ABSTRACT. The relationships between bacterial and phytoplankton variables, and particulate organlc carbon (POC), were examined along a longitudinal transect in the St. Lawrence Estuary Free bacteria, bacterial biomass and heterotrophic activity decreased in a seaward direction, whereas chlorophyll, phytoplankton biomass and photosynthetic act~vity had minimum values in the mid-estuanne region Values of POC and attached bacteria were h~g h e s t at the upper limit of the salinity intrusion, in the zone of ~naximum turbidity. Free bacteria predominated largely over attached bacteria, which were probably of benthic origin. It is hypothesized that free and attached bacteria formed 2 distinct, independent communities Correlation analysis showed that, in general, bacterial and phytoplankton vanables were highly correlated and that these variables were linked to salinity The effect of salin~ty on these correlations was controlled u s~n g partial correlat~on analys~s and the results showed that only attached bactena of the maxlmum turb~dity zone were signif~cantly col-related w~t h chlorophyll and POC concentrations. Elsewhere, phytoplankton and bacterial variables were not correlated. Careful estlmation revealed that bacterial b~o m a s s largely exceeded that of phytoplankton everywhere, except at the most seaward station, where coastal rather than ~s t u a r~n e condit~ons prevailed This study thus suggests that phytoplankton and POC did not to a large extent d e t e r m~n e bacterial distribut~on in the Upper St. Lawrence Estuary and that bacteria may have a relatively important trophic role In t h~s system

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The growth response of freshwater bacteria from the St. Lawrence River, exposed to brackish waters (salinity of 0 to 20‰) from the upper estuary, was assessed by a methodology requiring the combined use of dilution cultures and diffusion chambers. The longitudinal distribution of bacterial abundance in waters within this salinity range was also examined. Growth of the freshwater bacteria was reduced by 15 and 50% after exposure to salinities of 10 and 20‰, respectively. At lower salinities, no growth reduction was observed, and at a salinity of 2‰, growth was even stimulated. Longitudinal distribution data showed that bacterial abundance also peaked at this salinity. In contrast with an earlier hypothesis, this study shows that the decline of bacterial abundance in the low-salinity waters of the estuary is not caused by salinity-related mortality of freshwater bacteria, because the mixing time between fresh and marine (>20‰) waters is relatively long (days). However, results suggest that mortality of freshwater bacteria can be an important process in estuaries with shorter mixing times (hours). The combined use of diffusion chambers and dilution cultures proved to be a valuable methodology for assessing growth (or mortality) of bacteria exposed to environmental gradients.

The objective of this study was to evaluate the effect of agricultural pollution on periphyton in streams and rivers of southern Québec. We sampled benthic algae incubated from mid-July to mid-August on artificial substrates at 29 sites and analysed the variations in community structure and total community biomass. Diatom community structure as well as total benthic algae community were analysed. Water samples were taken to provide background chemical information, and land use data were also obtained. Preliminary tests showed that colonisation of the artificial substrates (unglazed ceramic tiles) resulted in biomass levels (Chlorophyll a and ash-free dry weight) and species composition that were not statistically different from those on natural rock substrates. The canonical correspondence analyses showed that pH, conductivity and suspended solids were the most significant environmental variables accounting for variations among sites and diatom community structure. No additional resolving power was obtained by including cyanobacteria, green algae and flagellates. This total community analysis substantially increased variance and sample processing time while reducing the relationship with environmental variables. These results indicate that an analysis based exclusively on diatoms provided the optimal approach. Traditional nutrient measurements (phosphorus and nitrogen) did not explain a significant part of the variance in the species composition among sites. The ordination analyses clearly separated agriculturally-impacted streams from reference sites, but no significant grouping was observed related to the intensity and type of agriculture, indicating the greater importance of local farming practices. The use of periphyton as a bioindicator provides an integrated measurement of water quality as experienced by the aquatic biota, and therefore offers a useful addition to physico-chemical water quality monitoring strategies.

The distribution of free bacteria in the St. Lawrence estuary was interpreted by means of bacterial fluxes and hydrodynamic residence times, derived from a circulation model, and growth and grazing rates, obtained from cultures. Bacterial abundances decreased exponentially along the salinity gradient, from 3.9 x lo9 liter-' in freshwater to 0.5 x lo9 liter-l at salinity ~20. The rates of growth (0.02-0.08 h-l) and grazing (0.0 l-O.09 h-l) were strongly correlated (r = 0.82), which suggests that bacteria tended toward a state of trophodynamic equilibrium (growth E grazing). However, negative bacterial fluxes obtained from the model imply that the estuary is a sink for bacteria. Results from the circulation model and the cultures show that physical processes are important controlling factors of the distribution of bacteria and point IO predation as the process causing bacterial losses in the estuary. Calculations show that small imbalances between growth and grazing can account for the observed distribution.Rates of dispersion and growth and grazing are of the same order of magnitude, which shows that biological processes are nevertheless significant relative to physical processes. Although the distribution of free bacteria appears largely under hydrodynamic control, the bacterial community is characterized by significant biological rates and can experience large transformations during estuarine transit.