Sub‐lethal effects on fish provide insight into a biologically‐relevant threshold of hypoxia

Abstract

Hypoxia (low dissolved oxygen) is a mounting concern for aquatic ecosystems as its prevalence increases with rising anthropogenic nutrient inputs. Hypoxia is most commonly defined as 2.0 mg l–1 of dissolved oxygen, although this level varies widely across studies and agency regulations. Such definitions may be too conservative, as ecologically‐relevant non‐lethal effects (e.g. consumption and growth) of hypoxia on important aquatic species, such as fish, often occur at oxygen levels much higher than 2.0 mg l–1. In addition, many mechanisms that regulate hypoxia tolerance in fish have been proposed, including temperature, habitat, location in the water column, and body size, but there is ongoing debate over which mechanisms are most important. Using a structured meta‐analysis of published studies, we showed consistent, significant negative effects on fish growth and consumption below 4.5 mg l–1. While the total amount of variation explained was generally low, below 4.5 mg l–1 of dissolved oxygen, phylogenetic relationships accounted for most of the explained variation in fish growth. Ecological factors including body size, location in the water column (pelagic, demersal, or benthopelagic), habitat (freshwater, marine, or diadromous), and temperature explained very little of the effect of hypoxia on fish growth and explained only a moderate level of variation in consumption. Our results suggest a dramatically higher threshold for sub‐lethal effects of hypoxia on fish than oxygen levels generally set for regulation purposes, and provide little support for accepted ecological mechanisms thought to influence hypoxia tolerance.

When things don't add up: quantifying impacts of multiple stressors from individual metabolism to ecosystem processing

“…; Hrycik et al . ). Ecological models, such as ours, can be used when two or more metabolic processes are targeted, either as a combination of separate stressors or single stressors with multiple modes of action.…”

“…Even though our stressors were hypothetical, we know there are stressors that inhibit feeding (DeMott ; Hrycik et al . ), increase the energy required for maintenance, survival or detoxification (Rowe et al . ; Muller et al .…”

“…; Hrycik et al . ). We here focus on pMoAs rather than specific chemicals to provide a generic approach to studying stressor impacts across levels of biological organisation.…”

“…Here, we define responses to pairs of hypothetical stressors with physiological modes of action (pMoAs) (Jager 2012;Martin et al 2014) specifically targeting feeding, maintenance, growth and reproduction in individual amphipods. Exposure to certain chemicals has been shown to impact all these processes, either in isolation or in combination (Heugens et al 2001;Hrycik et al 2016). We here focus on pMoAs rather than specific chemicals to provide a generic approach to studying stressor impacts across levels of biological organisation.…”

Impacts of hypoxia on estuarine macroinvertebrate assemblages across a regional nutrient gradient

“…Hypoxia and dissolved oxygen supersaturation clearly impact animal behaviour and vital rates (Fidler 1988;Deegan 2002;Miller et al 2002;Vaquer-Sunyer and Duarte 2008;Riedel et al 2014;Hrycik et al 2017), yet they are not always fully integrated into studies examining the consequences of eutrophication. Given that dissolved oxygen is highly variable, infrequent point measurements are not likely representative of the conditions to which biota are exposed, potentially masking its influence.…”