WINTER OXYGEN CONDITIONS IN ICE-COVERED RIVERS - THE IMPACT OF PULP-MILL AND MUNICIPAL EFFLUENTS

Depressions in dissolved oxygen (DO) commonly occur in ice-covered riv ers and can be further exacerbated by anthropogenic inputs. To assess the impact of pulp mill and municipal effluents on under-ice oxygen, t emporal and spatial patterns in DO were examined for the Athabasca Riv er, Alberta, Canada. Start-up of a bleached kraft mill in 1957 was ass ociated with the lowest late-winter (February-March) DO concentrations ever recorded. Improvements in mill technology since 1977 coincided w ith increases (P < 0.05) in late-winter DO concentrations at two of th ree downstream sites and an amelioration in both the magnitude and dow nstream extent of the DO sag. During recent years (1988-1993), effluen t loading resulted in sag and recovery zones over small spatial scales (tens of kilometres) and also contributed to large-scale (hundreds of kilometres) linear declines in DO. A review of oxygen conditions in i ce-covered rivers throughout the world likewise showed that DO concent rations decreased linearly with distance below effluent outfalls for m ost river reaches with effluent concentrations >1%. Our observation th at depressions in DO in ice-covered rivers increase with distance down stream raises concerns about safeguarding oxygen levels in northern ri vers, especially in view of increasing development in these areas.