Respiratory activity and C02 production rates of microorganisms in the lower St Lawrence Estuary

The spatio-temporal distribution of the respiratory activity of microorganisms (<200 μm) in the lower St Lawrence Estuary was investigated by monitoring their ETS (electron transport system) activity. Three stations located in the Laurentian Trough were investigated three times during the early summer (June-July 1990; COUPPB90-1 cruise). The largest ETS activity values were always found in surface waters and then decreased rapidly through the top 25 m. Nevertheless, ETS activity in the aphotic zone exhibited a relative peak in the minimum temperature layer and tended to increase close to the bottom. ETS activity maxima were also associated with high NH3 levels and increasing concentrations of seston particles. During the cruise, subsurface peaks in ETS activity were mainly correlated with the vertical distribution of the seston particle size fraction <5 μm. In the aphotic layer, rates of respiration and CO2 production were calculated in two ways, one assuming that microorganisms were all in the senescent phase and one assuming that they were all in log-phase growth. The calculated daily metabolic CO2 production, integrated through the water column below 25 m, revealed 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 explanations are presented to account for which R/ETS ratio more closely described qualitatively and quantitatively the production/respiration budget during the cruise and to reduce the range of uncertainty.