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Abstract

We present the first direct measurements of apparent diffusivity within diatom aggregates using a diffusivity microsensor. Transparent exopolymer particles (TEP) and aggregate dry mass (TEP and cells) were determined in the same aggregates after diffusivity measurements. Carbon in TEP comprised 8‐12% of aggregate dry mass. The (wet) volume fraction of TEP in aggregates, however, was on average 7.2‐fold larger than that of cells, and it decreased with increasing aggregate size similar to that of cells. The exchangeable pore‐water content occupied 87‐98% of the aggregate volume. The average apparent diffusivities of gases within aggregates ranged between 0.90 and 0.95 times the free diffusion coefficient in seawater. Using a diffusion‐reaction model, we analyzed silicic acid concentrations within marine snow. An apparent diffusivity of silicic acid within marine snow being 0.9 times its free diffusion coefficient in seawater, and a specific net silica dissolution rate of 0.002 h21 could explain the observation that concentrations of silicic acid are significantly higher within marine snow compared to that of the ambient water.