Seasonal variability of stable carbon isotopes (δ13CDIC) in the Skagerrak and the Baltic Sea: Distinguishing between mixing and biological productivity

Abstract We documented the annual cycle of the carbon isotopic composition of dissolved inorganic carbon (δ 13 C DIC ) in the water columns of the Skagerrak and Baltic Sea to obtain an increased understanding of the processes involved controlling the carbon isotopic distribution in shelf seas. The lowest δ 13 C DIC values (− 4.9‰) were found in the low-oxygen, brackish Baltic bottom water whereas the highest values (+ 1.8‰) were observed in the surface water of the Skagerrak during late summer. Photosynthesis drove the high δ 13 C DIC values (between 1.0 and 1.8‰) noted in the surface waters of both the Skagerrak and the Baltic. The δ 13 C DIC values below the halocline in the Baltic reflect mixing of brackish water and the more saline water from the Skagerrak, and foremost organic matter remineralization processes that release significant amounts of low-δ 13 C CO 2 . Similarly, in the stagnant fjord basins, little deep water exchange and the degradation of terrestrial and marine organic matter set the δ 13 C composition. Deep-water renewal in the fjord basins resulted in rapid increases of the δ 13 C DIC on the order of 1‰, whereas remineralization processes caused a decrease in δ 13 C DIC of 0.1–0.3‰ per month depending on location. The combined effects of water mixing and remineralization processes (estimated using apparent oxygen utilization (AOU) values) yielded the expression: δ 13 C DIC  = 0.032 ∗ S − 0.01 ∗ AOU − 0.12 for the Baltic – Skagerrak region at water depths below the halocline.