Introduction

A significant proportion of all geological and biological processes in the sea is concerned with the behaviour and fate of particles. Living and nonliving matter exist, move about, and interact insuspension and along fluid boundaries. Most particles have no independent powers of locomotion but are dependent on the dynamic forces of waves and current to counteract their negative buoyancy. As a result, their dimensions are controlled and constrained by the physical environment. Understanding the nonfluid component of the oceans depends on research on the physical properties that control the dynamic behaviour of particles.

The principal physical properties of particles are density, shape, and size. Size varies by at least three orders of magnitude: from clay grains and bacteria, the smallest particles in the sea, to sand grains and diatoms, the largest passively suspended, single grains in the sea. In contrast, rock-forming silicate minerals and calcium carbonate grains – the principal constituents of inorganic sediment – vary in density from ∼ 2.3−2.7 g/cm3. Organic matter has a density range of <1 to ∼ 1.2 g/cm3. Thus effective density varies by less than an order of magnitude. Of even smaller relative importance are differences in dynamic properties resulting from variation in shape. According to Komar & Reimers (1978), shape may change the settling rate of fine-grained inorganic sediment at most by a factor of 2. Even the very extreme diversity of shapes exhibited by phytoplankton does not alter the basic dependence of their settling rate on particle size.