BUDGETS OF SEDIMENT NITROGEN AND CARBON CYCLING IN THE SHALLOW-WATER OF KNEBEL VIG, DENMARK

Sediment was sampled from a shallow coastal area (Knebel Vig, Denmark) . The vertical distribution of pigments, Pb-210 and Cs-137 indicated t hat organic matter was mixed into the sediment. On an area basis, sedi ment acid hydrolyzable amino acids accounted for 24 % of the particula te organic carbon pool and 53 % of the particulate organic nitrogen po ol. Similarly, porewater add hydrolyzable amino acids were an importan t component of dissolved organic carbon and dissolved organic nitrogen (9 and 27%, respectively). It was inferred that ribonucleic acids pot entially were an important component of dissolved organic nitrogen. Th e estimated efflux of dissolved organic nitrogen from the sediment was higher (3.9 mmol N m(-2) d(-1)) than the estimated efflux of dissolve d inorganic nitrogen (<2 mmol N m(-2) d(-1)). The high efflux of disso lved organic nitrogen was explained by organic matter hydrolysis close to the sediment surface. The low rates of inorganic nitrogen efflux t ogether with a high carbon oxidation rate suggested degradation of org anic matter with a low average nitrogen content and possible bacterial nitrogen assimilation. This was further supported by mass balance cal culations on nitrogen incorporation into microbial biomass. The calcul ated average C/N ratio in the organic matter degraded suggested that o nly part of the sediment acid hydrolyzable amino acids were available for bacterial degradation. The efficiency of bacterial carbon incorpor ation was 0.33 and within the range previously encountered in sediment s of Danish waters.