Advection of freshwater phytoplankton in the St. Lawrence River estuarine turbidity maximum as revealed by sulfur-stable isotopes

Abstract

While estuarine turbidity maxima (ETM) are commonly known for their high biological productivity, the high abundances of primary producers in these light-limited environments have long been questioned. In the St. Lawrence River ETM, we performed sampling cruises during August and October 2006 to test the hypothesis that advection of upstream algae is responsible for high phytoplankton biomass within the ETM. The ETM was situated at the freshwater end of the estuary, where salinity ranges from 0.06 to 1.10. This abrupt peak in turbidity (up to 89.9 and 49.5 nephelo- meter turbidity units (NTU) during August and October, respectively) coincides with peak chloro- phyll a concentrations (54.8 and 22.9 µg l -1 during August and October, respectively); surface and depth concentrations were similar in the ETM and upstream, where the water column was vertically mixed. These biomasses represented an increase of >1 order of magnitude compared to upstream and downstream sites, while no growth proximal factor such as light or nutrients could explain this pattern. Sulfur-stable isotopic ratios from planktonic algae in the ETM differed from those of local periphyton, but corresponded to those of attached algae grown upstream in freshwater. Phytoplank- ton community structures were similar throughout the salinity gradient, and the vast majority of iden- tified algae corresponded to genera common to freshwater, ETM and downstream environments. This study provides evidence that the observed peak in phytoplankton biomass within the ETM is largely due to high inputs of phytoplankton from upstream freshwater, rather than to local growth.