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 rivers and can be further exacerbated by anthropogenic inputs. To assess the impact of pulp mill and municipal effluents on under-ice oxygen, temporal and spatial patterns in DO were examined for the Athabasca River, Alberta, Canada. Start-up of a bleached kraft mill in 1957 was associated with the lowest late-winter (February-March) DO concentrations ever recorded. Improvements in mill technology since 1977 coincided with increases (P < 0.05) in late-winter DO concentrations at two of three downstream sites and an amelioration in both the magnitude and downstream extent of the DO sag. During recent years (1988-1993), effluent 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 ice-covered rivers throughout the world likewise showed that DO concentrations decreased linearly with distance below effluent outfalls for most river reaches with effluent concentrations >1%. Our observation that depressions in DO in ice-covered rivers increase with distance downstream raises concerns about safeguarding oxygen levels in northern rivers, especially in view of increasing development in these areas.