Midsummer plankton development along a large temperate river: the St. Lawrence River

Abstract

Midsummer planktonic communities were sampled in 1997 along a 425-km reach of the St. Lawrence River between Lake Ontario and Trois-Rivières (Quebec) to describe longitudinal patterns of plankton development. Chlorophyll a (Chl a) quickly declined as water flowed from Lake Ontario into the river. Thereafter, in the reach upstream from Montreal, Chl a was ~1 µg·L-1and total phosphorus (TP) was ~10 µg·L-1. Downstream from Montreal, TP increased to 20-40 µg·L-1and Chl a rose to 2-5 µg·L-1. Although Chl a increased concomitant with TP, phytoplankton may have been light limited at the more turbid downstream stations. Zooplankton biomass decreased from 40 µg dry mass·L-1in Lake Ontario to 16 µg dry mass·L-1at Brockville, 90 km downstream. Zooplankton biomass continued to decline and downstream from Montreal remained <10 µg dry mass·L-1, despite the increased TP and Chl a. Rotifers dominated the zooplankton community, but as in other temperate rivers, total zooplankton biomass was low (<10 µg dry mass·L-1). There was no longitudinal pattern in heterotrophic bacterial or flagellate abundance. A comparison of the inflow and outflow biomass of two fluvial lakes showed the lakes to contribute little to the downstream river plankton during summer.