RESPIRATORY ACTIVITY AND CO2 PRODUCTION-RATES OF MICROORGANISMS IN THE LOWER ST-LAWRENCE ESTUARY

The spatio-temporal distribution of the respiratory activity of microo rganisms (<200 mum) in the lower St Lawrence Estuary was investigated by monitoring their ETS (electron transport system) activity. Three st ations located in the Laurentian Through were investigated three times during the early summer (June-July 1990; COUPPB90-1 cruise). The larg est ETS activity values were always found in surface waters and then d ecreased rapidly through the top 25 m. Nevertheless, ETS activity in t he aphotic zone exhibited a relative peak in the minimum temperature l ayer and tended to increase close to the bottom. ETS activity maxima w ere also associated with higher NH3 levels and increasing concentratio ns of seston particles. During the cruise, subsurface peaks in ETS act ivity were mainly correlated with the vertical distribution of the ses ton particle size fraction <5 mum. In the aphotic layer, rates of resp iration and CO2 production were calculated in two ways, one assuming t hat microorganisms were all in the senescent phase and one assuming th at they were all in log-phase growth. The calculated daily metabolic C O2 production, integrated through the water column below 25 m, reveale d a range of 26-48 mg C m-2 d-1 assuming senescence and a range of 154 -278 mg C m-2 d-1 assuming log-phase growth rate. Different explanatio ns are presented to account for which R/ETS ratio more closely describ ed qualitatively and quantitatively the production/respiration budget during the cruise and to reduce the range of uncertainty.