NP Budgets for three Japanese Bays

S. V. Smith and T. Yanagi

13 April 1997

Figure 1. Map of the island of Honshu, Japan, showing the locations of the three bays used for these budget calculations.

Water, salt, inorganic P, and inorganic N budgets can be calculated for three large embayments in Japan (Figure 1): Tokyo Bay (1,000 km²; 17 m average depth; 35.5 °N, 139.9 °E); Ise Bay (1,950 km²; 17 m average depth; 34.8 °N, 136.8 °E); and Osaka Bay (1,500 km²; 28 m average depth; 34.5 °N, 135.2 °E). The three bays are described in a single write-up, because their close geographic proximity and similarities in terms of both nutrient loading and general physiography provide an interesting replication of estimated biogeochemical function under similar circumstances. These systems are being studied as part of the Japanese TOICZ program (Tokyo, Osaka, and Ise bays Coastal Zone Study). All three systems are adjacent to major urban areas and receive relatively high nutrient loading.

Phytoplankton are apparently the major primary producers in the three systems, and some data are available for primary production in this system. Tokyo Bay experiences very high primary production (~2,100 g C m^-2 yr^-1) compared to the other two systems (Ise ~ 500; Osaka ~300 g C m^-2 yr^-1). All three appear to be highly productive.

The three systems are subjected to relatively low freshwater input from runoff and a near-balance between precipitation and runoff (Figure 2). Salt and water budgets can be used to establish water exchange rates (see salt and water budgets). The residual water flow (V_R) all three systems is outward, as shown on the figure. Residual salinity (S_R) is calculated as the average of the bay and ocean salinity for each system. Slight salinity gradients between the bays and the adjacent ocean data allow calculation of V_X, the exchange volume between the bays and adjacent coastal ocean, under the assumption of steady state salinity and water volume. Water exchange time (t ) is then calculated as the bay volumes (V_SYSTEM) divided by the sum of (V_X + |V_R|), the total water exchange at the bay mouths. As shown in the figure, the exchange time varies from 0.07 yr (i.e., about 25 days) for Ise Bay, through 0.09 yr (33 d) for Tokyo Bay, to 0.17 yr (62 d) for Osaka Bay.

Figure 2. Hydrographic exchange estimates for Tokyo, Ise, and Osaka Bays, based on water and salt budgets and calculated as described elsewhere

Figure 3. Estimates of exchange and nonconservative fluxes of dissolved inorganic P (DIP) for Tokyo, Ise, and Osaka Bays, calculated as described elsewhere

Figure 4. Estimates of exchange and nonconservative fluxes of dissolved inorganic N (DIN) for Tokyo, Ise, and Osaka Bays, calculated as described elsewhere

Figures 3 and 4 summarize the budgets (see discussion on nutrient budgets) for dissolved inorganic P (DIP) and dissolved inorganic N (DIN) in these three systems. Hydrographic and mixing fluxes for the nutrients, as well as the nonconservative nutrient fluxes (D Y’s, where "Y" is either DIP or DIN) for the three systems can be calculated using the values established for V_R and V_X (above). Loading estimates from land are shown on the figure as "V_QY_Q", even though the data were actually reported directly as mass loading of N or P. It is assumed that most of that loading is inorganic, although we do not have the data to be sure of that. Atmospheric loading data are available for DIN, and are assumed to be 0 for DIP.

The figures are instructive in bringing out the magnitudes of the nonconservative fluxes with respect to the conservative fluxes in each of the systems individually (see discussion on scaling). The primary point to note is that, while all of the bays seem to take up both DIP and DIN (i.e., D DIP and D DIN are negative), these nonconservative fluxes are large with respect to the conservative fluxes only in the case of Tokyo and Osaka Bays. About half the terrestrial input of DIP into Tokyo Bay is apparently taken up; about a third is taken up in Osaka Bay. Only 6% is taken up in Ise Bay. We can conclude that, for Tokyo and Osaka Bays, the budgets are robustly demonstrating nonconservative behavior of these nutrients; conclusions for Ise Bay are less robust. On average, the three bays take up about a third of the terrigenous N and P discharged into them.

Another feature of the data becomes more apparent if the nonconservative fluxes are normalized with respect to bay area (Table 1); then we can compare the biogeochemical performances of these three systems. Stoichiometric arguments presented elsewhere (see stoichiometry) biogeochemical significance of the nutrient fluxes. Organic matter being produced and consumed in this system is assumed to be dominated by plankton, with a C:N:P ratio (C:N:P)_part assumed to be about the Redfield ratio of 106:16:1. This ratio is used to calculate net metabolism (p-r) from D DIP.

Table 1 summarizes the rates of primary production (p; from independent data), net ecosystem production ([p-r]; where this quantity is estimated as -106 x DDIP), and respiration (r; by difference between the previous terms). In addition and also following the stoichiometric arguments, estimates are provided for net nitrogen fixation - denitrification ([nfix - denit]), by comparing the observed nonconservative fluxes of DIN with those fluxes expected from DDIP. All of these data are normalized per unit area of the systems, for ease of intercomparison among the systems.

Table 1. Estimated rates of nonconservative nutrient fluxes, primary production (p), respiration (r), (p-r) and (nfix-denit) for the three systems.

In all three cases, net ecosystem metabolism (that is, [p-r]) is small, relative to primary production. That is, net production appears to be a sink for 10% or less of primary production. One uncertainty in these calculations is the possibility that some of the P and N loading attributed to inorganic material is actually organic. Another uncertainty is that the budgets are for dissolved inorganic nutrients; we do not have inventories of the organic nutrient fluxes. These two sets of adjustments might alter the interpretation of net metabolism in these systems.

All three systems appear to be denitrifying more nitrogen than they are fixing (i.e., [nfix-denit] is a negative quantity). If we assume (reasonably) that nitrogen fixation is insignificant in these systems, then the rates of [nfix-denit] should approximate denitrification alone. The estimated rates are very similar to rates of denitrification estimated by assay techniques in other coastal ecosystems (e.g., Seitzinger, 1988).

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