Seasonal variations in dissolved organic matter composition using absorbance and fluorescence spectroscopy in the Dardanelles Straits – North Aegean Sea mixing zone

• Published in: Continental shelf research 2017-10, Vol.149, p.82-95
• Main Authors: Pitta, Elli, Zeri, Christina, Tzortziou, Maria, Mousdis, George, Scoullos, Michael
• Format: Article
• Language: English
• Subjects: Carbon specific absorbance; Chromophoric Dissolved Organic Matter (CDOM); Fluorescence indices; Mediterranean Sea; Parafac model; Spectral slope
• ISSN: 0278-4343; 1873-6955
• DOI: 10.1016/j.csr.2016.07.013

The Dardanelles Straits – North Aegean Sea mixing zone is the area where the less saline waters of Black Sea origin supply organic material to the oligotrophic Mediterranean Sea. The objective of this work was to assess the seasonal dynamics of dissolved organic matter (DOM) in this region based on the optical properties (absorbance and fluorescence). By combining excitation-emission fluorescence with parallel factor analysis (EEM-PARAFAC), four fluorescent components were identified corresponding to three humic – like components and one amino acid – like. The latter was dominant during all seasons. Chromophoric DOM (CDOM) and dissolved organic carbon (DOC) were found to be strongly coupled only in early spring when conservative conditions prevailed and the two water masses present (Black Sea Waters – BSW and Levantine Waters – LW) could be identified by their absorption coefficients (a300) and spectral slopes S275–295. In summer and autumn the relationships collapsed. During summer two features appear to dominate the dynamics of CDOM: i) photodegradation that acts as an important sink for both the absorbing DOM and the terrestrially derived fluorescent humic substances and ii) the release of marine humic like fluorescent substances from bacterial transformation of DOM. Autumn results revealed a source of fluorescent CDOM of high molecular weight, which was independent of water mass sources and related to particle and sedimentary processes. The removal of the amino acid-like fluorescence during autumn provided evidence that although DOC was found to accumulate under low inorganic nutrient conditions, dissolved organic nitrogenous compounds could serve as bacterial substrate. •Optical properties describe water mass mixing only during winter-spring conditions.•Changes in surface circulation pattern can be traced by CDOM absorbance.•CDOM molecular signatures indicative of photodegradation in surface waters.•CDOM molecular signatures indicative of sedimentary sources in subsurface waters.•Bacterial reworking contributes to the visible and diminishes the UVA fluorescence.