Cold-water species deepen to escape warm water temperatures

Aim. Whether marine species can respond to ocean warming by changing their depth remains controversial. While some evidence suggests that species can deepen to cope with warming climates, other studies found ecologically constrained depth distributions. Our study focuses on generalizing species depth response to warming, and elucidating whether some species display larger depth change than others. This may help in understanding 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 species depth records from bottom-trawl surveys encompassing 236 marine species across the steep climatic gradient of the Mediterranean Sea. This data, represents 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 species depth change across different climatic gradients. We then test whether species traits (e.g. thermal preference, depth affinity, and taxonomic relation) explain the variation in depth-response.

Results. We revealed 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, as 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 towards shallower maximal depths (deep depth limits) with warming, but this pattern was 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, as different species will respond differentially, the future vertical distribution of marine communities will change in complex ways.

Methods
We extracted data on the depth range of Mediterranean marine species from published literature based on bottom trawl surveys across multiple depths. We used the search terms “depth” and “Mediterranean” and “trawl*” on the ‘Web of Science’ database in September 2020, resulting in 437 publications. These publications were screened for papers containing data on the depth ranges (minimum and maximum) of surveyed species.