Quantifying oxythermal fish habitat quality in a large freshwater ecosystem

Abstract

The global occurrence and severity of aquatic hypoxia has been exacerbated by anthropogenic activities such as excess nutrient loading and climate change. There are various mechanisms by which hypoxia may affect aquatic organisms and impacts of hypoxia may occur synergistically with other environmental conditions (e.g., temperature). Nonetheless, aquatic resource managers often lack appropriate indicators for measuring the strength and severity of hypoxic conditions. Without standard methods to index hypoxic conditions, it is difficult to evaluate whether management practices are resulting in water quality and habitat improvements. We developed a novel framework for the application of fish habitat quality modeling to assess the severity of hypoxia in a large lake ecosystem. The framework produces three-dimensional measures of fish bioenergetic growth-rate potential in response to spatially-explicit oxythermal conditions and can be applied to other aquatic systems where vertical profile data are collected. We applied this framework to the seasonally hypoxic central basin of Lake Erie using a long-term water quality dataset, then compared our biological indicator to recognized drivers and indices of hypoxic conditions in Lake Erie. Our model application summarized spatiotemporally complex habitat quality conditions for six different fish species and life stages. Annual summary indices of habitat quality varied among years and species. Furthermore, our annual indicator was not strongly correlated with annual nutrient loading and an existing index of hypoxia severity. We suggest that quantification of this habitat quality index is a useful approach for studying the annual impact of hypoxia on biota in aquatic systems.