Dynamique des populations planctoniques et bactériennes au cours d'une production expérimentale de phytoplancton marin naturel. II. Structure et physiologie des populations et leurs interactions

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Abstract
During spring, an experiment was conducted on the production of natural phytoplankton in a continuous flow layout of large capacity, and an analysis was made of the simultaneous evolution of microplanktonic populations (taxonomic composition, diversity, class ranges and energetic charge) and of bacterial populations (structure, diversity, dominances and catabolic potentialities). Oligotrophic initial conditions were characterized by a poor (0.22 .mu.g of chlorophyll a/l) and diversified (pigment diversity = 4.5) phytoplankton with nanoplanktonic dominance associated with several benthic type diatoms. Bacterial flora, in which pseudomonads were dominant (54.5%), were also diversified (Shannon index .hivin.H = 3.57). Metabolic potentialities of these bacterial communities were several. Enrichment of the milieu caused the selection and development of a small number of algal species dominated by the diatoms Skeletonema costatum (64 .times. 106/l) and Chaetoceros (3.2 .times. 106/l) leading to a paucispecific population with a large biomass, a pigment diversity of 1.97 and a chlorophyll a concentration of 25.08 .mu.gl. The bacterial community then became diversified (.hivin.H = 4.12) and, although pseudomonads remained dominant (62%), a larger use of organic micromolecules was noted. The establishment of this phytoplanktonic community was followed by a zooplanktonic development in which larger species succeeded smaller ones (ciliates, tintinnids, rotifers and copepods) corresponding to the global increase of the phytobiomass. Although this system may be in a phase of relative stability, a decrease in the energetic charge showed a disturbance in the physiological state of the planktonic populations. These phenomena led to the vanishing of the diatom''s dominance (Chaetoceros and then S. costatum) and its replacement by nanoplankton (123 .times. 106 cells/l) and dinoflagellates (300,000 cells/l). A modification in the organic wealth of the milieu with the addition of a particular complex material that was not or little degraded caused deep disturbances in the bacterial populations. Their diversity reached a very low level (.hivin.H = 1.56-2.25) with a proliferation of vibrions (up to 74%) well equipped with exoenzymes, but more restricted in their nutritional versatility (marked preference for organic macromolecules). During the final phase of experimentation, despite a change in qualitative composition and a more important biomass, microplanktonic and bacterial populations returned to a diversified state nearing that of the initial conditions (pigment diversity = 3.2; .hivin.H = 3.37).