Effects of bioturbation in oxic and hypoxic conditions: a microcosm experiment with a North Sea sediment community

Sediment cores of 20 cm diameter containing the natural benthic fauna were subjected to low oxygen conditions in a laboratory microcosm system. After several days of oxic conditions ('oxic stage') the oxygen content of the water was reduced to 25% saturation for 15 d ('hypoxic stage&#...

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Bibliographic Details
Published in:Marine ecology. Progress series (Halstenbek) 1995-01, Vol.116 (1/3), p.153-161
Main Authors: Forster, Stefan, Graf, Gerhard, Kitlar, Jarmila, Powilleit, Martin
Format: Article
Language:English
Subjects:
Anaerobic conditions
Bioturbation
Bromides
Hypoxia
Marine
Oxygen
Sea water
Sediment core samples
Sediment transport
Sediments
Solutes
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Summary:Sediment cores of 20 cm diameter containing the natural benthic fauna were subjected to low oxygen conditions in a laboratory microcosm system. After several days of oxic conditions ('oxic stage') the oxygen content of the water was reduced to 25% saturation for 15 d ('hypoxic stage'), followed by a 'reoxygenation stage'. Effective solute transport rates were calculated using measurements with the conservative tracer ion bromide. Profiles of oxygen and ΣCO₂ were measured and molecular diffusive as well as effective fluxes, accounting for effective solute exchange, were calculated. The overall response of the benthic community was to compensate for low oxygen content of the overlying water by increased pumping activity. On average, effective diffusion coefficients (Deff) were 3 times higher in hypoxia than under oxic conditions. Deff reached 1.5 × 10⁻⁴ cm² s⁻¹, a value 30 times that of molecular diffusion. During hypoxia we observed low molecular diffusive O₂ flux, higher effective O₂ flux, as well as an increase in ΣCO₂ within the sediment. We interpret this as a shift of transport away from diffusion within the bulk sediment interstices (oxic conditions) to the advective transport pathways along burrows during hypoxia. This facilitates fast transport of oxygen and bromide along burrows and contrasts with the slower transport of CO₂ from the interstices governed by molecular diffusion. In this transient situation calulations based on gradients result in an unrealistic molar ratio of fluxes (CO₂/O₂) as high as 11.
ISSN:0171-8630
1616-1599
DOI:10.3354/meps116153