Transformations of dissolved organic matter in the Marmara Sea traced by its optical signature

Abstract

Abstract The Marmara Sea is a semi-enclosed basin connecting the Black and the Mediterranean Seas through the Bosporus and Dardanelles Straits (Turkish Straits System- TSS). It is characterized by a permanent two layered counter-flow caused by the exchange of water masses between the Black and Mediterranean Seas. In this study we focus on the role of the Marmara Sea in altering the optical characteristics of dissolved organic matter (DOM) during its transport through the TSS. We use detailed absorbance and fluorescence measurements followed by parallel factor analysis (PARAFAC). PARAFAC identified four components: a primary terrestrial humic C1 (ex/em 255/448 nm), a marine humic C2 (ex/em 300/402 nm), a secondary terrestrial humic C3 (ex/em 280, 370/480 nm) and a protein like component C4 (ex/em 275/336 nm). The above halocline waters (≤25 m) of Black Sea origin are characterized by high DOC (114–181 μmol L−1); high CDOM absorption (aCDOM(300): 1.97–2.92 m−1), low absorption spectral slope (S275–295: 0.0280–0.0307 nm−1) and high humic content. In this water layer, interrelationships with salinity, DOC and optical parameters revealed that although bulk DOC is diluted towards the west, transformations of the DOM pool are related to several processes. In the sub-halocline waters (>25–1250 m) of Mediterranean origin, all measured parameters are significantly lower (DOC: 55–65 μmo L−1; aCDOM(300): 0.46–0.80 m−1; S275–295: 0.0241–0.0376 nm−1). No relationship of DOC with apparent oxygen utilization (AOU) could be established, likely due to the proliferation of particles. Nevertheless, qualitative changes of DOM could be traced by the negative relationship of S275–295 with AOU and the two fold increase of the intensities of the humic-like components towards the eastern part of the basin. This suggests that the optical signature of the Mediterranean DOM is altered in the deep Marmara Sea despite its constant DOC content.