A global system of harmonized observations is needed to inform scientists and policy-makers.

At two intertidal sites (one sandy and one silty, in the Scheldt estuary, The Netherlands), the fate of microphytobenthos was studied through an in situ 13 C pulse-chase experiment. Label was added at the beginning of low tide, and uptake of 13 C by algae was linear during the whole period of tidal exposure (about 27 mg m Ϫ2 h Ϫ1 in the top millimeter at both sites). The 13 C fixed by microphytobenthos was rapidly displaced toward deeper sediment layers (down to 6 cm), in particular at the dynamic, sandy site. The residence times of microphytobenthos with respect to external losses (resuspension and respiration) were about 2.4 and 5.6 d at the sandy and silty stations, respectively. The transfer of carbon from microphytobenthos to benthic consumers was estimated from the appearance of 13 C in bacterial biomarkers, handpicked nematodes, and macrofauna. The incorporation of 13 C into bacterial biomass was quantified by carbon isotope analysis of polar lipid derived fatty acids specific for bacteria. The bacterial polar lipidderived fatty acids (i14:0, i15:0, a15:0, i16:0, and 18:17c) showed rapid, significant transfer from benthic algae to bacteria with maximum labeling after 1 d. Nematodes became enriched after 1 h, and 13 C assimilation increased until day 3. Microphytobenthos carbon entered all heterotrophic components in proportion to heterotrophic biomass distribution (bacteria Ͼ macrofauna Ͼ meiofauna). Our results indicate a central role for microphytobenthos in moderating carbon flow in coastal sediments.

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[1] Ocean acidification resulting from human emissions of carbon dioxide has already lowered and will further lower surface ocean pH. The consequent decrease in calcium carbonate saturation potentially threatens calcareous marine organisms. Here, we demonstrate that the calcification rates of the edible mussel (Mytilus edulis) and Pacific oyster (Crassostrea gigas) decline linearly with increasing pCO 2 . Mussel and oyster calcification may decrease by 25 and 10%, respectively, by the end of the century, following the IPCC IS92a scenario ($740 ppmv in 2100). Moreover, mussels dissolve at pCO 2 values exceeding a threshold value of $1800 ppmv. As these two species are important ecosystem engineers in coastal ecosystems and represent a large part of worldwide aquaculture production, the predicted decrease of calcification in response to ocean acidification will probably have an impact on coastal biodiversity and ecosystem functioning as well as potentially lead to significant economic loss.

Examination of amino acids in particulate samples from a variety of marine environments (fresh phytoplankton to deep‐sea sediments) revealed systematic compositional changes upon progressive degradation. These consistent trends have been used to derive a quantitative degradation index (DI) that is directly related to the reactivity of the organic material, as indicated by its lability to enzymatic decay and its first‐order degradation rate constant. This direct link between molecular composition and degradation rate allows us to quantify the quality of organic matter based solely on its chemical composition.

The rate and factors controlling denitrification in marine sediments have been investigated using a prognostic diagenetic model. The model is forced with observed carbon fluxes, bioturbation and sedimentation rates, and bottom water conditions. It can reproduce rates of aerobic mineralization, denitrification, and fluxes of oxygen, nitrate, and ammonium. The globally integrated rate of denitrification is estimated by this model to be about 230–285 Tg N yr−1, with about 100 Tg N yr−1 occurring in shelf sediments. This estimate is significantly higher than literature estimates (12–89 Tg N yr−1), mainly because of a proposed upward revision of denitrification rates in slope and deep‐sea sediments. Higher sedimentary denitrification estimates require a revision of the marine nitrogen budget and lowering of the oceanic residence time of nitrogen down to about 2×103 years and are consistent with reported low N/P remineralization ratios between 1000 and 3000 m. Rates of benthic denitrification are most sensitive to the flux of labile organic carbon arriving at the sediment‐water interface and bottom water concentrations of nitrate and oxygen. Denitrification always increases when bottom water nitrate increases but may increase or decrease if oxygen in the bottom water increases. Nitrification is by far the most important source of nitrate for denitrification, except for organic‐rich sediments underlying oxygen‐poor and nitrate‐rich water.

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The deep-sea floor has long been considered a 'food desert' but recent observations suggest that episodic inputs of relatively fresh organic matter (phytodetritus) occur and that benthic processing of this material may be rapid. Although the responses of the total community in terms of oxygen consumption and of some individual benthic groups have been identified, the quantitative role of the different groups in the short-term response remains largely unknown. We examined the short-term response in major benthic compartments in an in situ experiment in the NE Atlantic (2170 m water depth) using 13 C-enriched diatoms as a tracer of labile carbon. Within 35 h, 6 mg C m -2 was processed by the benthos, with the majority of the processed carbon recorded as respiration (45%). Among the fauna retained on a 300 µm sieve, Foraminifera were rapid consumers which, together with Bacteria, accounted for 50% of the processing. Therefore, although Bacteria dominate long-term carbon mineralization (as suggested by their general dominance in the benthic biomass), some faunal components, in this case Foraminifera, may play a central role in the rapid initial processing of fresh organic carbon in deep-sea sediments.