Cumulative effects of pulp mill and municipal effluents on epilithic biomass and nutrient limitation in a large northern river ecosystem

Large-scale patterns in epilithic biomass and nutrient status were evaluate d at 33 sites located upstream and downstream of point-source anthropogenic effluents in the Athabasca and Wapiti-Smoky rivers in Alberta, Canada. Mul tiple regression showed that epilithic chlorophyll a was significantly (p < 0.0001) related to concentrations of dissolved inorganic nitrogen and marg inally (p = 0.06) significantly related to soluble reactive phosphorus. Epi lithic biomass was up to 50 times higher immediately downstream of point-so urce inputs compared with sites upstream and those 20-150 km downstream. Da ta from nutrient diffusing substrata showed that the epilithon at 18 of the 33 sites was nutrient limited, while 14 sites showed no nutrient limitatio n; interpretation of the remaining site was inconclusive. Of the 18 nutrien t-limited sites, six were nitrogen limited, five were phosphorus limited, a nd seven were co-limited. Multiple discriminant function analysis showed th at the combined concentration of soluble reactive phosphorus and dissolved inorganic nitrogen was a significant discriminator between deplete and repl ete sites.