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The percentage of dividing individuals and temporal reproductive patterns were determined for natural populations of several planktonic protists including five species of tintinnids, a dinoflagellate, and a diatom. To obtain these data, a method was used in which the nuclei of planktonic ciliates and phytoplankters can be fluorescently stained with acridine orange at the time of collection and fixation. The technique is simple and can be used routinely in studies of reproduction or other life cycle phenomena of natural protistan populations. For the tintinnids, often more than half of the individuals were in some recognizable stage of fission; periodicity in the division process was only observed once and apparently followed a pulse of conjugation in the population. With the diatom Ditylum brightwellii the fluorescent staining technique yielded data on the extent and timing of division which were consistent with, but more complete than, previous enumerations of paired cells.

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The presence of nitrogen–Fixing cyanobacterial endophytes, such as Richelia intracellularis Schmidit commonly observed within several species of Rhizosolenia, in nitrogen‐limited oceanic waters has obvious implications both for the host organism(s) and for the entire planktonic assemblage. Recently, epi‐fluorescent examination of pllankton samples collected off Oahu, Hawaii, in september 1982 revealed R. intracellularis within the diatoms Hemiaulus membranaceus Cleve and H. hauckii Grunow as well as its expected presence within Rhizosolennia spp. Richelia intracellularis coccurred with within Hemiaulus spp. at frequenccies (ca.80%)comparable to those noted for Rhizosolenia spp. Standard bright field or phaseconrast microsocopy could not reliably distinguish the cyanobacteria within Hemiaulus spp. If this association is common, the occurrence of R.intracellularis (and its significance in the nitrogen dynamics of the ocean)may have been greatly underestimated in previous studies.

Enclosures of various sizes and configurations have been employed to maintain natural planktonic communities and to examine their responses to pollutants (e.g. Menzel & Steele, 1978; Steele, 1979). Studies on feeding, growth, and mortality of larval fishes, at times conducted in conjunction with pollution experiments, have also been successfully conducted in large enclosures (Koeller & Parsons, 1977). Implicit in these and other comparable studies has been the expectation (or hope) that a balance could be achieved between the advantages and difficulties inherent in more traditional field and laboratory studies. Field studies offer the realism of working with the natural assemblage with its many interacting components and links, but also suffer the disadvantage associated with a turbulent and advective system, which generally makes the repetitive sampling of the same populations impossible. Laboratory studies reverse the balance; control and definition of the components are gained at the expense of realism. The use of large containers represents a hybrid approach characterized by a partial control over a moderately realistic ecosystem. In the ideal case, the use of large containers allows sufficient control to permit experimental manipulation of (and the testing of hypotheses concerning) planktonic assemblages which are sufficiently realistic to permit extension of the experimental results to the ‘real world’.