In Situ Sediment–Seawater Solute Flux Determined with a Unisense A/S MiniChamber Lander: Effects of Stirrer Speed and Particle Resuspension

Abstract

Benthic chambers measure the sediment–seawater solute exchange. In cohesive sediments, this exchange may scale with the sediments’ diffusive boundary layer (DBL) thickness and thus the intensity of chamber water agitation. To investigate this, we deployed a customised Unisense A/S MiniChamber lander with an opaque, stirred benthic chamber on the subtidal seafloor of Queen Charlotte Sound, New Zealand. We deployed the chamber at 18 sites over six days, during the morning, midday, and the afternoon, using four stirrer speeds (10, 17, 26, 35 rpm). All but the highest stirrer speed allowed particles resuspended during chamber deployment to settle. Sediment total O<inf>2</inf> uptake (TOU) ranged from 614 to 1125 µmol m⁻² h⁻¹. Ammonium, phosphorus, and nitrate fluxes indicated consistent sediment release, except for two deployments that showed nitrate uptake. Statistical analyses revealed a negative effect of the initial chamber turbidity on TOU, but failed to detect effects of stirrer speed or time of day on any measured flux. Variation in TOU was minimal across stirrer speeds at midday but higher during the morning and afternoon. We discuss these observations in the context of sediment disturbance during deployment and diel variations in infaunal activity. Overall, our results align with those of previous studies using different chambers, indicating that chamber-derived sediment–seawater solute flux estimates may be unaffected by chamber water agitation intensity.