3. White Sea

3.3. Results and discussion

The mean monthly distributions of Chl, the particle backscattering and yellow substance absorption coefficients, TSM and Sea Surface Temperature (SST), derived from MODIS-Aqua data, are presented on color maps. Analysis of these maps shows that the areas of highest values of all of the considered bio-optical parameters (Chl, bbp, ag , and TSM) are associated with the North Dvina, Onega, Mezen Rivers runoff.

With respect to TSM, it can be seen from the maps, that the areas of turbid waters near the deltas of Onega and Mezen are larger than near North Dvina, although the discharge of North Dvina is greater than of two others. This can be explained by intensive tidal currents in the Onega and Mezen Bays, responsible for seashore abrasion and re-suspension of bottom sediments. It is well seen that turbid North Dvina waters propagate mainly along the eastern White Sea coast and TSM decreases rapidly towards the central part of the Dvina Bay. This is explained by along-shore cyclonic circulation in the White Sea, preventing propagation of turbid waters towards the central part of the sea. Only in May when rivers runoff is most intensive, North Dvina waters partly propagate towards the central part of the White Sea. Due to cyclonic circulation relatively transparent Barents Sea waters propagate along the western White Sea coast. The lowest values of TSM are observed in the central part of the sea, in the Kandalaksha Bay (where the river runoff is small). Values of TSM in these areas in August-September are less than 0.5-0.8 mg/l. Note that in the north-western part of the White Sea near the boundary with the Barents Sea high values of TSM are observed in August-September (2003-2005). This is associated with coccolithophore bloom in the Barents Sea (see 2.3) which waters partly propagate into the White Sea.

Spatial variability of chlorophyll concentration is usually weaker than of TSM. As a whole spatial distributions of Chl are quite similar to distributions of TSM. The highest values of Chl (usually more than 2 mg/m3) are observed in Dvina, Onega and Mezen Bays and the lowest – in central part of the sea.

The seasonal variability of suspended matter concentration is well pronounced in the central part of the sea, in Kandalaksha and Dvina Bays. Maximum values of TSM are observed in May (sometimes in June) during spring flood-time when river waters occupy the most part of Dvina Bay and partly propagate into the central parts of the sea. With decrease of river discharge the area of turbid waters in the Dvina Bay is reduced, and they propagate mainly along the eastern coast of the White Sea. This leads to decrease of TSM in the open parts of the sea and in Kandalaksha Bay.

This qualitative reasoning is confirmed by quantitative estimation of suspended matter content in different regions of the White Sea. Seasonal changes of TSM, as well as of Chl, bbp, ag, and SST in different sub-regions during the observational period (2002-2010) are shown in Fig.9, 10, 11, 12, 13. The most intensive seasonal changes (about 50%) are observed in the Dvina Bay in accordance with above reasoning. Maximal values of TSM here take place in spring and sometimes in autumn, minimal – in summer. In the central part of the White Sea and in Kandalaksha Bay increased values of TSM are observed only in May when river waters penetrate to the open sea. In other sub-regions seasonal changes are less. In addition to seasonal changes of river runoff they are influenced by other processes mentioned above.

The seasonal (May-September) mean values with their standard deviations for Chl, the particle backscattering (bbp) and the yellow substance absorption (ag) coefficients, TSM, and SST in different regions of the White Sea in 2002-2010 are given in Table 2 . Spatial and temporal variability (seasonal and inter-annual) of chlorophyll and TSM concentrations in the White Sea from satellite MODIS-Aqua data is considered in detail in [25-26].