THE ROLE OF WATER-MOVEMENT AND SPATIAL SCALING FOR MEASUREMENT OF DISSOLVED INORGANIC NITROGEN FLUXES IN INTERTIDAL SEDIMENTS

Fluxes of dissolved inorganic nitrogen (ammonium End nitrate) across t he sediment-water interface were determined at intertidal locations in I(Konigshafen, northern Wadden Sea, North Sea. Three different incuba tion techniques were compared: closed sediment cores (small scale), cl osed bell jars (medium scale) and an open flow system (Sylt flume, lar ge scale). Water movement in the two closed systems was maintained bel ow the resuspension limit by spinning magnets (cores, incubated in the laboratory) or by transfer of wave action through flexible plastic fo il (bell jars, in situ), whereas in the flume system (in situ) water m ovement was unidirectional, driven by currents and waves. Data sets fr om several years of core measurements (1992-94), bell jar measurements (1980) and flume campaigns (1990-93) served as the basis for a compar ison of dissolved inorganic nitrogen fluxes. Fluxes of ammonium and ni trate were within the same order of magnitude in closed cores and bell jars, while flume rates of ammonium were considerably higher.:The hig h flume rates were caused by advective flushing due to tidal water mov ement and wave action. The release of ammonium increased significantly with current velocity between 1 and 13 cm s(-1). Fluxes of ammonium w ere higher in sediments with Arenicola marina compared to those withou t this bioturbating species. The influence of benthic microalgae was e vident only in the small and medium scale core and bell jar systems as reduced ammonium release during light exposure. Nitrate was consumed by sediments in both closed systems at a rate proportional to the nitr ate concentration in the overlying water. Nitrate fluxes in the large scale Sylt flume were low with an average of only 7% of the ammonium f luxes, probably due to low concentrations in tidal waters during measu rements (summer), Both closed, small scale or open, large scale techni ques can be applied successfully for benthic flux studies, but the act ual choice depends on the purpose of the investigation Small enclosure s, either in the field I:bell jars) or in the laboratory (cores) are w ell suited to examine controlling factors for benthic microbial metabo lism and nutrient exchange across the sediment-water interface. Fluxes in benthic-pelagic subsystems need larger, open tools where the effec t of entire benthic communities and especially the influence of waves and currents can be evaluated. (C) 1998 Academic Press Limited.