Remineralization of terrestrial dissolved organic carbon in the Baltic Sea

Abstract

To assess the remineralization dynamics of terrestrial dissolved organic carbon (tDOC) in Baltic Sea water a set of incubation experiments was performed. Baltic Sea water was spiked with low molecular weight (LMW) and high molecular weight (HMW) tDOC extracted from water of the rivers Vistula and Oder. In parallel, incubation experiments were performed with seawater in order to determine the remineralization dynamics of Baltic Sea DOC (mDOC) that is characteristic for the marine environment but not necessarily of marine origin. At 10 consecutive time steps in the course of an incubation period of 188days, the concentrations of O2 and total CO2 (CT) were measured to assess the O2 consumption and CO2 production. The latter was used as a measure for the DOC mineralization. Three DOC fractions having different reactivity were identified. The refractory fraction was predominant in all samples and amounted to 66% in mDOC and about 80% in the tDOC. Two fractions of bioavailable DOC, which differed significantly concerning their reactivity, were identified in both the mDOC and tDOC. The more reactive labile fraction contributed to about 3% to LMW and HMW tDOC whereas 10% of the mDOC consisted of labile DOC. The half-life time for labile DOC ranged between 3.2d and 6.4d. The less reactive bioavailable fraction, the semi-labile DOC, showed half-life times of weeks to months. The total bioavailable fraction of the mDOC (34%) was higher than that of tDOC by a factor of about 2. Furthermore, it was characterized by a bulk half-life time of 10.6d that was significantly lower than those for tDOC from the two rivers (32.5d and 21.7d).The measurements of the oxygen consumption and total CO2 production during the remineralization yielded a ΔO2:ΔCT molar ratio close to 1.0 for the terrestrial dissolved organic matter (DOM) whereas an increased ratio of 1.2–1.5 was observed during the remineralization of the Baltic Sea DOM. This indicates that tDOM consists of carbohydrate-like compounds. The higher O2 demand for the mineralization of mDOM can be explained by a higher proportion of lipid-like compounds and by nitrification.