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Chapter 8
Exchange and Microdistribution of Solutes at the Benthic Interface: An In Situ Study in Aarhus Bight, Denmark 1
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Ronnie N. Glud and JensK.Gundersen 1
MarineBiological Laboratory, University of Copenhagen, Strandpromenaden 5, 3000 Eisinore, Denmark UnisenseA/S,Science Park Aarhus, Gustav Wieds Vej 10, 8000 AarhusC,Denmark
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In situ benthic exchange rates of O , DIC, NH and NO were measured along with microprofiles of O , S and pH at three coastal sites. The diffusive O uptake (DOU) increased while the O penetration depth decreased with increasing organic carbon content of the surficial sediment. Total O uptake (TOU) showed the same pattern, but values were significantly higher than the DOU at all stations. The difference was partly due to advective porewater transport and respiration of fauna and partly due to the microtophography all leading to underestimation of the actual oxic sediment area when using a simple one-dimensional diffusive flux calculation. Sulphide could not be detected in the upper 30mm of the two oxidized sediments, while H S and O coexisted in a narrow zone of intense H S oxidation just below the sediment surface at the reduced site. Nitrogen was mainly exported from the sediment as NH and urea. The importance of urea for the total Ν release rate increased with the surficial organic carbon content and reached a maximum of 66% at the reduced site. 2
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© 2002 American Chemical Society Taillefert and Rozan; Environmental Electrochemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
145 In coastal environments a significant fraction (25-50%) of the primary production reaches the seafloor (/, 2). Within the sediment the majorfractionof the organic material is being oxidized through a complex web of degradation processes, while a minor refractory part is being permanently buried in the sediment record (5). The oxidation of the organic material consumes the available electron acceptors in a sequential order eliminating the energetically most favorable first (i.e. 0 , N0 ", MnO , FeO , S0 ) (4). This leads to a laminated sequence of reduction processes within the sediment in which oxic respiration is followed by denitrification, metal reduction and sulfate reduction (5). Irrigating fauna may disturb this general pattern and create a more mosaiclike distribution of the various processes. The anaerobic degradation leads to the production of soluble reduced metal ions and H S that may interact with each other or with the various metal oxides and form reduced precipitates (e.g. FeS, FeS ) (
