The range of recorded sinking velocities of particles in the oceans spans from negative (particles float toward the surface) to several km per day (as with salp fecal pellets) When considering the sinking velocity of an individual particle, a first approximation can be obtained from Stokes law (originally derived for spherical, non-porous particles and laminar flow) combined with Whites approximation, which suggest that sinking velocity increases linearly with excess density (the difference from the water density) and the square of particle diameter (i.e., linearly with the particle area). Building on these expectations, many studies have tried to relate sinking velocity primarily to size, which has been shown to be a useful predictor for particles generated in controlled environments (e.g., roller tanks. However, strong relationships were only observed when all particles were generated using the same water/plankton community. When particles were made by different plankton communities, size alone was a bad predictor (e.g., Diercks and Asper, 1997) strongly supporting notions that particle densities and shapes vary widely depending on the source material.
Packaging and porosity contribute appreciably to determining sinking velocities. On the one hand, adding ballasting materials, such as diatom frustules, to aggregates may lead to an increase in sinking velocities owing to the increase in excess density. On the other hand, the addition of ballasting mineral particles to marine particle populations frequently leads to smaller more densely packed aggregates that sink slower because of their smaller size. Mucous-rich particles have been shown to float despite relatively large sizes, whereas oil- or plastic-containing aggregates have been shown to sink rapidly despite the presence of substances with an excess density smaller than seawater. In natural environments, particles are formed through different mechanisms, by different organisms, and under varying environmental conditions that affect aggregation (e.g., salinity, pH, minerals), ballasting (e.g., dust deposition, sediment load; van der Jagt et al., 2018) and sinking behaviour (e.g., viscosity;). A universal conversion of size-to-sinking velocity is hence impracticable.