Experimental dead zones: two designs for creating oxygen gradients in aquatic ecological studies

Abstract

Hypoxia is commonly defined as dissolved oxygen (DO) concentrations below 2 mg L−1. The occurrence of hypoxia within freshwater and marine ecosystems is stressful for respiring organisms across all trophic levels, leading to changes in community diversity, altered trophic interactions, and decreased physiological fitness of organisms. Whereas many field studies have identified changes in animal diets and species distribution due to hypoxia, the behavioral changes that drive these patterns are poorly understood. Additionally, many laboratory studies that subject organisms to a fixed DO concentration may not be entirely applicable to natural environments because many organisms are capable of sensing and avoiding hypoxic areas. Herein we describe two experimental tank systems developed to study the effects of oxygen gradients on fish behavior. These systems are novel in that 1) fish and potentially other aquatic organisms can freely move between hypoxic and well oxygenated areas, 2) the thermocline or oxycline is easily adjusted for multiple treatment levels, 3) they are large enough to study fish behavior on an ecological scale, and 4) are constructed with affordable, readily available materials and are easily maintained. In both systems, we were able to conduct fish behavior studies under stable thermal and oxic stratification comparable to conditions found in temperate freshwater lakes.