| | | 19.4 River runoff and atmospheric forcing |
At the beginning of the annual plankton succession, the spring flood period dominates the growth conditions of phytoplankton not only because of the nutrient supply factor, but also because of the physical conditions. The density difference between the high riverine runoff and the well-mixed basin water results in a strong, diffusive river plume. Salinity differences are the main agent for springtime stratification, and the buoyancy transport from springtime river runoff can re-stratify the Gulf after winter homogenisation in a period of about 5 weeks. During summer this would take about 1 year, indicating that the river runoff plays a major role in the creation of stratification only in spring [443].
The springtime river plume implies a formation of distinct horizontal gradients within the basin, whereas during summer conditions the weak river plume quickly dissolves and creates spatial variability in the southernmost basin only. The horizontal stratification gradients during spring have significant effects on the depletion of wintertime nutrients from the water column, and thus on the forthcoming growth conditions of planktonic communities during the stratified period. The areas in the middle and northern Gulf are mixed down to the bottom far later than the southern basin, which is under immediate river plume influence [443]. Therefore significant nutrient reserves remain in the intermediate and deep layers of the southern basin after spring, whereas the northern basin is depleted of nitrate and especially phosphate down to deep layers [456].
After the establishment of thermal stratification the surface fluxes take over the buoyancy forcing by lateral freshwater eddy fluxes, and the mixed layer starts to deepen due to more variable atmospheric forcing. This implies higher temporal variability in the growth conditions of the planktonic community, and also more variability of lateral heterogeneity than during spring. An especially important feature of the vertical processes is the relation between mixed layer deepening and the depth of the euphotic zone, which in the Gulf of Riga is typically around 10 m as in adjacent Baltic Sea. Towards early autumn, the depth of the mixed layer clearly exceeded the euphotic zone, with dramatic consequences for the nutritional and production preconditions of the planktonic biota [456][351].
| | | 19.4 River runoff and atmospheric forcing |