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Nitrogen cycling in two types of sediments of the southern North Sea (Frisian front, Broad Fourteens): field data and mesocosm results
Van Raaphorst, W.; Kloosterhuis, H.T.; Berghuis, E.M.; Gieles, A.J.M.; Malschaert, J.F.P.; Van Noort, G.J. (1992). Nitrogen cycling in two types of sediments of the southern North Sea (Frisian front, Broad Fourteens): field data and mesocosm results. Neth. J. Sea Res. 28(4): 293-316
In: Netherlands Journal of Sea Research. Netherlands Institute for Sea Research (NIOZ): Groningen; Den Burg. ISSN 0077-7579, more
Peer reviewed article  

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Keyword
    Marine

Authors  Top 
  • Van Raaphorst, W.
  • Kloosterhuis, H.T.
  • Berghuis, E.M.
  • Gieles, A.J.M.
  • Malschaert, J.F.P.
  • Van Noort, G.J.

Abstract
    Benthic nitrogen cycling was examined at a sandy station (Broad Fourteens, B14) and at a silty station (Frisian Front, FF) in the southern North Sea. Both stations are shallow (B14: 29 m, FF: 39 m) and had a vertically well-mixed water column throughout the period of investigation (January 1989 to April 1990). Both 02 consumption and NH4 + release were maximal during the summer, their rates at B14 were 20 and 0.5 mmol·m-2·d-1, respectively, and at FF, 40 and 1.7 mmol·m-2·d-1, respectively. At B14, NOx (nitrate + nitrite) fluxes were directed into the sediment during the first half of 1989 while an efflux was measured from August onwards. At FF, an efflux of NOx was observed throughout the year, though at very low rates during the period January-June. Maximum denitrification rates were measured during April and June, while minimum rates were observed in August and November. On an annual basis, at B14 ~50% and at FF ~85% of the ammonium produced by benthic mineralization was regenerated to the water column. Consequently, the N retention in the sediment (denitrification and burial), as a percentage of ammonification, was lower at the silty FF than at the sandy B14. It is argued that this is probably caused by lower O2 availability due to higher mineralization rates and consequently lower nitrification rates at FF as compared to the organic-poor B14. Benthic nitrification supplied more than 85% of the N03- annually needed for denitrification, thus demonstrating the importance of nitrification-denitrification coupling in these sediments. Parallel to the field measurements, the effect of a single addition of Phaeocystis material to the B14 and FF sediments was studied in mesocosms. Oxygen respiration strongly increased directly after addition of this organic substrate but after ~1 month, respiration decreased to values lower than before food addition. This pattern could be described with a simple multi-G model (n=3). Directly after the supply of organic matter to the mesocosms, nitrate concentrations in the pore water and denitrification rates substantially decreased. After ~2 months when most of the added material was respired, both nitrate concentrations and denitrification recovered.

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