3Benthic chamber experiments including the nonreactive solute tracer, Br -, indicated that sediment irrigation was very important to solute exchange at the study site.The enhancement of sediment-seawater exchange due to irrigation was determined for the nonreactive tracer (Br -), TCO 2 , NH 4 + , U and Mo. The comparisons between these solutes showed that reactions within and around the burrows were very important for modulating the Mo flux, but less important for U. The effect of these reactions on Mo exchange was highly variable, enhancing Mo (and, to a lesser extent, U) uptake at times of relatively modest irrigation, but inhibiting exchange when irrigation rates were faster.These results reinforce the observation that Mo can be released to and removed from pore waters via sedimentary reactions.The removal rate of U and Mo from seawater by sedimentary reactions was found to agree with the rate of accumulation of authigenic U and Mo in the solid phase. The fluxes of U and Mo determined by in situ benthic flux chamber measurements were the largest that have been measured to date. These results confirm that removal of redoxsensitive metals from continental margin sediments underlying oxic bottom water is important, and suggest that continental margin sediments play a key role in the marine budgets of these metals.4

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We tested the effect of additions of organic matter of different quality (C:N ratio) and quantity on benthic fluxes of ammonium, nitrate, silicate, oxygen and dissolved inorganic carbon (DIC). Additions were made to intact sediment boxes, according to a 2-level factorial design, and the magnitude and temporal scale of the response were evaluated with Partial Least Square (PLS) regression analysis. Response patterns were followed over a 40 d period following the addition, but only fluxes of ammonium, nitrate, silicate and oxygen for the first 2 d after addition, and nitrate fluxes for the first 8 d after addition, could be used for predictive modelling. The results showed that the response of the microbial community to different qualities and quantities of organic matter may initially be related to its capacity to sequester organic matter, and in the long term, to its growth in response to the respective nutritional supply. The results also suggest that such an experimental approach can be a useful tool for classification of areas with potential risk of eutrophication.

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