Multiple stressor effects of hypoxia and metals on ventilation, activity, bioaccumulation, and loss of equilibrium in fathead minnows.

Abstract

Hypoxia is an increasing concern in freshwater ecosystems, driven by warming, stratification, and nutrient enrichment. Fish cope with hypoxia through behavioural and ventilatory adjustments, yet growing evidence suggests that certain metals can impair these critical compensatory mechanisms. It is currently unknown whether lead (Pb) and nickel (Ni), released in part through expanding mining operations, may alter these critical survival responses. We tested whether 48 hr Pb and Ni exposure impaired fathead minnows' (Pimephales promelas) responses to hypoxia by measuring their gill metal accumulation, ventilation rates, behaviour (movement), and hypoxia tolerance (as measured by % dissolved oxygen [DO] at loss-of-equilibrium [LOE]). Notably, Pb (43 μg/L) exposure during hypoxia resulted in 3.5 × greater gill bioaccumulation, indicating changes in gill uptake dynamics. Opercular movements were used as a proxy for the hypoxic ventilatory response (HVR) and the increase in ventilation in Ni exposed fish was 48% less than the increase in unexposed fish, indicating a blunted HVR. Ni (318 μg/L) increased fish activity, as measured by total distance moved, in both normoxia and hypoxia. Both metals increased hypoxia tolerance as measured by LOE. Together, it appears that co-exposure to Pb and hypoxia may increase gill Pb accumulation due to a maintained HVR, and short-term exposure to Ni can increase locomotion, without affecting ventilatory responses to hypoxia. These results underscore the importance of multiple-stressor research to better understand the interplay of hypoxia and metals to inform water quality guidelines.