Elimination of fine suspensoids in the oceanic water column

Fine suspended materials in the particle-size range 1 < r < 6 μm (r, equivalent sphere radius) from the equatorial North Atlantic, between the ocean surface and greater depths (30-5100 m), can be consistently described by a size-distribution relationship dN/dr = Ar^-b, where the parameters A and b are independent of r. The shape of the particle-size distribution is essentially constant and independent of depth, as reflected in the values of the logarithmic slope of the size distribution b = 4.0 ± 0.3. The particle-number and mass concentration, however, decrease strongly with depth, most of the decrease taking place in the surface layer 200 m deep. In the surface layer, mass concentration decreases exponentially with the half-concentration depth z _{1 2} {reversed tilde equals} 60 m; in the deep layer (200-5000 m), z _{1 2} {reversed tilde equals} 1000 m. The computed removal half-lives of the particulate material are 0.8 ± 0.6 yr in the surface layer, and 60 ± 20 yr in the deep layer, both half-lives being considerably shorter than Stokes settling residence times. The fast turnover or removal rate in the surface layer is compatible with the rates of zooplankton growth and carbon assimilation.