Cycling of dissolved and particulate nitrogen and carbon in the Framvaren Fjord, Norway: stable isotopic variations

The reaction pathways of nitrogen and carbon in the Framvaren Fjord (Norway) were studied through stable isotope analysis (delta(15)N and delta(13)C) Of dissolved inorganic and particulate organic matter (POM). The variations in the isotopic compositions of the various C and N pools within the water column were use to evaluate the historical deposition of material to the sediments. The high delta(15)N-NH4+ at the O-2/H2S interface, as a consequence of microbial uptake between 19 and 25 m, results in extremely depleted delta(15)N-particulate nitrogen (PN) of approximately 1 parts per thousand within the particulate maximum at approximately 19 m. The carbon isotopic distribution of dissolved inorganic carbon (DIC) and particulate organic carbon (POC) within the interface suggests that the distinct microbial flora (Chromatium sp. and Chlorobium sp.) fractionate inorganic carbon to different degrees. The extremely light delta(13)C-POC within the interface (-31 parts per thousand) appears to be a result of carbon uptake by Chromatium sp. while delta(13)C-POC of - 12 parts per thousand is more indicative of Chlorobium sp. Nitrogen isotopic mass balance calculations suggested that approximately 75% of the material sinking to the sediments was derived from the dense particulate maximum between 19 and 25 m. The sediment distribution of nitrogen isotopes varied from 2 parts per thousand at the surface to approximately 6 parts per thousand at 30 cm. The nitrogen isotopic variations with depth may be an indicator of the depth or position of the O-2/H2S interface in the fjord. Low sediment delta(15)N indicated that the interface was within the photic zone of the water column, while mon enriched values suggested that the interface was lower in the water column potentially allowing for less fractionation during biological incorporation of dissolved inorganic nitrogen. Results indicate that the dense layers of photo-autotrophic bacteria in the upper water column impart unique carbon and nitrogen isotopic signals that help follow processes within the water column and deposition to the sediments. (C) 1999 Elsevier Science B.V. All rights reserved.