Phytoplankton and zooplankton development in a lowland, temperate river

The longitudinal and seasonal patterns of plankton development were examined over 2 years in a lowland, temperate river: the Rideau River (Ontario, Canada). Following an initial decrease in phytoplankton and zooplankton biomass as water flowed from the headwaters into the Rideau River proper, there was an increase in chlorophyll a (chl a) and zooplankton biomass with downstream travel. At approximately river km 60, both phytoplankton and zooplankton reached their maximum biomass of 27 μg l−1 (chl a) and 470 μg l−1 (dry mass), respectively. Downstream of river km 60, the biomass of both planktonic communities declined significantly despite increasing nutrient concentrations and favorable light conditions. These downstream declines may be due to the feeding activity of the exotic zebra mussel (Dreissena polymorpha) which was at high density in downstream reaches (>1000 individuals m−2). There was no evidence for longitudinal phasing of phytoplankton and zooplankton, as increases and decreases in chl a and zooplankton biomass appeared to coincide. Overall, chl a was best predicted by total phosphorus (R2=0.43), whereas zooplankton biomass was best predicted by chl a (R2=0.20). There was no evidence for significant grazing effects of zooplankton on phytoplankton biomass.