Baseline studies in the Slave River, NWT, 1990–1994: Part IV. Evaluation of benthic invertebrate populations and stable isotope analyses

Abstract

The Northwest Territories (NWT) portion of the Slave River is downstream of the Athabasca and Reace Rivers and the potential for contamination from industrial and agricultural processes from these sources and from long-range transport is very real. In response to this threat, the Slave River Environmental Quality Monitoring Program was established cooperatively between the government of the Northwest Territories and federal government agencies in 1991. A 5-year program was conducted to examine the quality of the water, suspended sediment, and fish in the territorial portion of the Slave River and to establish a baseline data set for comparison purposes in future monitoring programs. Additional facets of the study on the Slave River included an assessment of the benthic invertebrate population, since it was recognized that such a survey could be important for biomonitoring purposes. Abundance of organisms, taxon diversity, and presence or absence of sentinel species could all be used to assess environmental contamination. The final focus of the program was the examination of stable isotope ratios of carbon, sulphur, and nitrogen in the Slave River fish. Ecologically, these measurements can be used to determine the feeding and migration pattern of fish, and to define their trophic position in the food chain. From a toxicological point of view, these stable isotope ratios can help explain contaminant sources and transfer through the food chain. The benthic invertebrate survey was conducted in 1990 and 1991. The study concluded that the abundance of benthic invertebrates at the numerous sites examined in the Slave River was very low and organisms that had been used in other biomonitoring studies (e.g. bivalve molluscs, large oligochaetes) were rare or absent. Over 90% of the invertebrates collected from the Slave River were chironomids or small oligochaetes and comparisons of benthic invertebrate communities in the Slave River Delta indicated that few changes in percent composition or diversity had occurred over a 10-year period. The benthic invertebrate survey provided a baseline for future population analysis. Analysis of the stable isotope ratio of sulphur in fish from the Slave River indicated at least two significant food sources. One source is probably from Great Slave Lake, while the other is probably upstream of Fort Smith (Athabasca and Peace Rivers). Also, the stable isotope of carbon indicated that the food source is via different pathways and may include benthic as well as pelagic origins. It is difficult to make any conclusions regarding the organic concentrations found in fish tissues and their possible significance to isotope analysis, since the overall contaminant body burdens were generally very low or below analytical detection.