Tubificids consumed O 2 in bottom water, so that the sediment was anoxic, allowing a direct flux of dissolved reduced species into overlying water. Compared to control experimental units, this recycled material represented at the end a several centimetre-thick layer enriched in water content, dissolved nitrate and sulphate, and depleted in oxygen, ammonium and dissolved Mn(II). Worm activity transferred reduced particles and pore water at the sediment surface at a rate of 0.14 cm day −1. Bioturbation intensity was determined using fluorescent tracers, and the distribution of redox-sensitive compounds was studied from replicate experimental units handled 7, 14, 21, 28 and 56 days after tubificid colonization. The bioturbating organisms were tubificid worms, which consume sediment at depth and deposit undigested material at the sediment–water interface. The effects of freshwater infaunal invertebrates on sediment geochemical properties were studied through an experimental approach using indoor microcosms during a 56-day experiment.

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