Many factors affect the dissolved oxygen (DO) content of surface waters, including photosynthesis, respiration, temperature, ice cover, mixing and input of organic materials. In northern Alberta, the discharge of pulp mill wastes with a high biochemical oxygen demand into river systems can result in depletion of DO supplies in winter and increased concentrations of contaminants in the water. A need therefore exists to examine DO requirements of northern rivers fishes considering all life stages and interactions with other stressors, and to document the combined effects of low dissolved oxygen and contaminants on fish populations.
A number of major reviews have been conducted on the general subject of dissolved oxygen and fish. The goal of this project was to establish dissolved oxygen requirements for fish species found in northern Alberta rivers, concentrating on environmental conditions found there, particularly the presence of pulp mills and winter ice cover. Material for this review was compiled from an on-line computer search of major pertinent data bases and from printed literature reference services. The report is subdivided into a number of sections that discuss: (1) physiology of fish respiration, (2) limnology of dissolved oxygen in surface waters, (3) effects of adverse DO conditions on fish and invertebrates, (4) modifying effects of other environmental factors, and (5) individual species and life stage requirements of northern rivers fishes.
The primary site of gas exchange in teleost fish is the gill membrane; the diffusion of oxygen into the blood is aided by the high oxygen-binding capacity of hemoglobin. Gas exchange depends on the oxygen tension gradient, or partial pressure, between blood and water. Oxygen tension in the water, in turn, depends not only on the DO concentration, but on other physical conditions, particularly water temperature and atmospheric pressure. Large rivers generally maintain dissolved oxygen levels at or near saturation during open-water periods by reaeration. Under winter ice cover, however, DO levels often decline as a result of decomposition of organic matter; DO sags increase with downstream distance in the river.
The detrimental effects of low dissolved oxygen on fish are well known. The fundamental effect at the cellular or tissue level is hypoxia, which occurs when the rate of oxygen delivery to the cells is less than that required to meet metabolic needs. Death results when insufficient oxygen delivery to the brain and other tissues results in cellular dysfunction. At the sublethal level, low environmental dissolved oxygen may cause alterations in physiological processes, blood chemistry and hematology, and result in histopathological damage. Moreover, exposure of fish to prolonged hypoxic conditions can lead to increased incidence of disease, reduced growth and decreased swimming performance, as well as altering predator avoidance, feeding, migration, reproduction and other behaviours. Just as important, adverse DO conditions may depress long-term production of fish populations by altering benthic invertebrate communities, especially food species.
The detrimental effects of suboptimal dissolved oxygen levels are exacerbated by the presence of contaminants in the environment, including those typically found in pulp mill effluent. This effect of contaminants may be direct, such as by interfering with normal oxygen uptake across the gill membrane, or indirect, by elevating metabolic rate, thereby increasing oxygen demand and reducing the fish’s scope for activity. Fish exposed to reduced environmental dissolved oxygen display increased susceptibility to other toxicants and contaminants, notably those in pulp mill wastes. Increased gill ventilation in response to low DO further increases the rate of contaminant uptake.
All species and life stages of fish have both acute and chronic dissolved oxygen requirements. Acute requirements are the minimal levels of dissolved oxygen necessary to avoid short-term mortality. However, chronic requirements are more important ecologically as long-term exposure to suboptimal DO levels, even those well above levels necessary for survival, create stress and decrease growth and population production. Any reduction in the natural DO content of surface waters is likely to have some detrimental effect on fish production; thus, the assumption of a "no effect" DO level below saturation is not valid.
Acutely lethal DO levels are relatively straightforward to measure and, based on available evidence on tolerances of adult fish, northern rivers species are grouped into categories of DO sensitivity as follows: (1) Sensitive (acute limit >2 mg/L DO; includes all salmonids, longnose sucker and burbot); (2) Intermediate (acute limit 1-2 mg/L DO; includes all cyprinids except fathead minnow, walleye, white sucker, brook stickleback and goldeye); (3) Tolerant (acute limit < 1 mg/L DO; includes fathead minnow, northern pike and yellow perch); and (4) Unknown (insufficient information available; includes largescale sucker, ninespine stickleback, all sculpins and trout-perch).
The establishment of chronic dissolved oxygen requirements, which are more important to the long-term maintenance of healthy fish communities, is more problematic. Until more complete regional data are available, general DO criteria recommended in earlier reviews of this subject (e.g., >6 mg/L for salmonids, >5 mg/L for nonsalmonids; Taylor and Barton 1992) are probably applicable to fish of the northern rivers of Alberta. Although considerable information exists on DO requirements for rainbow trout, and to a lesser extent for walleye, northern pike and fathead minnow, information on the requirements for the other northern river fish species is either very limited or lacking completely. Thus, it is recommended that future research efforts concentrate on these other species, as they are integral components of the fish community. Rainbow trout and bull trout (adults) and mountain whitefish (eggs and larvae) are recommended as sentinel species to monitor dissolved oxygen conditions. Burbot and longnose sucker might also be considered if then- suspected DO sensitivities can be confirmed and they are sufficiently common in the northern rivers.