Assessment of Salinity-Related Mortality of Freshwater Bacteria in the Saint Lawrence Estuary

ThegrowthresponseoffreshwaterbacteriafromtheSt.LawrenceRiver,exposedtobrackishwaters(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 salinityrangewasalsoexamined.Growthofthefreshwaterbacteriawasreducedby15and50%afterexposure tosalinitiesof10and20‰,respectively.Atlowersalinities,nogrowthreductionwasobserved,andatasalinity 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,becausethemixingtimebetweenfreshandmarine(>20‰)watersisrelativelylong(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. Low-salinity (,10‰) estuarine areas have been identified as regions of severe physiological stress for both freshwater and marine organisms (10). These waters are also important sites for biological and chemical reactions (16, 18). The impact of these processes on bacterial dynamics has never been well documented. Morris et al. (18) hypothesized that mass mortalityoffreshwaterphytoplanktonresultsinabuild-upofeasily degradable dissolved organic matter, which, in turn, supports localized and active bacterial populations. Therefore, low-salinity estuarine waters should favor bacterial growth and metabolism. However, there is field and experimental evidence suggesting that bacterial activity is adversely affected at low salinity, in particular because of low halotolerance of freshwater bacteria. In the Tamar Estuary, Mantoura (16) reported a drastic drop in glucose heterotrophic activity between salinities of 0 and