Influence of physical forcing on microclouds of dissolved organic matter and nutrients in the ocean

A theory is developed on the statistical field description of plankton microzones or micropatches. The theory, which is based upon stochastic geometry, predicts length scales and volume-fraction measures associated with plankton dynamics. The theory is exemplified by considering how microclouds might modulate the transformation of dissolved organic carbon (DOC) into heterotrophic biomass. Volume fraction, covariance functions and spherical contact distributions for assemblages of Baltic Sea phytoplankton were computed using real data on phytoplankton cell density and size with conjectured microzone dimensions. By comparing microcloud length scales with molecular diffusion, turbulent diffusion and uncorrelated velocity length scales, we determined that variability in the turbulent kinetic energy (TKE) dissipation rate, which might be induced by global and basin-scale wind forcing (for example), could influence the structure and functioning of the carbon microclouds. Microclouds, in contrast to microzones, are subject to the straining effects of turbulent diffusion, in addition to molecular diffusion. The microclouds themselves are often too small to be subject to uncorrelated velocities. However, the relative motion among microclouds is driven by the uncorrelated velocities of homogeneous and isotropic turbulence. The concentrations of DOC suggest that the microclouds are temporary loci that enhance interaction among phytoplankton cells, bacteria and microzooplankton.