Size spectra and aggregation of suspended particles in the deep ocean

  • a School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
  • b Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A.

Abstract

Work of the last 10 years has demonstrated that oceanic particle size distribution by volume tends to be flat at mid-water depths (equivalent to a cumulative particle number distribution with a slope of −3) and is peaked in nepheloid layers with active resuspension and in surface waters with active biological production. The observed loss of fine peaks from the suspensions to yield flat distributions requires aggregation of the material, as the fines settle slowly. Mechanisms leading to particle collision are examined; for interactions between particles of similar size, Brownian motion dominates below 1.5 to 8 μm. However, if large particles (such as ‘marine snow’) are present at realistic concentrations, they become important in the removal of fine particles by shear-controlled coagulation. The coagulation times calculated for shear are too long for steady state to be presumed while the size distributions evolve under the influence of coagulation mechanisms. Therefore suggestions that the flat size distributions are quasi-stationary results of shear-controlled coagulation are rejected, and the notion that there is sub-equal production of particles at different points in the spectrum is favoured. Such production and the subsequent scavenging of small particles by large settling ones confers great importance on components of biological origin in both providing elements of the total size spectrum and determining the distribution and sedimentation of others of lithogenic origin. In surface waters, filtration rates by zooplankton indicate that aggregation rates of particles above submicron sizes are biologically determined.

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