Measurement techniques for quantification of pumping activity of invertebrates in small burrows

Abstract

The quantification of exchange processes across sediment-water interfaces needs precise measurements of burrow ventilation of tube-dwelling invertebrates, including small species which can occur in high densities. Since most studies have focused on relatively large organisms, established techniques are usually suitable for burrow diameters clearly > 2 mm. Our aim was to test video analyses, colour tracers, O 2 and flow velocity microelectrodes, thermal flow sensors, particle image velocimetry, and conductive exchange experiments, for quantifying pumping activity in burrows ≤ 2 mm in diameter in muddy sediment. To assure comparability, all measurements were performed with the same species (Chironomus plumosus, 4 th instar larvae). On average we determined a flow velocity of 14.9 mm s -1 , an individual pumping time of 33 min h -1 , an individual pumping rate of 61 ml h -1 (1.7 mm burrow diameter), and a population pumping rate of 1.09 m 3 m -2 d -1 (745 larvae m -2 , 20-23 °C). Our values are higher than mostly reported for other small invertebrates. The differences might be caused by the artificial set-ups used in previous studies, and by the fact that C. plumosus conducts pumping not only to meet its oxygen demand such as other species but also to filter-feed. All seven techniques tested in this study are applicable for burrow diameters of 1.7-2 mm in muddy sediments. We recommend colour tracers and particle image velocimetry to measure flow velocity, and flow velocity microelectrodes and thermal flow sensors to determine the individual pumping time. Conductivity exchange experiments are suitable to directly measure individual pumping rates.