The development of sensitive fluorimetric techniques for continuous in vivo measurements of chlorophyll ‘a’ (Lorenzen, 1966) has made it possible to study in detail the vertical distribution of phytoplankton in lakes and seas (e.g. Strickland, 1970; Berman & Pollingher, 1974; Lasker, 1975; Fasham & Pugh, 1976; Pingree et al. 1976). However, for marine environments there is little compatible data, such as given by Lasker (1977), on the species composition of localized phytoplankton populations. This is an important gap in our knowledge since the variations shown by different algal taxa in attributes such as nutrient requirements, growth rates, adaption to light, and palatability to zooplankton have wide ecological implications.