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This paper investigates and compares experimentally determined water velocity field above natural macrozoobenthos burrows generated by Chironomus Plumosus larva during their bio-irrigation activity. All experiments were carried out using particle image velocimetry and performed in mesocosms filled with sediment burrowed by larvae, and the water velocity fields near the inlets and outlets of the U-shaped burrows were measured. From water velocity data the average volumetric flow rates between 54.6 and 61.1 mm3/s were calculated. Assuming an average burrow diameter of 2.25 mm, the volumetric flow rates suggest the average flow velocities through burrows during the pumping period between 13.7 and 15.4 mm/s. Two additional interesting phenomena could also be shown by analyzing the flow field generated by the larva. The analysis of the amount of tracers used for visualizations revealed that some of the tracer particles added to the water must have been consumed along their path from the inlet toward the outlet, hinting clearly to the so-called filter-feeding action of C. plumosus. The second phenomenon is due to the form of motion C. plumosus generates. By careful flow visualizations it was found that unlike other organisms such as Urechis caupo that use peristaltic body contractions, C. plumosus worms its body sinusoidally catapulting the fluid far into the overlying water body. This action is of ecological advantage for it avoids generating short oxygen circuits for their respiration and filter feeding.
