Influence of the Danube River inputs on C and N stable isotope ratios of the Romanian coastal waters and sediment (Black Sea)

The main aim of this study was to estimate the influence of the seasonal variations of the Danube River particulate organic matter (POM) inputs on the Black Sea surface seawater POM and upper layer of sediments along the Romanian coast. Ratios of carbon (13C/12C) and nitrogen (15N/14N) stable isotopes allowed differentiating river and marine organic matter sources. Danube River POM presented significantly lower average values of δ13C (−27.52 ± 0.88‰) and δ15N (4.88 ± 1.45‰) than seawater POM (δ13C = −24.70 ± 2.37‰ and δ15N = 6.75 ± 1.96‰), whereas surface sediment presented average values similar to seawater POM (δ13C = −24.02 ± 2.39‰ and δ15N = 7.29 ± 2.16‰). Stable isotope values showed that the Danube River influence on marine ecosystems decreased from the North to the South of the Romanian coast. Strong seasonal variations of C and N isotopic signatures were observed in all compartments studied with generally higher values in spring when the river was flooding.