Invasion stress mitigates climate stress in a brackish marsh amphipod

Abstract Organisms in coastal brackish ecosystems face not only highly variable environmental conditions, but the intensity and stochasticity of these environmental conditions are anticipated to increase due to climate change. While environmental changes are often presented unilaterally as stressors, there are some anthropogenic environmental transformations, such as the introduction of habitat forming plant species, that may mitigate physiological stress imposed by warming. In Suisun Marsh in the California Delta, native plant canopies allow light and heat to penetrate to the understory yet the canopies of the introduced Phragmites australis reed block out sunlight and heat for organisms living below. We set out to understand the physiological performance of mud‐dwelling invertebrates in this context: can invasion stress mitigate climate stress? We raised field collected Gammarus amphipods in the laboratory under simulated ‘native canopy’ and ‘ Phragmites canopy’ temperature/light conditions based on data from our field environmental loggers. We assessed survival, body mass, fecundity, glycogen and protein content every 2 weeks from our lab population of amphipods. We found that amphipods raised under the Phragmites conditions had better survival, greater stores of protein, and similar body mass and stores of glycogen, which could positively impact available food resource quality for juvenile fishes. We support this with field data showing higher use of Phragmites canopies by amphipods, especially in the water column, across the year of sampling. Synthesis and applications : Understanding the potential benefits of Phragmites as climate stress refugia can inform management decisions around its mitigation in future restoration.