The factors regulating the development and trophic interactions of planktonic communities were determined in 31 temperate rivers. In addition, the Rideau River, Ontario was studied in detail over three seasons. Variables measured included: phytoplankton biomass as measured by chlorophyll-A (chl-A), zooplankton biomass, heterotrophic bacterial and flagellate abundance, nutrient concentrations (phosphorus and nitrogen), dissolved organic carbon concentration, discharge, water residence time, depth, temperature, and light attenuation.

Phytoplankton was abundant in eutrophic rivers (> 15 micrograms per liter of chl-A) and was most strongly related to nutrients, primarily total phosphorus (TP) which explained up to 76% of the variation in chl-A. Phytoplankton biomass in the rivers was not related to the hydrological parameters of water residence time or discharge, possibly due to the short generation time of phytoplankton. Light did not appear to limit phytoplankton biomass due to shallow depths and extensive vertical mixing of the water columns. In the Rideau phytoplankton biomass exhibited longitudinal heterogeneity, but in general increased downstream, concomitant with increases in nutrients. Phytoplankton biomass did not appear to be affected by zooplankton grazing in the rivers. However, phytoplankton may have been negatively impacted by benthic filter feeders, in particular the invasive zebra mussel (DREISSENA POLYMORPHA), in the downstream reaches of the Rideau River.

Zooplankton biomass in the rivers was low (usually < 20 micrograms per liter dry mass) and small taxa dominated the zooplankton communities (rotifers, bosminids). Large zooplankton taxa, such as DAPHNIA sp., were much less abundant. Due to longer generation times, zooplankton biomass was primarily related to water residence time which explained 33% of the variation. Zooplankton appeared susceptible to advective loss in the rivers. A positive resource effect of either nutrients or phytoplankton on zooplankton biomass, typically observed in lakes, was weaker in the rivers. In comparison to lakes, zooplankton appeared less tightly coupled to phytoplankton. As with phytoplankton, zooplankton biomass in the Rideau increased downstream and appeared to be negatively affected by benthic filter feeders.

Heterotrophic bacteria were abundant in the rivers (4.5 million cells per milliliter) and as in lakes, bacteria were most strongly related to nutrients (TP) and chl-A. In contrast to lakes, no relationship between river bacterial abundance and dissolved organic carbon was observed, possibly due to the more allochthonous, refractory nature of river dissolved organic carbon. Heterotrophic flagellates were also abundant (4.0 thousand cells per ml) and were most strongly related to bacterial abundance and nutrients (TP). Neither bacterial nor flagellate abundance was related to water residence time. A negative relationship between zooplankton biomass and bacterial or flagellate abundance was not observed, possibly because of the low grazing pressure of zooplankton in the rivers. Due to the scarcity of zooplankton in rivers, there may be little transfer of energy from the planktonic microbial food web to planktonic metazoans.