Climate change-induced alteration of flow regimes is a major threat for organisms inhabiting mountain streams, particularly those experiencing earlier snowmelt. Yet, whole-ecosystem responses to prolonged low flow remain difficult to anticipate. This is mainly because stream food web components can respond differently to prolonged drought, and interactions among them can vary over the drought period. Here, we used large-scale flow-through mesocosms in California’s Sierra Nevada to assess how food webs will respond to end-of-century flow regimes (i.e. advancing snowmelt resulting in earlier flow recession to baseflow levels). We manipulated the flow regime (current versus early low flow treatments) crossed with brown trout presence or absence, across 24 channel sections. Extended low flow increased water temperature and reduced dissolved oxygen. The onset of early low flow immediately conferred a 3-fold increase in the abundance of emerging insects, but this effect was temporary. Flow alteration had a positive and long-lasting effect on cumulative abundance of emerging insects, suggesting that fine-scale changes occurring early in the season influenced the community trajectory. Surprisingly, insects emerged more when trout were present, indicating potential cascading effects on the ecosystem. Our results emphasise that the magnitude and timing of cross-ecosystem subsidies are likely to shift with climate change in mountain streams. Large-scale experiments may provide a realistic model to understand the full range of stream ecosystem responses to projected climate change.

Primary Presenter: Charlotte Evangelista, UPPA/INRAE (charlotte.evangelista0@gmail.com)

Authors:

Mathieu Buoro, e2s UPPA, INRAE, UMR 1224 Ecobiop, Université de Pau & des Pays de l'Adour, Saint-Pée sur Nivelle, France  (mathieu.buoro@inrae.fr)

Stephanie Carlson, Department of Environmental Science, Policy, and Management, University of California, Berkeley, California, USA  (smcarlson@berkeley.edu)

Kyle Leathers, Department of Environmental Science, Policy, and Management, University of California, Berkeley, California, USA  (kyle_leathers@berkeley.edu)

Tatiana Tronel, e2s UPPA, INRAE, UMR 1224 Ecobiop, Université de Pau & des Pays de l'Adour, Saint-Pée sur Nivelle, France  ()

Stephane Glise, e2s UPPA, INRAE, UMR 1224 Ecobiop, Université de Pau & des Pays de l'Adour, Saint-Pée sur Nivelle, France  ()

Albert Ruhi, Department of Environmental Science, Policy, and Management, University of California, Berkeley, California, USA  (albert.ruhi@berkeley.edu)