Winter and Spring Characterization of Particulate and Dissolved Organic Matter in the Danube-Black Sea Mixing Zone

Abstract The mixing zone between the Danube and the Black Sea was investigated at the front of the Chilia and Sulina branches of the Danube delta, in April–May 1997, during two periods corresponding to conditions at the end of winter (Chilia and Sulina branches) and the beginning of spring (Sulina branch). The distribution of the organic matter in the particulate, colloidal and truly dissolved pools along the salinity gradient was characterized at both global and molecular levels using biochemical parameters. The distribution of particulate organic carbon (POC) paralleled that of total suspended matter in winter, whereas decoupling was observed in spring, reflecting active biological mechanisms in the mixing zone off the Sulina branch during this period. This is supported by distribution patterns of polyunsaturated fatty acids (PUFA) and branched fatty acids (BrFA), which are indicators of phytoplankton and bacteria activities, respectively. The δ 13 C isotopic signature of POC in the Danube–Black Sea mixing zone varied of between −29·1 and −24·6‰, which is consistent with the usual temperate range of estuarine systems. In spring, the observed shift towards lower values in the area of salinity 0–5 suggested a gradual change in the composition of POC, such as that caused by cell lysis of riverine planktonic populations induced by a salinity increase. A subsequent increase of δ 13 C values towards higher salinities indicated phytoplankton growth in the estuarine/marine zone, as assessed by high concentrations of PUFA. Indirect evidence of sedimentation of terrestrial particles in the upper part of the salinity gradient in the range of 2–3 was suggested by the decrease of the content of particles and POC in fatty acids (C 24 –C 32 ) and n-alkanes (C 25 –C 35 ), both constituents of cuticular waxes of higher plants. A tight coupling was suggested between phytoplankton and bacteria by similar distributions of PUFA and BrFA along the salinity gradient. The distribution of dissolved organic carbon (DOC) varied according to the season. In spring, the DOC concentrations remained constant across the salinity gradient in front of the Sulina branch at about 2·7 mg l −1 , whereas higher concentrations, >4 mg l −1 , were observed in winter at stations located in the low salinity (2–4 and 5–8) area. A few data were obtained for δ 13 C of the DOC pool, with an average of −26·9‰ in the Danube River end-member and values close to those of POC off the various branches. The low molecular weight (LMW) fraction of DOC isolated from water by sequential cross-filtration was evaluated by summing the pool of the combined and free amino acids and that of the combined and free carbohydrates. The distribution of LMW DOC along the salinity gradient was similar in the three investigated areas. Off the Sulina branch, proportions of LMW DOC fluctuated between 17 and 37‰ in winter and between 23 and 35‰ in spring. Two zones of high LMW DOC were located in two ranges of salinity—between 2 and 4 and between 5 and 8—probably originating from the release of decaying diatoms, assessed by high molecular proportions of serine in the combined amino acids. LMW DOC indicated a high potential for microbal activity, as shown by concomitant distributions of branched fatty acids.