ABSTRACT: A free operating benthic flux chamber lander (ELINOR) was used to measure in situ denitrification rates in the Aarhus Bight, Denmark (16 m depth). After insertion of the flux chamber into the sediment ¹⁵NO₃^- was automatically injected into the enclosed water phase. After 3 to 4 h of incubation ELINOR was brought back to the surface with an intact water and sediment phase. Dinitrogen was extracted and later analyzed for ¹⁵N enrichment by mass spectrometry. Parallel sediment cores were sampled for laboratory incubation under in situ conditions. In situ denitrification rates in June and November were 350 and 470 umol N m^-2 d^-1 respectively, and were not significantly different from the laboratory measured rates of 290 and 500 umol N m^-2 d^-1, respectively. Oxygen uptake was 40% lower in the laboratory incubated cores compared to in situ measurements, apparently due to lowered activity of the dominating species of infauna, the bivalve Abra alba. Nitrate concentrations were low in the bottom water (<8 uM) and most of the denitrification activity (65 to 100%) was coupled to nitrification in the sediment. A laboratory test showed that the addition of ¹⁵NO₃^- did not significantly impede the reduction rate of the natural unlabelled NO₃^-. The results support the validity of laboratory measurements for coastal waters and demonstrate the potential of combining benthic flux chamber landers and the isotope pairing technique for accurate measurements of denitrification in shelf, deep sea, and freshwater environments.

KEY WORDS: Denitrification . Sediment . Benthic lander . Flux . ¹⁵N . Nitrogen . Nitrification

Published in MEPS Vol. 137 (1996) on June 27
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1996