Article révisé par les pairs
Résumé : Algal and bacterial biomass and activities have been studied in the Loire estuary (France) by means of particulate organic matter proteins and chlorophyll pigments determination, electron transport system activity measurement, 14C-bicarbonate incorporation and tritiated thymidine incorporation determination. These data collected throughout the estuary in various hydrological conditions allow to characterize two opposite typical situations: (i) During winter and early spring, when discharge increases beyond 1000 m3s-1, heterotrophic activity always dominates over primary production but remains moderate because of low temperature and non-biodegradable quality of organic carbon; (ii) at drought situations, phytoplankton production develops in the river causing increases of pH and oxygen concentration. In these situations, accumulation of phytoplanktonic material in the highly turbid inner estuary results in very high heterotrophic activities causing pH decrease and complete depletion of dissolved oxygen. Budgets of POC show that the anthropogenic inputs contribute only for less than 5% to the organic load of the inner estuary. © 1988.