Chemical and isotopic composition of the organic matter sources in the Gulf of Gdansk (Southern Baltic Sea)

Particulate organic matter (POM) in coastal areas and in estuaries, originates from heterogeneous allochthonous and autochthonous sources. Each POM source may contribute substantially to the total input, although the relative importance of these sources may vary spatially and temporally within an individual estuary. In this work we used combined indicators of origin of organic matter in order to characterize its sources: atomic C/N ratio (C-at/N-at), organic carbon to chlorophyll a ratio (C/Chla), as well as carbon (delta C-13) and nitrogen (delta N-15) isotopic composition of organic matter. We show that in the Gulf of Gdansk, two major organic matter sources, responsible for around 95% of total POM inflow (autochthonous primary production and riverine discharges), can be easily distinguishable based on their carbon isotopic composition (mean delta C-13 equal, respectively, to -22.3 parts per thousand and -28.5 parts per thousand). We hypothesize that isotopically depleted nutrients from atmospheric deposition often support phytoplanktonic primary production in the Southern Baltic leading to low delta N-15 values of POM. We show that isotopic composition of riverine POM is highly seasonally variable due to the fact that riverine suspension is composed of a mixture of organic matter originated from riverine primary production, terrigenous vegetation and POM from industrial and domestic waste waters. However, elemental composition of the Vistula's POM (both low C-at/N-at and C/Chla) suggests that during most of the year it is composed of freshwater phytoplanktonic material. Based on the magnitude of organic matter inputs and associated delta C-13 value we estimate a weighted mean carbon isotopic composition of surface water POM in the Gulf of Gdansk, which gives - 24.9 parts per thousand, and approximates well the real measured value in the open waters of the Gulf in 1996. We conclude that a combined approach using all studied parameters and including analysis of isotopic composition of dissolved constituents, which are utilized during primary production (nutrients and dissolved inorganic carbon-DIC), is necessary to obtain true characterization of organic matter origin in the Gulf of Gdansk. It results from the fact that the most important source of POM in the Gulf-autochthonous phytoplanktonic primary production-is highly dependent on the discharges of nutrients and DIC from allochthonous sources: riverine discharges and atmospheric deposition. (C) 2000 Academic Press.