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An attempt has been made to evaluate the accuracy of ATP as a measure of living phytoplankton carbon in estuaries. Phytoplankton carbon estimated from ATP was compared to estimates from cell counts. In high biomass samples the agreement between the two estimates was quite good. In the low biomass samples the cell count method underestimated phytoplankton carbon relative to the ATP estimate. This was interpreted to be due in part to low cell counts in these samples. Contamination by microzooplankton (essentially only tintinnids) was, in 14 out of the 18 samples, 1 μg chlorophyll a/L for the St. Lawrence). Since carbon to chlorophyll ratios in the high biomass samples varied considerably, a constant ratio appears inapplicable for transforming chlorophyll to carbon. Key words: ATP, phytoplankton biomass, estuaries, carbon to chlorophyll ratio

A transect across the lower St. Lawrence estuary was occupied to estimate volume transport and nutrient distributions (total dissolved nitrogen and phosphorus, and silicate) during February and May through September. Seaward transport was restricted to the upper 50 m and varied from 17 × 103 m3/s (August) to 129 × 103 m3/s (May). Nutrient and volume transport data are combined to estimate seasonal variability in monthly surface nutrient transport towards the Gulf of St. Lawrence. The estimates vary as follows: 0.72–6.72 × 103 tons N/mo, 0.05–0.48 × 103 tons P/mo and 0.50–6.73 tons Si/mo. Ratios between monthly freshwater input into the estuary and the corresponding values for net seaward transport, which varied from 1.1 to 9.4, are combined with annual data (1960–73) on monthly freshwater input into the estuary to estimate potential annual variability in seaward transport for each of the above months. Observed nutrient data, with the above transport estimates, suggest the possible range in monthly seaward nutrient transport that would be associated with the annual variability in monthly freshwater input into the estuary. Seaward nitrogen transport from May through September, estimated at 2.3 × 104 tons, is considered in relation to published data on the primary production in the western part of the Gulf of St. Lawrence. The estuary supplied a smaller amount of nutrients to the Gulf than has previously been assumed.

The study involved two data series, a weekly sampling program at a single station near Rimouski from June through September, 1974, during which semidiurnal tidal effects were considered, and a 5-d survey of the lower St. Lawrence estuary during July 1975. The biomass and species composition distributions during the survey appeared to be a function of the residual circulation and mixing characteristics of the lower estuary. Active growth in phytoplankton populations was limited to a 2-mo midsummer period. The impoverishment up to late June was probably due to high turbidity, the 1% light level being about 4 m in depth, rather than due to advective flushing. Strong temporal fluctuations in water column phytoplankton biomass and production were similar to the distribution in density stratification. The confidence limits of the derived parameters [photosynthesis at light saturation normalized by chlorophyll (θ) and by carbon (μpot), and the carbon/chlorophyll ratio] were broad with respect to their seasonal range. Due to the lack of coherence in the physiological state indices considered, and the importance of species composition differences in "explaining" the observed distributions of the indices, the utility of the derived parameters as measures of physiological state of mixed populations is questioned. Key words: phytoplankton, St. Lawrence estuary, primary production, physiological state, tidal mixing