Thermal effluents from power plants boost performance of the invasive clam Corbicula fluminea in Ireland's largest river

Highlights: @@@ Elevated temperatures can improve performance of warm-adapted non-native species. @@@ We tested this on the highly invasive Asian clam in two thermal plumes in Ireland. @@@ Increases in body size and abundance lead to an over 50-fold greater biomass. @@@ We found strong increases in degree-days for growth and larval development. @@@ Thermal plumes form sanctuaries, and stepping-stones for warm-adapted invaders. -- Abstract: Elevated temperatures due to anthropogenic activities can improve the performance of non-native species that are adapted to higher temperatures than resident species. Ecosystems may experience higher temperature due to global stressors, such as climate change, or local stressors, including thermal effluents and urban heat islands. Using field surveys of population density and body size of the highly invasive and hot-adapted clam Corbicula fluminea in and out of two thermal effluents from power plants along the River Shannon, Ireland, we tested the hypothesis that C. fluminea performs better in thermal plumes. Shell length and body mass of C. fluminea in thermal effluents were 1.8 and 4.4 times higher, respectively, compared with adjacent unheated river sections. Density of C. fluminea was 13.7 times higher in heated, compared with unheated river reaches, leading to an increase in combined biomass per unit area of >50-fold. Our temperature data suggest an up to 2.5-fold increase of degree-days for growth and up to 5.2-fold increase of degree-days for larval incubation in the thermal plumes in River Shannon, compared with unheated conditions. Through enlarged body size, the elevated temperatures likely increase fecundity within the plumes. These findings illustrate that, in temperate climates, thermal plumes can form sanctuaries, where C. fluminea likely alters habitat, outpaces competitors and potentially dominates the energy flow through food webs. Furthermore, thermal plumes can act as stepping-stones and propagule banks for further proliferation of C. fluminea and other warm-adapted invaders.