Dissolved organic matter along the eutrophication gradient of the Darβ–Zingst Bodden Chain, Southern Baltic Sea: I. Chemical characterisation and composition

Abstract

The Darβ–Zingst Bodden Chain (DZBC) is a shallow tideless estuary of 4 separate basins containing extremely high contents of particulate organic matter (POM) because of an eutrophication for more than 30 years. Therefore the open question to be addressed in this study was whether and how high POM is reflected in amounts and composition of dissolved organic matter (DOM). The pelagial was sampled at different stations along the eutrophication gradient during the growing season 1996, as well as in winter 1995/96 and 1996/97 to assess seasonal variations. The chemically utilisable compounds (uDOM) like dissolved free amino acids (DFAA) and dissolved free carbohydrates (DFCHO), as well as the corresponding polymer fractions dissolved combined amino acids (DCAA) and dissolved combined carbohydrates (DCCHO) were analysed together with the total dissolved organic carbon (DOC) and total dissolved organic nitrogen (DON). The concentrations of these fractions were used to calculate the respective percentage contribution of the DOC and DON and to compare with the C:N ratio in the dissolved pool. In the DZBC very high absolute amounts of DOM components of up to 30 μmol C l − 1 for DFAA, 288 μmol C l − 1 for DCAA, 67 μmol C l − 1 for DFCHO, 196 μmol C l − 1 for DCCHO were determined. On average, proportions of 0.8% DFAA, 3.6% DFCHO, 6.5% DCAA and 12.6% DCCHO of DOC were measured in the growing season of 1996. Consequently, at least 23.5% of DOC in the DZBC were classified as readily utilisable. The concentrations of these bio-available compounds fluctuated strongly over the course of the year, but the dissolved monomers more than the corresponding polymers. With the exception of DFAA at significantly higher concentrations in winter, a clear seasonality in DOM compounds was absent. In spite of the high amounts of uDOM, most likely being able to sustain many microheterotrophs, the quality of the total DOM has to be assessed as recalcitrant due to an average molar C:N ratio of ca. 19. This fact makes the Darβ–Zingst Bodden Chain quite unique concerning persistent high amounts of utilisable and refractory organic matter. So far, it was not possible to prove whether the high DOM originates directly from primary production, is mostly the result of microbial degradation in the pelagial or stems from anaerobic degradation in the sediment. However, the high DOM concentrations, its element composition and the accumulation of glycine and serine in the dissolved amino acid pool suggest that the degradation of DOM was advanced. Such analyses seem to be an additional tool to describe the ecological state of highly polluted waters and estimate the origin and fate of organic matter.