Relationships between bacteria, phytoplankton and particulate organic carbon in the Upper St. Lawrence Estuary

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~ghes 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~omass 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 de t e rm~ne bacterial distribut~on in the Upper St. Lawrence Estuary and that bacteria may have a relatively important trophic role In t h ~ s ystem