The relative densities of Baltic Sea waters have been measured from 3.5 to 20‰ salinity and 0.36 to 20°C with a vibrating densimeter. The directly measured densities were compared with those determined from the seawater equation of state (Millero, Gonzalez and Ward, Journal of Marine Research, 34, 691-693, 1976) at the same true salinity given by S(‰)T = a + bS(‰), where a is related to the river water input of dissolved solids, b = (35.171-a)/35.000 and S(‰) = 1.00566 Cl(‰). By adjusting a to 0.123±0.005 g kg-1 the differences between the directly measured and calculated densities had a minimum standard deviation of 8.7 × 1--6 g cm-3. The value of a determined from the density data is excellent agreement with the value (0.121 ± 0.019 g kg-1) determined from the composition of these same samples (Kremling, 1969, Kieler Meeresforschungen, 24, 1-20, 1970; Deep-Sea Research, 19, 377-383, 1972b). the measured densities have been fitted by at least squares method to the equation: d = d0 + AS(‰)T + BS(‰)T sol;2 3, where d0 is the density of pure water (Kell, Journal of Chemical and Engineering Data, 12, 66-69, 1976), A and B are temperature dependent parameters. The densities fit this equation to a standard deviation of 7.1 × 10-6 g cm-3. The smoothed densities are in good agreement (± 6ppm) with the results of Knudsen et al. (Kongelige Danske Videnskabernes Selskabs, 1, 1-151, 1902) providing the comparisons are made at the true salinity. These results demonstrate that the densities of a natural estuary are equal (within experimental error) to those of seawater diluted with pure water when compared at the same total dissolved solid concentration, which is in agreement with theoretical calculations (Millero, Marine chemistry in the coastal environment. A.C.S. Symposium Series Vol. 18, pp. 25-55, 1975) and measurements on an artificial estuary (Millero, Lawson and Gonzalez, Journal of Geophysical Research, 18, 1177-1179). The physical chemical properties of the Baltic or any estuary can thus be determined from those of seawater diluted with pure water by using only the river input of total solids (a).
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