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

Midsummer planktonic communities were sampled in 1997 along a 425-km reach of the St. Lawrence River between Lake Ontario and Trois-Rivieres (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, Chi a was -1 mu g.L-1 and total phosphorus (TP) was -10 mu g.L-1. Downstream from Montreal, TP increased to 20-40 mu g.L-1 and Chl a rose to 2-5 mu g.L-1. Although Chi a increased concomitant with TP, phytoplankton may have been light limited at the more turbid downstream stations. Zooplankton biomass decreased from 40 mu g dry mass.L-1 in Lake Ontario to 16 mu g dry mass.L-1 at Brockville, 90 km downstream. Zooplankton biomass continued to decline and downstream from Montreal remained <10 mu g 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 mu g 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.