Mathematical modelling indicates that POC and DOC concentrations depend on light, water temperature and nutrient availability (Dzierzbicka-Głowacka

et al., 2010, Almroth-Rosell et al., 2011 and Segar, 2012). Organic substances are exchanged horizontally through the Danish Straits with the North Sea (Thomas et al., 2005 and Kuliński and Pempkowiak, 2011). The OC concentration depends on distance from the land – coastal and estuarine areas are more abundant in organic matter than the open sea (Witek et al., 1997, HELCOM, 2005 and HELCOM, 2006). Plankton activity may contribute to large seasonal find more fluctuations in both POC and DOC (Dzierzbicka-Głowacka et al. 2011). Although numerous studies have been carried out regarding the organic carbon concentration and its dynamics in Baltic seawater, most factors affecting its spatial and temporal distribution still require quantification. For example, nothing is known about the differences in carbon concentrations in the different sub-basins of the Baltic Sea. As changes in both particulate and dissolved organic matter concentration are to be expected

in the near future (Dzierzbicka-Głowacka et al. 2011), the acquisition of basic knowledge regarding this important component of seawater is a matter of primary importance. POC and DOC concentrations in Baltic seawater and the factors impacting on both in seawater were the subject of this study, carried out in the southern Baltic in the period 2009–2011. The following questions were addressed: 1) What Avasimibe is the dynamics of the DOC and POC components in the Baltic Sea? 2) Do the dynamics and

concentrations of both components differ in the sub-basins of the Baltic Sea? 3) What factors influence POC and DOC concentrations? The answers obtained are given in this paper. One of the largest brackish seas in the world, the Baltic Sea lies between latitude 54°N and 66°N and between Amylase longitude 10°E and 30°E. This inland shelf sea is flanked by the Scandinavian Peninsula in the north and the east, continental Europe in the south and the Danish islands in the west. It is connected with the North Sea by the shallow Danish Straits, and the Kattegat and Skagerrak. The salinity of the surface sea water layer in the Baltic Proper is ca 7.1. This is a consequence of the large freshwater runoff from the catchment area and the limited exchange of water with the North Sea. Other factors contributing to the low salinity are the abundant precipitation and the shallowness of the sea (average depth = 53.2 m). The considerable inflow of nutrients from rivers and the atmosphere makes the Baltic one of the most productive marine ecosystems in the world. Occasional inflows of highly saline water masses from the North Sea lead to water stratification – the halocline lies at a depth of 70 m. The inflows also contribute to a north-eastward decrease in salinity (Hakanson 1991, Hagström 2001, Thomas et al.