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[1] Field measurements by eddy correlation indicate an average CO 2 uptake of 1.9 g C m À2 d À1 by the intertidal Wadden Sea estuary in spring 2008. The flux did not show a dependency on the tide and fluxes during high and low tide were comparable. We hypothesize that biological production in the water column and in microbial mats that cover sediments lead to an undersaturation of CO 2 that is strong enough to support the observed fluxes. The total carbon uptake by this intertidal estuary from day of the year 101 -168 is estimated to be À1.7 Tg C. Extrapolation of this flux over three months in spring suggests that the uptake of CO 2 by this estuary over this period is comparable to 24% of the yearly carbon flux over the North Sea and the European estuaries. Citation:

16To reduce the spread of aquatic invasive species, the discharge of ballast water by ships will soon be 17 compulsorily regulated by the International Maritime Organization (IMO) 46To comply with the upcoming discharge regulations, most ships will have to be fitted with ballast water 47 management systems (BWMSs), to disinfect ballast water before discharge. 76Although metabolic activity does not guarantee viability it is considered to be a good viability indicator for 89In the present study, Clean Trace™ ATP assay (3M, Minnesota, USA) was applied. 100Esterase enzymes are exclusively produced by living organisms and thus considered a proxy for the 106in combination with ATP analysis in ballast water compliance testing will be discussed.

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