The qualitative and quantitative analyses of submicron particles from coastal surface seawaters off California involved their separation by ultracentrifugation, visual examination by transmission electron microscopy (TEM), and the determination of elemental composition by energy dispersive spectroscopy. Particles fell into three classes: organic, both living and detrital; inorganic, such as iron colloids; clay minerals. The greatest abundance of particles occurred in the smaller than 120 nm size fraction. The mass concentration for particles sized between 5 and 120 nm is estimated to range between 0.03 and 0.09 mg l−1. Particle numbers in this size range were greater than 109 particles ml−1 with the total surface area being 8 m2 per cubic meter of seawater, or more. Particle size distributions in the smaller than 120 nm fraction showed numbers to increase nearly logarithmically with decreasing size. TEM examination indicates that many of these particles are in fact aggregates of granules 2–5 nm in size.
