Ocean winter warming induced starvation of predator and prey.

Ocean warming impacts the fitness of marine ectothermic species, leading to poleward range shifts, re-shuffling of communities, and changes in ecosystem services. While the detrimental effects of summer heat waves have been widely studied, little is known about the impacts of winter warming on marine species in temperate regions. Many species benefit from low winter temperature-induced reductions in metabolism, as these permit conservation of energy reserves that are needed to support reproduction in spring. Here, we used a unique outdoor mesocosm system to expose a coastal predator-prey system, the sea star Asterias and the blue mussel Mytilus, to different winter warming scenarios under near-natural conditions. We found that the body condition of mussels decreased in a linear fashion with increasing temperature. Sea star growth also decreased with increasing temperature, which was a function of unaltered predation rates and decreased mussel body condition. Asterias relative digestive gland mass strongly declined over the studied temperature interval (ca twofold). This could have severe implications for reproductive capacity in the following spring, as digestive glands provide reserve compounds to maturing gonads. Thus, both predator and prey suffered from a mismatch of energy acquisition versus consumption in warmer winter scenarios, with pronounced consequences for food web energy transfer in future oceans.