Loadings of dissolved organic matter and nutrients from the Neva River into the Gulf of Finland – Biogeochemical composition and spatial distribution within the salinity gradient

Abstract

We studied the loadings of dissolved organic matter (DOM) and nutrients from the Neva River into the Eastern Gulf of Finland, as well as their distribution within the salinity gradient. Concentrations of dissolved organic carbon (DOC) ranged from 390 to 840μM, and were related to absorption of colored DOM (CDOM) at 350nm, aCDOM(350), ranging from 2.70 to 17.8m−1. With increasing salinity both DOC and aCDOM decreased, whereas the slope of aCDOM spectra, SCDOM(300–700), ranging from 14.3 to 21.2μm−1, increased with salinity.Deviations of these properties from conservative mixing models were occasionally observed within the salinity range of approximately 1–4, corresponding to the region between 27 and 29°E. These patterns are suggested to mostly reflect seasonal changes in properties of river end-member and hydrodynamics of the estuary, rather than non-conservative processes. On the other hand, observed nonlinear relationships observed between aCDOM*(350) and SCDOM(275–295) emphasized the importance of photochemistry among various transformation processes of DOM.Dissolved inorganic nitrogen was effectively transformed in the estuary into particulate organic nitrogen (PON) and dissolved organic nitrogen (DON), of which DON was mostly exported from the estuary, enhancing productivity in nitrogen limited parts of the Gulf of Finland. DON concentrations ranged from 12.4 to 23.5μM and its estuarine dynamics were clearly uncoupled from DOC. In contrast to DOC, estuarine DON dynamics suggest that its production exceeds losses in the estuary.Total nitrogen (TN) and phosphorus (TP) loadings from the Neva River and St. Petersburg were estimated as 73.5GgNyr−1 and 4.2GgPyr−1, respectively. Approximately 59% of TN and 53% of TP loads were in organic forms. DOC and DON loadings were estimated as 741.4GgCyr−1 and 19.0GgNyr−1, respectively. Our estimate for DOC loading was evaluated against a previously published carbon budget of the Baltic Sea. According to the updated model, the Baltic Sea could be identified as a weak source of carbon into the atmosphere.