Why do sinking mucilage aggregates accumulate in the water column?

The formation of nuisance scums of mucilage on the ocean surface requires that the sinking mucilage aggregates which eventually form the scums first become neutrally buoyant so as to accumulate in the water column. We hypothesize that neutral buoyancy is attained by an aggregate sinking from a lower density surface layer into a higher density halocline if some proportion of the aggregateʹs volume is occupied by lower density mucus which is resistant to diffusion of salts and water. A model, based on the porosities, excess densities, and sinking rates of natural diatom aggregates obtained empirically, demonstrates that only 0.5–2% of the volume of an aggregate need be occupied by mucus for it to attain neutral buoyancy and accumulate at even modest pycnoclines. The model predicts accumulations at vertical density gradients increasing by only 1–3 σt, similar to vertical changes in density previously observed at the depths of mucus accumulations in the Adriatic Sea.