Gross primary production, community respiration and reaeration coefficient were determined during an annual cycle on the Viroin River (South Belgium), based on the daily variations of dissolved oxygen concentration.Reaeration coefficient remains remarkably constant (0.26 h-') during the year in spite of discharge variations. The autotrophic community is dominated by 'Ranunculusfluitans'. Primary production parallels the variations of total solar radiations. It ranges from 0 in winter to 8 g 02 m 2 d-in summer. In spring and summer, respiration variations parallel those of primary production (average value: 10 g 02 m 2 d-1 ); in the dry autumn, decomposition of dying macrophytes considerably enhances the community respiration (15 g 02 m 2 d-l).A P/R diagram is used to characterize the trophic state of the Viroin.

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