We would like to thank Elena Couñago for her help in preparing the cartography and to Cristina Santa Marta

and Lobo Orensanz for their careful and critical reading of the manuscript. “
“The presence in seawater of dissolved and suspended organic substances, treated collectively as organic matter, means that this medium is not just a solution of inorganic salts. Organic matter plays a key role in a variety of natural (physical and biological) processes occurring in the marine environment, especially in DAPT chemical structure shelf seas like the Baltic, where its concentration is substantial (Seager and Slabaugh, 2004 and Kuliński and Pempkowiak, 2008). These processes include oxygen depletion, as well as complex formation with both organic

and inorganic compounds, which facilitates the downward transport of chemical substances (C, N, P, heavy metals, organic pollutants) in the water column. Organic matter influences the chemical Torin 1 concentration and physical properties of seawater, including the light field and alkalinity (Dera, 1992, Hedges, 2002 and Kuliński et al., 2014). Aquatic organic matter is commonly divided into particulate organic matter – POM and dissolved organic matter – DOM. Both fractions are important components of the carbon cycle. POM in the marine environment is composed of phytoplankton, zooplankton, bacteria and dead organic material (detritus), while dissolved organic matter comprises molecules of both high and low molecular weight. Both POM and DOM originate from internal and external sources (river run-off, atmosphere, sediments) (Emerson & Hedges 2008). Organic matter is most often measured as organic carbon (OC), which makes up some 45% of organic matter (Chester 2003). In the oceans, the OC concentration is < 1.5 mg dm− 3, but in coastal areas it amounts to as much as 8 mg dm− 3 (Hansell, 2002 and Gardner et al., 2006). Like organic matter, organic carbon

is for practical purposes divided into two principal fractions: particulate organic carbon (POC) and dissolved organic carbon (DOC). MTMR9 Both fractions can be separated by passing seawater through, for example, 0.4 μm glass-fibre filters. The POC and DOC concentrations in the Baltic Sea have been a subject of interest for many years (Jurkovskis et al., 1976, Pempkowiak, 1983, Pempkowiak et al., 1984, Emelyanov, 1995, Ferrari et al., 1996, Grzybowski, 2003 and Grzybowski and Pempkowiak, 2003, Burska 2005, Pempkowiak et al., 2006, Kuliński and Pempkowiak, 2008, Dzierzbicka-Głowacka et al., 2010, Dzierzbicka-Głowacka et al., 2011 and Szymczycha et al., 2014). Concentrations of DOC and POC in Baltic seawater have been reported to range from 3.2 to 7.7 mgC dm− 3 (Jurkovskis et al., 1976, Grzybowski and Pempkowiak, 2003 and Kuliński and Pempkowiak, 2011) and from 0.1 to 1.4 mgC dm− 3 (Burska 2005, Kuliński & Pempkowiak 2011).