This article reviews the sensitivity, responses, response thresholds, and minimum oxygen requirements of marine and freshwater organisms with strong emphasis on Canadian species. The analysis attempts to define low dissolved oxygen thresholds which produce some physiological, behavioral, or other response in different species.Oxygen availability is discussed with reference to seasonal, geographical, or spatial variation in dissolved oxygen. Factors affecting availability of dissolved oxygen include atmospheric exchange, mixing of water masses, upwelling, respiration, photosynthesis, ice cover, and physical factors such as temperature and salinity. Dissolved oxygen terminology is summarized and tables are included for both fresh and saltwater O2 solubility at different temperatures.Incipient O2 response thresholds are used in a statistical analysis to develop oxygen criteria for safeguarding various groups of freshwater and marine fish. These include mixed freshwater fish populations including or excluding salmonids, freshwater salmonid populations, salmonid larvae or mature salmonid eggs, marine anadromous and nonanadromous species. Criteria are based on threshold oxygen levels which influence fish behavior, blood O2 saturation, metabolic rate, swimming ability, viability and normal development of eggs and larvae, growth, circulatory dynamics, ventilation, gaseous exchange, and sensitivity to toxic stresses. The criteria provide three levels of protection for each fish group and are expressed as percentage oxygen saturation for a range of seasonal temperature maxima.Oxygen tolerances and responses of aquatic invertebrates to low oxygen are reviewed for freshwater and marine species according to habitat. No invertebrate criteria are proposed owing to the capacity for many invertebrate species to adopt anaerobic metabolism during low O2 stress. It is suggested that the criteria proposed for fish species will provide a reasonable safeguard to most invertebrate species. It appears likely, however, that a change in oxygen regime to one of increased O2 scarcity will probably influence invertebrate community structure.It is suggested that criteria for protection of aquatic life be implemented by groups of experienced individuals. The group should consider the natural oxygen regime for a specific water body and its natural variability, the aquatic life therein and its value, importance, relative O2 sensitivity, and the possibility of interactions with toxicants and other factors that may compound the stress produced by low O2 on aquatic life. Each water body and its aquatic life should be considered as a unique situation and criteria application should not encompass diverse areas, habitats, or biological associations as if they were identical.
