Spreading of juvenile freshwater in the Baltic proper

In low‐saline estuaries, “juvenile freshwater,” that is, freshwater trapped by the seasonal thermal stratification, may be used as an almost neutrally buoyant natural tracer for dispersion studies. Most of the juvenile freshwater supplied to the Baltic proper enters the northern parts by runoff from rivers and outflow of low‐saline water from the Bothnian Sea. The mean monthly spatial distribution of juvenile freshwater in the Baltic proper in the period April to August is studied using historical hydrographic data. Month‐to‐month changes of the distribution reveal a fast southward spreading that only partly can be explained by the quite slow mean circulation in the Baltic proper. Most of the spreading is probably due to currents forced by fluctuating winds. In early spring, when the surface temperature is around the temperature of maximal density, the spreading of juvenile freshwater may initiate the seasonal stratification, which in turn starts the spring bloom. The spreading route of juvenile freshwater in early spring, as deduced in this paper, is in qualitative agreement with the spatial progress of the observed time for the start of the spring bloom of phytoplankton. The use of juvenile freshwater as a tracer should help in the interpretation of the spreading of tracers (e.g., pollutants) from local sources, as well as helping to verify recipes for calculating the spread of tracer clouds by different mechanisms. The monthly averaged spatial distributions of juvenile freshwater in the Baltic proper presented in this paper should be useful for testing dispersion properties of state‐of‐the‐art three‐dimensional ocean circulation models.