Using Mesocosms to Explore Confounding Factors Influencing Longnose Dace (Rhinichthys cataractae) Responses to Kraft Mill Effluent on the Wapiti River, AB, Canada

摘要

Development of mesocosm systems for assessing the effects of pulp mill effluents (PME) on fish has been an ongoing research effort in Canada over the past decade. Mesocosms have been used to assess the effects of kraft and thermomechanical PMEs on small bodied fish species in marine, estuarine and freshwater habitats. Mesocosms have also been developed as a technique for investigating the in-mill waste stream sources contributing to reproductive-endocrine effects in fish. Most recently, mesocosms were applied in the Wapiti River, AB, Canada in an effort to separate the confounding effects of a secondary treated bleached kraft PME from a municipal sewage effluent (MSE) on survival, growth, condition, and reproductive endpoints in adult and juvenile longnose dace (Rhinichthys cataractae). Longnose dace were exposed to the following treatments for 42 d: reference river water, PME (3%), PME (10%), MSE (1%), and MSE (1%) + PME (3%). The objective of the dose response exposure to PME was to examine the response pattern to PME in isolation under low and high concentrations. The MSE and mixture treatments were representative of conditions upstream (MSE 1%) and downstream (MSE 1% + PME 3%) of the PME discharge in the Wapiti River. Results showed that 10% PME slightly reduced juvenile condition and altered some reproductive hormones in adults. Exposure to 3% PME slightly increased juvenile condition suggesting nutrient enrichment at lower PME concentrations. No effects on survival, growth, liver size, gonad size, or stage of gonadal development were observed with PME exposure. MSE affected reproductive endpoints such as male gonad size, female fecundity and some hormone levels in males and females. Hormone changes after exposure to 10% PME were of similar magnitude to changes measured after exposure to 1% MSE. This study specifically examined the effects of waterborne exposure to PME and MSE on a forage fish after 42 d in a field-based mesocosm. Results suggest contaminant mediated responses may be associated with MSE exposure. However, nutrient enrichment in the Wapiti River due to PME and MSE has increased the food supply for fish and may mask or compensate for these effects in the field. Follow up studies might be considered using mesocosms with self-sustaining food webs as a food supply for fish to determine if response patterns differ from waterborne exposures. Additional studies might also be considered if nutrient loads in the Wapiti River are reduced resulting in decreased algal and benthic invertebrate abundance. These results illustrate how controlled exposure in mesocosms can help interpret fish response patterns in rivers confounded by multiple stressors.