Role of sub-micrometre particles in the ocean

Abstract

PARTICULATE matter plays an important part in biogeochemical cycles in the ocean; as particles settle out of the water column, they carry with them carbon and other adsorbed chemicals. Because particles of sub-micrometre size (at the boundary between 'particulate' and 'dissolved' materials) do not have appreciable settling rates in natural waters, they have generally been considered unimportant in the downward flux of particles and the nature and origin of the particles are largely unknown^1–5. Here we present results from epifluorescence microscopy and from particle counting which have allowed us to determine the vertical distribution of sub-micrometre particles (size range 0.38–1 µm). We find that >95% of these particles are non-living and occur in the upper layers of the ocean (50 m) in concentrations of the order of 10 million per millilitre. Many of the non-living particles seem to be fragile and flexible, and seem to have a high water content and to be composed largely of organic material. The size distribution of these sub-micrometre particles leads us to conclude that a significant portion (at least 10%) of 'dissolved' organic material may in fact be in the form of these small particles, as suggested by Sharp⁶.