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| Last Review/Updated: July 5, 2002 |
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NRBS - HomeTable of Contents |
Northern River Basins Study Final Report
3.0 Major Findings
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Introduction Dissolved Oxygen Patterns in the Athabasca River Fish Requirements Dissolved Oxygen Modelling Relevant Documents |
| Related NRBS Question: | |
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| 7. | What concentrations of dissolved oxygen are required seasonally to protect the various life stages of fish, and what factors control dissolved oxygen in the rivers? |
| Table 3.8.1 | Canadian Council of Ministers of the Environment Objectives for Dissolved Oxygen |
As with land-based animals, most aquatic organisms require oxygen to survive. Instead of breathing oxygen from the atmosphere, these organisms extract oxygen that is dissolved in the water. To protect sensitive species and life stages, federal and provincial regulations list minimum dissolved oxygen requirements for surface waters. In Alberta, the level required to protect aquatic organisms in surface waters is set at 5 mg/L under the Alberta Surface Water Quality Objectives. Federally, the Canadian Council of Ministers of the Environment (CCME) list higher objectives for cold- and warm-water organisms (Table 3.8.1), especially for early life stages of fish and aquatic organisms.
Oxygen enters a water body through contact with air in a process referred to as reaeration. The amount of oxygen that remains dissolved in water is dependent upon temperature, pressure and salinity. Generally speaking, more oxygen can be dissolved in cold freshwater at higher pressures. When the amount of dissolved oxygen in the water equals the capacity of the water to hold oxygen based on temperature, pressure and salinity, the water is said to be saturated. During the winter, when water temperatures are approximately 0oC, the Athabasca River is saturated at about 13.5 mg/L oxygen.
Winter is a critical time for dissolved oxygen levels. While cold water can hold more dissolved oxygen than warm water, ice cover blocks contact with air and reduces the potential for reaeration. Effluent addition during low winter flows causes dissolved oxygen concentrations in many northern rivers to dip to near critical levels for many oxygen-sensitive organisms. Rapids and turbulent stretches of a river are important reaeration zones during these winter months. Grand Rapids, located upstream of Fort McMurray, remain unfrozen throughout the winter and provide a site of reaeration for the Athabasca River.
Many factors contribute to oxygen loss. Plants and animals take up oxygen in respiration. More importantly, bacteria consume oxygen as they break down dead plant and animal matter. The amount of oxygen used by bacteria to break down dead organic matter, plus that consumed by the chemical oxidation of organic matter in the water column is referred to as biochemical oxygen demand (BOD). The amount of oxygen-demanding organic material released into a river system by natural or human sources is often referred to as the BOD load and is a significant factor in the oxygen level declines in some of the Study area Rivers.
The Nutrients Component coordinated studies to understand and predict the impact of human activities on dissolved oxygen levels within the Study area. Much of this work focused on the Athabasca River. Long stretches of winter ice cover and low flows, combined with the number of industrial and municipal effluent it receives, makes the Athabasca River more susceptible to oxygen problems than the Peace and Slave River systems.
| Figure 3.8.1 | Dissolved Oxygen Trends in the Athabasca River (1992) |
Figure 3.8.1 illustrates dissolved oxygen trends along the Athabasca River for an average winter with existing effluent discharges. Without these discharges, the oxygen levels throughout the river would be higher in many regions, except in the headwaters above human influence. During winter ice cover, the river experiences dissolved oxygen concentrations less than saturation for its entire length downstream of Hinton, except in the reaeration zone at Grand Rapids. Dissolved oxygen levels rarely dip below the Alberta surface water objective of 5 mg/L or the Canadian guideline of 6.5 mg/L for adult cold-water fish, but in certain areas they do drop below the 9.5 mg/L objective for early life stages of bull trout and whitefish.
The dissolved oxygen patterns in Figure 3.8.1 are affected by both natural and man-made BOD sources. In most winters, dissolved oxygen levels tend to drop downstream of the Weldwood pulp mill in Hinton in response to effluent loadings. At times, levels also decline below the junction of the Pembina River with the Athabasca River. Effluent and tributary BOD loadings result in another drop downstream of Fort McMurray.
Historical information suggests that the impact of the Weldwood mill has been decreasing. Figure 3.8.2 illustrates the changes in dissolved oxygen at four sites on the Athabasca River over a period of 40 years. As the figure illustrates, winter levels of dissolved oxygen upstream of Hinton have remained relatively constant. While there are few data for the early years, dissolved oxygen levels downstream of Hinton appear to have decreased in the last two decades following the 1957 startup of the bleached kraft mill. Since then, dissolved oxygen levels have increased in response to mill improvements that decreased the BOD load discharged to the Athabasca River.
| Table 3.8.2 | Acute Dissolved Oxygen Needs for Adult Fish in Northern Rivers |
The direct and indirect effects of low dissolved oxygen on fish are well documented. Extremely low oxygen levels lead to cellular breakdown and rapid death in fish. Sub-optimal oxygen levels affect the ability of fish to survive and reproduce by increasing susceptibility to disease, slowing growth, hampering swimming ability and altering survival behaviour such as predator avoidance, feeding, migration and reproduction. Low dissolved oxygen levels can also influence fish indirectly by reducing the survival of organisms they eat.
Environmental contaminants can aggravate the effects of low dissolved oxygen in several ways. Fish that are already weakened by low oxygen conditions are more susceptible to the harmful effects of contaminants. Contaminants may raise the metabolic rate of fish, resulting in a greater need for oxygen. This leads to increased ventilation and greater exposure to contaminants.
NRBS researchers conducted a literature review of dissolved oxygen requirements of fish in northern rivers. These requirements vary among individual fish species and life stages. Acute dissolved oxygen requirements (i.e., the minimal amount of oxygen necessary to avoid short-term mortality, usually under two days) for adult fish in northern rivers are listed in Table 3.8.2. Less information is available regarding chronic dissolved oxygen needs (i.e., the amount of oxygen required for long-term health and survival). Generally accepted chronic requirements are "greater or equal to" 6 mg/L for adult fish belonging to the salmon family and "greater or equal to" 5 mg/L for all other fish species.
Adequate dissolved oxygen levels are critical for fish in their early life stages. Since many fish species in the northern river basins spawn in the spring or summer, these early life stages occur at times when dissolved oxygen levels are high. However, eggs laid in the fall give rise to young that develop during winter when dissolved oxygen levels may be lower. NRBS studies on several fish that develop during the fall or winter (including mountain whitefish and bull trout) showed that these species can survive in dissolved oxygen levels as low as 3 mg/L. However, mountain whitefish eggs took longer to hatch, and recently hatched bull trout were under developed at low dissolved oxygen concentrations. Burbot may also experience delayed hatching under lower dissolved oxygen (6 mg/L). Since the riverbed is usually lower in dissolved oxygen than the water column, levels of 6 mg/L or greater may be required in surface waters to achieve 3 mg/L in the spawning beds. The CCME objective is 9.5 mg/L for this life stage in bull trout and whitefish but this level is not met in all parts of the river.
NRBS studies also showed that low dissolved oxygen levels also affect the health of an aquatic insect (the mayfly Baetis tricaudatus)—a common fish food source. Mayflies exposed to low dissolved oxygen (5 mg/L) ate less and exhibited a lower survival rate. Benthic invertebrates such as these live in or adjacent to the river bed, where oxygen concentrations can be up to 3 mg/L less than in the water column. Therefore, oxygen levels in the Athabasca River may currently be affecting animals at localized sites.
These results suggest that the current Alberta dissolved oxygen guideline of 5 mg/L will not adequately protect sensitive aquatic species, particularly during their early lifestages. The more conservative CCME guidelines may be a more suitable guideline for setting effluent licence conditions in Alberta's northern waters. Alternately, governments could explore the concept of reach-specific guidelines, which would consider regional differences in aquatic habitat and nutrient loadings. These policy options are examined further in the Board's recommendations to the Ministers (Section 4.0).
| Figure 3.8.2 | February / March Dissolved Oxygen Levels in the Athabasca River Upstream of Selected Towns (1955-1993) |
Dissolved oxygen models are useful tools to evaluate "what if" scenarios. Models can be used to estimate the impact of new industries and larger communities on dissolved oxygen levels, to evaluate the effectiveness of proposed regulations and to assess cost-effective options for pulp mill and sewage treatment plant upgrades.
Accurate models require a thorough understanding of the complex natural processes that control oxygen levels. Since each river has slightly different characteristics, models must be fine-tuned to the unique properties of each situation. NRBS work has focused on quantifying the natural processes that govern dissolved oxygen levels in the Athabasca River. The values determined for the major processes controlling oxygen levels in the Athabasca River were examined by mathematical modelling using the DOStoc (Dissolved Oxygen Stocastic) model. The model was relatively successful in predicting large-scale trends in average oxygen concentrations for the Athabasca River, but was unable to capture local oxygen sags downstream of certain pulp mills in some winters. The knowledge generated by this endeavour can now be used to optimize more sophisticated dissolved oxygen models.
Chambers, P.A. and T. Mill. 1996. Dissolved Oxygen, Fish and Nutrient Relationships in the Athabasca River. Northern River Basins Study Synthesis Report No. 5.
NRBS Technical ReportsChambers, P.A., Pietroniro, A., Scrimgeour, G.J. and M. Ferguson. 1995. Assessment and Validation of Modelling Under-Ice Dissolved Oxygen Using DOStoc, Athabasca River 1988 to 1994. Northern River Basins Study Technical Report No. 95.
Culp, J.M. and P.A. Chambers. 1994. Proceedings of a Workshop on Water Quality Modelling for the Northern River Basins Study, March 22-23, 1993. Northern River Basins Study Technical Report No. 37.
Giles, M.A. and M. Van der Zweep. 1996. Dissolved Oxygen Requirements for Fish of the Peace, Athabasca and Slave Rivers: A Laboratory Study of Bull Trout (Salvelinus confluentus) and Mountain Whitefish (Prosopium williamsoni). Northern River Basins Study Technical Report No. 120.Giles, M.A. et al. 1996. Dissolved Oxygen Requirements for Fish of the Peace, Athabasca and Slave River Basins: A Laboratory Study of Burbot (Lota lota). Northern River Basins Study Technical Report No. 91.
Lowell, R.B. and J.M. Culp. 1996. Effects on Mayfly of Dissolved Oxygen Level Combined with Bleached Kraft Mill Effluent and Municipal Sewage Assessments Using Artificial Streams. Northern River Basins Study Technical Report No. 98.
MacDonald, G. and A. Radermacher. 1993. An Evaluation of Dissolved Oxygen Modelling of the Athabasca River and the Wapiti - Smoky River System. Northern River Basins Study Technical Report No. 25.
Monenco Inc. 1993. Sediment Oxygen Demand Investigations: Athabasca River, January to March 1992. Northern River Basins Study Technical Report No. 3.
Noton, L.R. 1996. Investigations of Streambed Oxygen Demand, Athabasca River, October 1994 to March 1995. Northern River Basins Study Technical Report No. 94.
Shaw, R.D. and G. MacDonald. 1993. A Review of Rate Coefficients and Constants Used in Nutrients and Dissolved Oxygen Models for the Peace, Athabasca and Slave River Basins. Northern River Basins Study Technical Report No. 18.
Other Relevant DocumentsAlberta Environment. 1977. Alberta Surface Water Quality Objectives. Water Quality Branch, Standards and Approvals Division.
Canadian Council for Ministers of the Environment (CCME). 1993. Canadian Water Quality Guidelines. Task Force on Water Quality Guidelines. Eco-Health Branch, Ecosystem Science and Evaluation Directorate, Environment Canada, Ottawa
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