Diffusion and advection within and around sinking, porous diatom aggregates

Fluid motion within and around sinking aggregates is an important factor in particle scavenging and solute exchange between sinking aggregates and the surrounding water and, hence, vertical fluxes and remineralization processes in the ocean. In the present study, we analyzed O 2 uptake rates in >2‐mm porous diatom aggregates and in model aggregates impermeable to flow by measuring, on the same aggregates, the interface diffusive uptake rates with microsensors and the total (diffusive + advective) O 2 uptake rate by the Winkler method. The uptake rates were measured in a flow field similar to that experienced by sinking aggregates. The ratio of total O 2 uptake rate to diffusive uptake rate was 0.97 ± 0.10 (n = 14) in model aggregates impermeable to flow. In contrast, total O 2 uptake was similar to or higher than diffusive uptake rate calculated from the O 2 gradients at the aggregate‐water interface in 85% of all field‐sampled and roller tank diatom aggregates examined. The highest ratio of total O 2 uptake rate relative to diffusive uptake rate measured in <1‐cm field‐sampled diatom aggregates was 3.91 ± 1.39 (n = 22). Hence, diffusive O 2 uptake calculated from the O 2 gradients in aggregates is a conservative (minimum) estimate of total O 2 uptake. The estimated average fluid velocity through the cross‐sectional area of field‐sampled diatom aggregates, which could explain the measured differences in O 2 uptake, ranged between 5 and 40 µm s -1 . The average value was 16 µm s -1 , which was equal to 1.3% of aggregate sinking velocity.