The vertical distribution of chlorophyll in the sea generally shows a maximum concentration at a depth which may vary from at or just below the surface, to near or below the bottom of the euphoric zone. The easiest distributions to explain are those where the maximum chlorophyll concentration is at or very near the surface. These are found when a population is photosynthesizing actively during the first stages of an outburst, and the distribution corresponds roughly with the change with depth of the photosynthetic rate (Steele, 1957). However, these form a small proportion of observations, probably because outbursts are usually restricted to comparatively short periods of time. The more usual distributions show a maximum in the chlorophyll concentrations well below the surface and below the depth of maximum photosynthesis.