Dispersal transiently modifies the temperature dependence of ecosystem productivity after an extreme thermal fluctuation

Effects of warming on ecosystem productivity are typically summarized over broad time scales, yet they emerge from communities that can reorganize in a matter of days. Temperature accelerates ecosystem productivity through predictable effects on metabolic rates, but dispersal across thermally heterogeneous metacommunities may modify this effect by redistributing communities and their underlying thermal phenotypes. Using outdoor freshwater mesocosms, we tested the hypothesis that increasing dispersal modifies the thermal sensitivity of gross primary productivity through the redistribution of biomass, size spectra, and thermal phenotypes along spatial thermal gradients. High dispersal briefly weakened the thermal sensitivity of GPP after an unplanned heatwave through regional homogenization of phytoplankton community structure and the spread of poorly adapted thermal phenotypes reducing mass-specific GPP in the warmest mesocosms. Except immediately post-heatwave, the thermal sensitivity of GPP was robust to dispersal, despite evidence of metacommunity dynamics. These findings suggest that the temperature dependence of ecosystem metabolism can be briefly modified by dispersal after a perturbation, but remains robust under steady-state conditions.