disappearance of 6 to perhaps > 15 x 10⁹ mol/yr is likely to be due to the difference between nitrogen fixation and denitrification (nfix-denit). Over the area of the estuary, this is a net denitrification rate of about 300-700 mmol m^-2 yr^-1. Systems such as this typically exhibit little or no nitrogen fixation, so this rate can be assumed to represent gross denitrification. This range of rates of denitrification calculated by this budgetary procedure is close to rates reported by various authors (see summary by Seitzinger, 1988) for denitrification as measured by various assay techniques in estuarine systems.

Fluxes fluxes of dissolved inorganic nitrogen in the Rio de la Plata Estuary, and calculated nonconservative fluxes to balance the budgets.

Fluxes fluxes of dissolved inorganic phosphorus in the Rio de la Plata Estuary, and calculated nonconservative fluxes to balance the budgets.

Acknowledgments

The information in this report is based on information supplied by Dr. Raúl Guerrero, of the Universidad Nacionál de Mar de Plata, Argentina.

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