The relationship between Chironomus plumosus burrows and the spatial distribution of pore-water phosphate, iron and ammonium in lake sediments

1. To study the influence of chironomids on the distribution of pore‐water concentrations of phosphate, iron and ammonium, we conducted a laboratory experiment using mesocosms equipped with two‐dimensional pore‐water samplers, filled with lake sediment and populated with different densities of Chironomus plumosus. 2. Specially designed mesocosms were used in the study. A 6‐mm deep space between the front plate and the pore‐water sampler at the back plate was just thick enough to allow the chironomids to live undisturbed, yet thin enough to force all the burrows into a two‐dimensional plane. 3. The courses of the burrows were observed during the experiment as oxidised zones surrounding them, as well as being identified with an X‐ray image taken at the end of the experiment. 4. We investigated the relationship between C. plumosus burrows and spatial patterns of pore‐water composition. Concentrations of the three ions were significantly less around ventilated burrows (54% to 24%), as bioirrigation caused a convective exchange of pore‐water enriched with dissolved species compared with the overlying water, and also because oxygen imported into the sediment resulting in nitrification of ammonium, oxidation of iron(II) and a co‐precipitation of phosphate with Fe(III) oxyhydroxides. 5. In mesocosms with chironomids, new (redox) interfaces occurred with diffusive pore‐water gradients perpendicular to the course of burrows and the site of major phosphate, ammonium and iron(II) release shifted from the sediment surface to the burrow walls.