Essie Rodgers

Aquatic habitats encompass some of the most complex and dynamic environs on earth, leaving fish to navigate multiple, interacting stressors. Fish regularly contend with shifts in key environmental conditions in combination with biotic challenges and anthropogenic pressures. Stressors are becoming more numerous and severe owing to human pressures, and multi-stressor studies are critical to building an understanding of how fish physiology is affected by multivariate phenomena, like climate change. In this article, we explore how fish physiologically respond to multivariate changes in their environment, paying particular attention to non-additive stressor interactions where “ecophysiological surprises” are revealed.

An endless list of new chemicals are entering nature, which makes it an impossible task to assess all possible mixture combinations at all possible concentrations and conditions that are leading to the ubiquitous anthropogenic impacts on the aquatic environment resulting from deteriorating water quality. Therefore, ecotoxicology is moving more toward a mechanistic understanding of toxicological processes, using trait-based approaches and sublethal molecular and physiological endpoints to understand the mode of action of pollutants and the adverse outcomes at the organismal and population level. These molecular and physiological endpoints can be used as biomarkers, applicable in the field. This brings ecotoxicological research much closer to conservation physiology. Understanding the relationships between chemical reactivity in the water and in organisms, and assessing the consequences at higher levels, allows conservation physiologists and managers to take the right restoration measures for an optimal improvement of the aquatic habitats of concern. In this chapter we discuss the role which the promising approach of mechanistic-based Adverse Outcome Pathways (AOPs) can play in ecotoxicological research. It studies a pathway of events, from the direct interaction of a chemical with a molecular target, through subsequent intermediate events at cellular, tissue, organ and individual organism levels which then result in an Adverse Outcome (AO) relevant to ecotoxicological risk assessment and regulatory decision-making. In this context, we also discuss the importance of modeling, including bioavailability based and effect based models. Finally, we reflect on the possibilities that meta-analysis has to offer to detect unifying physiological processes, as well as interesting outliers.

Globally, freshwater fish numbers have declined substantially in part due to anthropogenic structures (e.g. dams) that impede fish movements. The environmental and societal benefits of balancing environmental health with human resource requirements have meant that there is increasingly a concerted effort to remove or remediate barriers to fish passage. However, this is technically, financially, and biologically challenging. Experimental approaches provide a controlled, iterative, integrative, and cost-effective approach to assess the physical, physiological, and behavioural limitations of fish that can be used to provide evidence-backed information to support or develop remediation practices. This chapter explores some of the physiological tools used to measure fish performance in modified environments, and how empirical studies are informing current issues in the management of freshwater fish passage.

Nitrate is a natural and important component of freshwater ecosystems. Yet, human activities, such as the extensive use of fertilizers, urban wastewater, and aquaculture operations, have significantly increased the concentration of nitrate entering freshwater environments. Nitrate concentrations can nowadays be 10 to 100 times above preindustrial levels, and elevated nitrate concentrations are expected to cause severe consequences to aquatic life. However, the responses of freshwater life to elevated nitrate concentrations are mixed, with some animals showing severe signs of toxicity while others appear unaffected by nitrate, even at extremely high concentrations. This chapter describes the main toxic actions of nitrate to aquatic life and provides a summary of sublethal effects (e.g., impacts on growth, development, histopathology, and endocrine disruption). This chapter also examines how nitrate toxicity is mediated by other environmental variables, such as water temperature, salinity, and other pollutants, to affect aquatic fauna. Finally, this chapter concludes by directing future research on the effects of nitrate on aquatic fauna.