Fire and rain: A systematic review of the impacts of wildfire and associated runoff on aquatic fauna

Abstract

Climate and land‐use changes are expected to increase the future occurrence of wildfires, with potentially devastating consequences for freshwater species and ecosystems. Wildfires that burn in close proximity to freshwater systems can significantly alter the physicochemical properties of water. Following wildfires and heavy rain, freshwater species must contend with complex combinations of wildfire ash components (nutrients, polycyclic aromatic hydrocarbons, and metals), altered light and thermal regimes, and periods of low oxygen that together can lead to mass mortality events. However, the responses of aquatic fauna to wildfire disturbances are poorly understood. Here we provide a systematic review of available evidence on how aquatic animals respond to and recover from wildfire disturbance. Two databases (Web of Science and Scopus) were used to identify key literature. A total of 83 studies from across 11 countries were identified to have assessed the risk of wildfires on aquatic animals. We provide a summary of the main ecosystem‐level changes associated with wildfires and the main responses of aquatic fauna to such disturbances. We pay special focus to physiological tools and biomarkers used to assess how wildfires impact aquatic animals. We conclude by providing an overview of how physiological biomarkers can further our understanding of wildfire‐related impacts on aquatic fauna, and how different physiological tools can be incorporated into management and conservation plans and serve as early warning signs of wildfire disturbances.

Loss of riparian forests from wildfire led to increased stream temperatures in summer, yet salmonid fish persisted

“…However, increases in stream temperature are a primary concern, potentially resulting in negative consequences for salmonid fishes (Bixby et al, 2015; Gomez Isaza et al, 2022; Gresswell, 1999). In previous studies, loss of riparian canopies due to forest disturbance has resulted in abrupt increases in stream temperatures from 0.8°C to nearly 15°C (Gomez Isaza et al, 2022; Wagner et al, 2014). For example, Dunham et al (2007) documented increases in maximum stream temperatures by 3–11°C in Idaho streams.…”

“…For example, Dunham et al (2007) documented increases in maximum stream temperatures by 3–11°C in Idaho streams. However, the effects of wildfire‐associated alterations to stream thermal regimes on salmonid fishes can be complicated and do not always confer negative impacts on cold‐water fish (Gomez Isaza et al, 2022). As such, it is critical to improve our understanding of the factors driving the direction and magnitude of fish responses to postfire changes in stream temperature.…”

“…This is a common question asked by scientists, resource managers, and the public, but it does not have a simple answer. Wildfires can have complex effects on aquatic ecosystems (Bixby et al, 2015; Jager et al, 2021; Silins et al, 2014), and the nature of fish responses to wildfires may depend on the time scale of the observations (Gresswell, 1999), severity of the fire, or on the ecological context of where the fire occurs, which collectively can lead to notably different and highly variable short‐ and long‐term responses (Bixby et al, 2015; Gomez Isaza et al, 2022; Jager et al, 2021). The response of fish will also depend on the focal species, with some groups potentially responding positively while others may respond negatively (LeMoine et al, 2020).…”

“…The response of fish will also depend on the focal species, with some groups potentially responding positively while others may respond negatively (LeMoine et al, 2020). Therefore, to answer the general question of how wildfires affect fish, we need to draw from an extensive suite of studies that document species‐ and context‐specific responses (Bixby et al, 2015; Gomez Isaza et al, 2022; Jager et al, 2021). With this in mind, our objective was to contribute to our understanding of fish responses to wildfire in forested landscapes by documenting how two cold‐water adapted salmonid species responded to substantial increases in stream temperatures that occurred in the summer after a severe wildfire in a headwater ecosystem in the western Cascades of Oregon.…”

“…Wildfires can elicit a series of complex effects on aquatic ecosystems (Bixby et al, 2015; Robinne et al, 2020). However, increases in stream temperature are a primary concern, potentially resulting in negative consequences for salmonid fishes (Bixby et al, 2015; Gomez Isaza et al, 2022; Gresswell, 1999). In previous studies, loss of riparian canopies due to forest disturbance has resulted in abrupt increases in stream temperatures from 0.8°C to nearly 15°C (Gomez Isaza et al, 2022; Wagner et al, 2014).…”