Nutrient limitation and herbivory: processes influenced by bleached kraft pulp mill effluent

Effects of nutrient enrichment from bleached kraft pulp mill effluent (BKPME) on water chemistry, nutrient limitation status, periphyton biomass, and Orthocladiinae communities were investigated in the Thompson River, Canada, 1991-1994. Spatial differences in phosphorus (SRP) concentrations occurred downstream of a pulp mill in the winter when river flows and effluent dilution were low. Concentrations of SRP were twofold higher at the near-field site (Walhachin) compared with the far-field site (Spences Bridge), resulting in a doubling of mean periphyton accrual at Walhachin (25 mu g . cm(-2)) in the winter. Individual orthoclads were 1-2 mu g . cm(-2) heavier at Walhachin in the winter compared with Spences Bridge, suggesting that nutrient enrichment from BKPME affected biomass at multiple trophic levels. Nutrient diffusing substrata (NDS) deployed at Walhachin showed that periphyton and chironomid biomass and chironomid density were not Limited by P availability in the winter when BKPME concentrations were high but were P limited in the autumn when effluent concentrations were low. Algal biomass was not higher on malathion-enriched substrata in the winter compared with the autumn, suggesting that grazers were unable to reduce algal biomass in the winter when BKPME concentrations were high and periphyton growth was not P limited.