The importance of bioturbation in mediating biogeochemical processes in the upper centimetres of oceanic sediments provides a compelling reason for wanting to quantify in situ rates of bioturbation. Whilst several approaches can be used for estimating the rate and extent of bioturbation, most often it is characterized by calculating an intensity coefficient (D b ) and/or a mixed layer depth (L). Using measures of D b (n = 447) and L (n = 784) collated largely from peer-reviewed literature, we have assembled a global database and examined patterns of both L and D b . At the broadest level, this database reveals that there are considerable gaps in our knowledge of bioturbation for all major oceans other than the North Atlantic, and almost universally for the deep ocean. Similarly, there is an appreciable bias towards observations in the Northern Hemisphere, particularly along the coastal regions of North America and Europe. For the assembled dataset, we find large discrepancies in estimations of L and D b that reflect differences in boundary conditions and reaction properties of the methods used. Tracers with longer half-lives tend to give lower D b estimates and deeper mixing depths than tracers with shorter half-lives. Estimates of L based on sediment profile imaging are significantly lower than estimates based on tracer methods. Estimations of L, but not D b , differ between biogeographical realms at the global level and, at least for the Temperate Northern Atlantic realm, also at the regional level. There are significant effects of season irrespective of location, with higher activities (D b ) observed during summer and deeper mixing depths (L) observed during autumn. Our evaluation demonstrates that we have reasonable estimates of bioturbation for only a limited set of conditions and regions of the world. For these data, and based on a conservative global mean (± SD) L of 5.75 ± 5.67 cm (n = 791), we calculate the global volume of bioturbated sediment to be > 20 700 km 3 . Whilst it is clear that the role of benthic invertebrates in mediating global ecosystem processes is substantial, the level of uncertainty at the regional level is unacceptably high for much of the globe.

Abstract. This paper presents new results from high temporal resolution observations over two years (2007 and 2008) from instrumented moorings deployed in the central North Sea, at the Oyster Grounds and on the northern slope of Dogger Bank (North Dogger). The water column was stratified in the summer at both sites, leading to limited exchange of the water in the bottom mixed layer. Data from these moorings revealed the variable nature of summer oxygen depletion at the Oyster Grounds. The combination of in situ and ship-based measurements allowed the physical and biological conditions leading to decreasing dissolved oxygen concentrations in bottom water to be examined. In 2007 and 2008, the concentration of dissolved oxygen in the bottom water at both sites was observed to decrease throughout the summer period after the onset of stratification. Depleted dissolved oxygen concentration (6.5 mg l −1 , 71% saturation) was measured at the North Dogger, a site which is not significantly influenced by anthropogenic nutrient inputs. Lower oxygen saturation (5.2 mg l −1 , 60% saturation) was measured for short durations at the Oyster Grounds. The seasonal increase in bottom water temperature accounted for 55% of the decrease in dissolved oxygen concentration at the Oyster Grounds compared to 10% at North Dogger.Dissolved oxygen concentration in bottom water at the Oyster Grounds was shown to be strongly influenced by short term events including storms and pulses of particulate organic matter input. In contrast, dissolved oxygen concentration in bottom water at the North Dogger reflected longer seaCorrespondence to: N. Greenwood (naomi.greenwood@cefas.co.uk) sonal processes such as a gradual temperature increase over the summer and a more steady supply of particulate organic matter to the bottom mixed layer. The differences between the study sites shows the need for an improved understanding of the mechanisms driving these processes if the use of oxygen in marine management and ensuring ecosystem health is to be meaningful and successful in the future. These high frequency observations provide greater understanding of the nature of the depletion in bottom oxygen concentration in the North Sea.

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13Natural seabed disturbance was quantified by estimating the number of days in a year that movement of the seabed 14 occurred due to waves and currents, taking into account the seabed characteristics. Disturbance over gravel 15 substrates was based on the concept of a critical threshold for bed movement. For mud substrates disturbance was 16 assessed on the basis of bed failure under extreme hydrodynamic stress. For sand beds the disturbance frequency 17 could be calculated as a function of disturbance depth below the sediment surface by relating disturbance to the 18 occurrence of small scale bedforms using established relationships for predicting ripple and megaripple height. The 19 method was applied to the northern European Continental Shelf (48° N to 58.5° N and 10° W to 10° E) using modelled 20 annual wave and current forcing with a temporal resolution of one hour and spatial resolution of approximately 11 21 km. At the broad scale highest levels of disturbance occurred in areas of high tidal stress where dune/megaripple 22 type bedforms were predicted and in shallow regions exposed to waves with large fetch. However, the detailed 23 distribution of disturbance showed a complex relationship between water depth, tidal stress, wave fetch and grain 24size. An assessment of the uncertainty in the results was made using a simple Monte Carlo approach and in general 25 indicated a large spread in disturbance frequency values. This suggests that present predictive relationships need 26 improvement if assessments of natural disturbance are to be made with confidence. Nevertheless the results give a 27 broad understanding of the location and intensity of natural physical bed disturbance including a measure of the 28 relative intensity between different regions. This has applications to management of the seabed where human 29 impacts have to be assessed in the context of the underlying natural disturbance. Recommendations are given for 30 further research that would help decrease the uncertainty in natural disturbance prediction. 31 32

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