Thermal variability drives synchronicity of an aquatic insect resource pulse

Abstract

Spatial heterogeneity in environmental conditions can prolong food availability by desynchronizing the timing of ephemeral, high‐magnitude resource pulses. Spatial patterns of water temperature are highly variable among rivers as determined by both natural and anthropogenic features, but the influence of this variability on freshwater resource pulse phenology is poorly documented. We quantified water temperature and emergence phenology of an aquatic insect (salmonfly, Pteronarcys californica) resource pulse in two rivers characterized by differing catchment topography and human impact. Along both rivers, salmonfly emergence occurred earlier where spring temperatures were warmer. Emergence events were brief (4–8 d) at sites in the more human‐impacted river, but occurred asynchronously along the entire river, lasting 27 d in total. In contrast, emergence events were more prolonged (6–11 d) at sites on the more natural and topographically complex river, but occurred synchronously along the entire river, lasting 13 d in total. These scale‐specific differences in subsidy duration could have opposing consequences for salmonfly consumers depending on their mobility and foraging habits. Asynchronous emergence at a large scale is potentially most important for mobile consumers like birds and fish that can migrate to feed on aquatic insects and track resource waves across a landscape, whereas prolonged emergence duration at a smaller scale may be most important for immobile or opportunistic consumers like spiders and ants. Relating environmental heterogeneity and resource pulse phenology across a gradient of human impact and at multiple spatial scales is needed for a better understanding of how food availability, aquatic–terrestrial linkages, and consumer–resource dynamics may change with climate variability and increasing human activity in the future.