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Figure 1 Nereis diversicolor crawling at the sediment surface.
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Figure 2 Nereis virens. Head and first 16 segments.
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Figure 3 Drawing of Nereis sp. and Arenicola marina in burrows. The dark areas represent reduced sediment and the light areas represent oxidized sediment (modified from Kristensen et al.[11]).
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Figure 4 Arenicola marina lying on the sediment surface.
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Figure 5 Sediment inhabited by Arenicola marina with numerous feeding funnels and faecal casts at the surface. Inset shows a close up of faecal casts.
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Figure 6 Irrigation patterns of Nereis virens, N. diversicolor (under non-feeding and suspension feeding conditions) and Arenicola marina (modified from Kristensen et al.[11]).
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Figure 7 Cross section of sediment bioturbated by Nereis diversicolor. Light patches are oxidized sediment surrounding older burrows. Note that the worm in the centre inhabits a newly constructed burrow without noticeable oxidized sediment.
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Figure 8 Simplified carbon cycle in sediment. Particulate organic carbon (POC) is delivered from the water column (blue) to the oxic sediment (orange) where part of it is oxidized to CO2 by aerobic heterotrophs. Part of the POC is further buried undegraded into anoxic sediment (black) where most of it is converted to dissolved organic carbon (DOC) by hydrolysing and fermenting bacteria. The produced DOC is then oxidized to CO2 by anaerobic respirers. A small fraction of POC is buried permanently in the sediment and CO2 is ultimately transported from the sediment to the overlying water by molecular diffusion.
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Figure 9 Release of CO2 from sediment microcosms. Intact sediment cores were either kept untreated (green) or defaunated on day 6 by deoxygenation (N2 purging). Defaunated cores were either kept without animals (black) or added Nereis diversicolor at a density of 1390 m-2 on day 13 (after Hansen and Kristensen[46]).
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Figure 10 Porewater profiles of TCO2 in sandy sediment inhabited by 600 Nereis diversicolor m-2 (Nereis) or defaunated (control) (after Kristensen and Hansen[53]).
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Figure 11 A hypothetical marine sediment with two adjoining sites. Left: defaunated. Right: with a normal density of Arenicola marina. Arrows indicate sedimentation of reactive organic matter at a rate of dGi/dt. Bulk sediment organic matter decomposition is dGu/dt at the defaunated site and dGb/dt at the faunated site (after Kristensen[55]).
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Figure 12 Vertical profiles of POC content in two adjacent sandy sediment sites (5 m apart) from Løgstør Broad, Denmark. The + A. marina site was inhabited by a population of A. marina at a density of 60 m-2. The defaunated site contained no A. marina (modified from Kristensen et al[11]).
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Figure 13 Clearance (water volume cleared of algae) in microcosms supplied with 10000 cells ml-1 of Rhodomonas sp. in the overlying water throughout a 27 day experimental period (modified from Christensen et al.[9]).
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Figure 14 Oxygen uptake in microcosm with and without addition of Rhodomonas sp. in the overlying water (modified from Christensen et al.[9]).
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Figure 15 Vertical profiles of anaerobic carbon oxidation in defaunated and Arenicola marina inhabited sandy sediment (modified from Kristensen et al.[11]).
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Figure 16 Decay pattern of fresh barley hay (upper panel) and 50 days predecomposed (aged) barley hay (lower panel) in oxic and anoxic marine sediment (modified from Kristensen and Homier[59]).
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Figure 17 A schematic presentation of the volumetric model used for quantification of carbon reaction rates in sandy coastal sediment with a depth of L cm: A, with irrigated burrows of the polychaetes Nereis diversicolor. The grey zones indicate oxic surface sediment (thickness Lox cm) and oxic burrow walls (thickness Box cm). All burrows have a radius of r cm and a length of Lb cm. B; with reworking by the headdown conveyor-belt feeding polychaetes Arenicola marina. The lightly hatched zone (Lox) indicates the oxic surface sediment. The darkly hatched zone (L1) indicates surface related labile material displaced into anoxic sediment. In both cases, the carbon oxidation rate (R1) of the labile detritus under both oxic and anoxic conditions is A1 times faster than the carbon oxidation rate (R2an) of the partly degraded detritus in the anoxic black sediment (R1 = A1 R2an), When deep subsurface sediment is exposed to oxygen in irrigated burrows or by reworking the reaction rate is enhanced A2 fold (R2ox = A2 R2an) (modified from Kristensen and Holmer[59]).
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Figure 18 Enhancement of total sediment carbon oxidation due to increased microbial degradation in the presence of irrigated burrows (E1 = Ciox/Cdox) and reworking (Er = Crox/Cdox) as a function of abundance of Nereis diversicolor and Arenicola marina. Other variables are fixed: L = 20 cm, Lb = 20 cm, Lox = 0.3 cm, r = 0.3 cm, Box = 0.2 cm, A1 = 20 and A2 = 10. See text for further details (modified from Kristensen and Homier[59]).
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Figure 19 Decay constants (k) for the mineralization of 14C labelled Fucus serratus detritus. Labile and refractory detritus were deposited at the surface (Surf.) and in subsurface (Sub.) sediment of defaunated and Nereis diversicolor bioturbated microcosms held at 15°C (Mikkelsen and Kristensen[62]).
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Figure 20 Volume specific carbon oxidation as a function of diffusion scale (plug thickness). The experiment was conducted under anoxic conditions using sediment plugs of a diameter of 5 cm and a length ranging from 0.1 to 10 cm (Valdemarsen and Kristensen[64]).
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Figure 21 Balance between production of nutrients by microorganisms and uptake by assimilation or precipitation as a function of concentration (proportional to diffusion scale). Reaction is normalized to the maximum production rate and concentration is normalized to half saturation (Km) (modified from Aller and Aller[23]).
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Figure 22 Idealized example of product and substrate inhibition of microbial reactions as a function of product and substrate concentration. Reaction is normalized to the maximum production rate and concentration is normalized to half inhibition (Ki) (modified from Aller and Aller[23]).
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