The Barra Lagoon (28°S and 43°W, 7 km², mean depth, 1m, salinities from 3 to 12 ) was investigated between October 1990 and August 1993. We report here a first synthesis of organic carbon metabolism, the phytoplankton community and their mutual interactions. Barra Lagoon exhibited a hypereutrophic state (high values of phytoplanktonic biomasses, diurnal metabolic rates, particulate and dissolved organic carbon concentrations). As with most shallow lagoons, Barra Lagoon was very sensitive to short-term meteorological changes which induced high day to day changes in organic carbon metabolism and severely dampened seasonal trends. But surprisingly, when consistent structural changes occurred, its total organic carbon metabolism was rapidly dampened by shifts from pelagic to benthic carbon metabolism and vice-versa. Seasonal auto/heterotrophic shifts of the ecosystem (generally autotrophy in summer and heterotrophy in winter) were not always verified. They were more strongly linked to growing and degeneration phases of pelagic and benthic algae. Biological activity was mainly controlled by the rate of nutrient in situ regeneration (low nutrient concentrations in water). In these conditions, small-size phytoplankton species predominated, as they are more efficient than large forms in sharing nutritive resources. Cyanophyceae were ecologically favoured by low transparency, nitrogen limitation and high nutrients regenerated in the water column, while small species of Dinophyceae and Chrysophyceae were promoted by a higher photic layer, nitrogen content and benthic nutrient regeneration. Extra input of nutrients (proceeding from fish-kill in February 1991, from decomposition of macro-algae after a temporary phase of colonization in August-September 1992, and from abundant rainfalls) induced an ephemeral growth of larger species at the expense of Cyanophyceae. Cyanophyceae lost their dominance also when temperature fell to 20°C.

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