Cold-water species deepen to escape warm water temperatures

Aim: Whether marine species can respond to ocean warming by changing their depth remains controversial. Some evidence suggests that species can deepen to cope with warming climates, whereas other studies have found ecologically constrained depth distributions. Our study focuses on generalizing the depth response of species to warming and elucidating whether some species display a larger change in depth than others. This might help us to understand the future distribution of marine species and communities. Location: The Mediterranean Sea. Time period: 1985–2017. Major taxa studied: Fish, malacostracans and cephalopods. Methods: We compiled depth records of species from bottom-trawl surveys encompassing 236 marine species across the steep climatic gradient of the Mediterranean Sea. These data represent the largest assessment, to date, of the potential of species to modify their depth distribution in response to spatially varying climate. Using environmental variables (e.g., sea surface temperature, bottom temperature and salinity), we elucidate the change in depth of species across different climatic gradients. We then test whether species traits (e.g., thermal preference, depth affinity and taxonomic relationship) explain the variation in depth response. Results: We reveal a significant deepening of minimum depths (shallow depth limits) with increasing sea temperatures across the Mediterranean Sea. Moreover, we show that this deepening is uneven among species, in that cold-water species and eurytherms deepen more than warm-water species and stenotherms. In addition, deep-water species deepen more than shallow-water species. We also find surprising changes toward shallower maximal depths (deep depth limits) with warming, but this pattern is not entirely supported by our sensitivity analyses. Main conclusions: These large changes across the Mediterranean Sea imply that progressively warmer oceans will compress the vertical distribution of marine organisms. However, given that different species will respond differentially, the future vertical distribution of marine communities will change in complex ways.