Using a two-dimensional simulation analysis, we investigated the effects of sediment flushing on denitrification and the implications for two methods commonly used to measure denitrification in intact sediment cores: the N 2 :Ar-ratio method and isotope pairing technique (IPT). Our simulations of experimental chamber incubations showed that advective flushing of the sediment can significantly increase sediment denitrification driven by NO 3 -from the water column (up to a factor of 5), but that nitrification and coupled nitrification-denitrification is reduced under conditions of sediment flushing (up to a factor of 6). N 2 fluxes across SWI may differ significantly from actual rates of denitrification for periods lasting from 1 up to more than 5 d after changes in parameters such as sediment flushing rate and water column NO 3 -concentrations. Simulations of the isotope pairing technique, showed that the rate of labeled N 2 production, after the addition of 15 NO 3 -may take up tõ 24 h to reach steady state, depending on NO 3 -concentrations in the water column and sediment flushing rate. Measurements of denitrification in sand using IPT confirmed that short term incubations (11 h) underestimated the actual denitrification. Furthermore, model simulations were able to give a good estimate of measured N 2 fluxes across SWI at different flushing rates under non-steady state conditions, confirming the ability of the model to realistically simulate experimental situations.

The physical mechanism that drives bioirrigation is strongly dependent on the permeability of the sediment. We advance two mechanisms, each described by a corresponding microenvironment model. In muds, burrow water cannot penetrate the sediment, so bioirrigation is intrinsically driven by diffusional transfer across the burrow wall. This ''diffusive'' mode of bioirrigation is accurately described by the classical tube irrigation model. In sands, ventilation flows can penetrate the surrounding sediment via dead end burrows. To quantify this ''advective'' mode of bioirrigation, we propose a novel two-dimensional pocket injection model. This model's principal features are that (1) organisms indent the sediment-water interface with burrow structures, (2) the specific structure of the burrow can be neglected except for the location of a feeding pocket, and (3) burrow water is injected from this feeding pocket into the surrounding sediment. We tested the adequacy of the pocket injection model in a detailed case study of the lugworm Arenicola marina, comparing model simulations and experimental data from core incubations. Simulation of two different sets of inert tracer experiments shows good agreement between model and data, indicating that our model captures the relevant aspects of lugworm bioirrigation in permeable sediments.

In sandy sediments, a strong connection exists between the physics of flow and the ecology of burrow-ventilating macrofauna. We developed a general modelling procedure that quantifies this link involving 3 steps. (1) Burrow-ventilating organisms can be described as mechanical pumps.(2) The pumping of burrow water into blind-ending tubes induces advective flow in the sediment. (3) The resistance to pore water flow is governed by the friction between solid and fluid, i.e. Darcy's law. This analysis allows the determination of the operation point of an 'organism pump' under in situ conditions, and we applied it in a detailed modelling study of the lugworm Arenicola marina. A 3-dimensional finite element model encompasses the lugworm's J-shaped burrow and represents a typical lugworm territory at in situ density. We simulated the associated flow patterns in the sediment and analysed the factors that influence the lugworm's ventilation rate. Since the lugworm's oxygen supply critically depends on the burrow ventilation rate, we advance the following 2 ecological hypotheses: (1) decreasing the permeability of the burrow lining greatly increases the efficiency of oxygen supply, as it prevents the re-entry of anoxic pore water; and (2) the permeability of the bulk sediment constrains the lugworm's habitat. When permeability falls below a critical threshold, the sediment's resistance becomes too high, thus resulting in an insufficient oxygen supply. Overall, we show that permeability exerts an important control on ventilation activity, and hence resource availability, in sandy sediment ecosystems. From an evolutionary point of view, we anticipate biological feedbacks on this physical control -in particular, behavioural adaptations that increase the permeability in the bulk sediment but decrease the permeability near the burrow wall. KEY WORDS: Bio-irrigation · Burrow ventilation · Arenicola marina · Permeable sediments · ModellingResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 303: 195-212, 2005 Harisson 1981, Ziebis et al. 1996, Koretsky et al. 2002. The metabolic need for oxygen is satisfied by ventilation of these burrows with oxygen-rich water from the overlying water column. Due to this 'active' burrow ventilation, the organisms are no longer restricted to the narrow zone near the sediment surface where oxygen penetrates via 'passive' diffusion.One important issue, which has not received much attention in past studies, is that the mechanism of burrow ventilation crucially depends on the permeability of the sediment. One can consider 2 end-member situations (Fig. 1). In muddy environments, the pressures required to force a flow of pore water through the sediment are typically beyond the physiological capabilities of benthic fauna. To enable burrow ventilation, burrows must have at least 2 connected openings to the sediment surface, to ensure a free conduit for the ventilated burrow water (Fig. 1a). So, in muddy sediments, advective flows are generated with...

The mapping approach is applied to study the distributive mixing in the, widely industrially used, Kenics static mixer. The flexibility of the mapping method makes it possible to study and compare thousands of different mixer layouts and perform optimization with respect to macroscopic homogenization efficiency and interface generation. In the paper two different designs of the mixer are investigated. The conventional mixer with sequentially different twisted blades and adesign where the twist direction is maintained constant. In both cases the blade twist angle is varied. Recommendations are given in the choice of the design of the mixer dependent on the desired structure of mixing.

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