Oxygen consumption in the Baltic proper’

Total oxygen consumption rate, averaged over 1957-1982, and its variations with salinity in the deep water of the Baltic proper are calculated by a system of diagnostic advectiondiffusion equations in a “natural” coordinate frame of reference. Although the data set used is not ideal for a more thorough investigation of the sensitivity of the model, an oxygen budget is formulated. A gross consumption rate of 3 x 10e9 mol rnd3 s-l is obtained. Further, a total consumption of about 0.14 x lop6 mol m-* s-l is found, which is equivalent to an annual carbon input of 50 g C me2 yr’. Direct measurements of the rate of oxygen consumption and its distribution over depth are rare in the Baltic. These rates are, however, essential in ecosystem and eutrophication studies. Because of the high costs in carrying out such field programs, it is tempting to try to extract information from data gathered during routine surveys over many decades. This situation is not unique to the Baltic. Hence the approach suggested below may have more general interest. Walin (1977) presented a method to extract such information for estuaries. He used conservation equations for various water properties in a “natural” coordinate frame of reference to derive basinwide average flux equations. Since isopycnal mixing is intense relative to diapycnal mixing, an approximate analysis in terms of isopycnically averaged properties may be justified. Given the inflow distribution of some substance, the corresponding advective and diffusive diapycnal fluxes are obtainable. Unfortunately, the practical and economic problems involved in determining these distributions with field measurements are considerable (Walin 198 1; Stigebrandt 1987). By assuming that various water L This work was supported by the Swedish Environmental Protection Board. properties are mixed by the same turbulence field, however, Walin (1977) was able to formulate a similar model without having to prescribe the inflow distribution. Hence a turbulent diapycnal flux function can be determined by applying this diagnostic model to the observed distributions of parameters in question, both at the entrance region and within the semienclosed estuary. This approach was used by Rahm ( 1985) in a study of salt flux in the Baltic. Salinity was chosen as the independent variable because it dominates the density field of the deep water in this region. The deep water temperature was assumed to behave like a conservative tracer in the same region. Exchange coefficients were estimated from these calculationstheir values are in accord with literature values for the Baltic Sea. The present study is dedicated to the corresponding oxygen fluxes in the same basin, using, as before, hydrographic data from ICES. Oxygen consumption is estimated by combining oxygen and temperature distributions and fluxes with regard to salinity. The two dependent variables are averaged over 19 5 71982 and hence represent the mean state of the system. Oxygen has, however, both a decreasing trend and strong variability during this period. Sometimes anoxic conditions occurred in the deeper parts (Matthgus 1984). These fluctuations are mainly due to infrequent inflows of deep water from the Kattegatt (Fig. 1). Before performing the calculations, the equations are presented. Details and derivations are found in Walin (1977), whose nomenclature is followed throughout. The flux equation for a conservative tracer like temperature, in an idealized, semienclosed estuary (Fig. 2), becomes in steady state, rn(rr T) + Fg = 0 (1)