Enclosed ecosystem experiments with marine plankton were carried out in Rosfjord, Nonuay, March-April, 1979. We report results from studies on planktonic bacteria in natural communities and cultures of phytoplankton contained in l -m diameter flexible plastic bags, and compare these to observations in the surrounding sea. Emphasis was on organotrophic bacteria and their diversity dynamics, particularly in relation to phytoplankton. Five previously isolated strains of bacteria were followed by immunofluorescent staining on membrane filters, and the populations of chromobacteria, bdellovibrios and luminescent bacteria were assayed by specific methods. Secondary productivity of total bacterial populations was estimated from growth rate measurements in filtered seawater and compared to net primary production. Possibly, as much as one third of the net production was ultimately consumed by the bacteria. The highly dynamic state of organotrophic bacterial populations and diversity in the sea are demonstrated, and the roles of substrate quality and concentrations and of potential bacterial grazers in population regulation are discussed.

Of 34 strains of marine bacteria isolated on a general seawater medium, 5 were selected for detailed studies of their population dynamics in the plankton. The isolates were characterized as Aeromonas sp., Chromobacterium cf. lividum, Vibrio sp., and two Pseudomonas spp. Specific antibodies were produced by immunization of rabbits, and bacterial cells were stained on black Uni-Pore membrane filters by an indirect immunofluorescent staining procedure. The method proved to be very specific and practical for use in a large-scale field sampling program. Growth of all five isolates was stimulated by high values for net primary production, chlorophyll a , and dissolved organic carbon. Calculation of a diversity index based on specific and total counts is proposed as a way of characterizing the dynamics of organotrophic bacterial populations in the sea.

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