Particle Size

Abstract

Suspended matter in natural waters – in the sea as well as in estuaries, rivers, and lakes – is mainly present in the form of flocs (or aggregates) which have been observed by many authors in situ in the water or in suspended matter samples collected on filters, or studied with a reversed microscope (Beebe 1934; Sherman 1953; Suzuki and Kato 1953; Nishizawa et al. 1954; Piccard and Dietz 1957; Berthois 1961; Krone 1962; Riley 1963; Riley et al. 1964, 1965; Holmes 1968; Sheldon 1968; Biddle and Miles 1972; Edzwald 1972; Schubel and Kana 1972; Kranck 1973; Paerl 1973 to name the earlier ones). Mostly the flocculated material was observed under a microscope and usually found mixed with single mineral particles. Nishizawa et al. (1954), however, already indicated that flocs in situ in the water are fragile and break up readily during sampling so that suspended flocs have to be studied by in-situ observation. Observations of suspended flocs while diving or from a submersible were recently made by Trent et al. (1978), Shanks and Trent (1980), Silver and Alldredge (1981), Syvitski et al. (1983) and Eisma (1986). Since diving and submersibles have a limited application, camera systems were developed for in-situ photography. A Benthos plankton camera was used by Eisma et al. (1983) and Kranck (1984a). Other camera types were developed by Honjo et al. (1984), Johnson and Wangersky (1985), Wells and Shanks (1987), and Eisma et al. (1990). Most camera systems do not allow in-situ observations of particles smaller than 50–100 µm; the one developed by Eisma et al. (1990) gives results down to about 3 µm. Bale and Morris (1987) used laser diffraction to determine in-situ particle size down to about 1 µm. An optical settling tube giving size distributions from a few microns to several hundred microns was deployed by Spinrad et al. (1989) in the deep ocean. In-situ flocs have a size up to 1–2 mm but large elongated flocs with a length of up to 10 cm have been observed and occasionally, in quiet waters, even much larger flocculated structures.