Effects of Lake Erie hypoxia on fish habitat quality and yellow perch behavior and physiology

Hypoxia is a concern in freshwater, marine, and estuarine systems worldwide including in Lake Erie, North America. Hypoxia may develop more frequently and for a longer duration as a result of excess nutrient loading from human sources, therefore, the incidence of hypoxic areas has increased with human population increases and intensive land-use practices. The effects of hypoxia on fish and other aquatic organisms have been well-studied, but most research examines the negative aspects of hypoxia development on habitats and the effect of severe hypoxia on individuals. In this study we focused on less studied and more subtle effects of hypoxia, including the possible benefits of prey increases that can occur concomitantly with increased hypoxia and the behavioral and physiological responses of yellow perch (Perca flavescens) to moderate hypoxia. First, we examined the balance between the potential beneficial and detrimental aspects of nutrient loading and hypoxia on habitat quality for four fishes and two life-stages using a growth rate potential model (GRP). Although nutrient loading did increase prey biomass in model simulations, it also caused a decline in overall habitat quality due to the spatial and temporal expansion of hypoxia. Nutrient loading also increased habitat quality, but only in areas where habitat quality was already above average and primarily for species and life-stages that preferred warmer, pelagic habitat conditions. Habitat quality for species that preferred cooler, benthic habitat conditions declined relatively steeply at intermediate nutrient loading levels. Therefore, although nutrient loading may benefit some species by increasing prey resources, hypoxia may alter the ability of many species, particularly benthic species, from accessing peaks in prey or other advantageous resources in the bottom of the water column. The second aspect of our study examined how moderate hypoxic concentrations affected the behavior and physiology of yellow perch. We found modest differences in the expression of some genes that may be altered in response to hypoxia, however, moderate hypoxia did not affect the majority of behavioral and physiological responses examined. This may indicate that yellow perch can utilize areas of moderate hypoxia and the resources therein without many deleterious effects. Overall, our studies demonstrated that the responses of individuals and ecosystems to hypoxia and nutrient loading are complex. Examining fish and ecosystem responses to changes in habitat conditions like hypoxia can improve understanding of current and future large-scale environmental changes