Depth distributions of benthic and pelagic species highlight the potential of mesophotic and deep habitats to serve as marine refugia

Abstract

Shallow marine ecosystems are threatened by human impacts. The prospect that deeper environments could serve as refugia is promising but remains uncertain, as it has been evaluated primarily in benthic taxa and on small spatial scales. We use a dataset of all biota from the Gulf of Mexico to evaluate the degree to which species’ occurrences extend across shallow (0-30 m), mesophotic (>30-150 m), and deep (>150 m) environments. High percentages of species inhabit multiple depth zones: 70% of benthic and 81% of pelagic species occurring in the mesophotic zone are also found in the shallow zone, and 45% of benthic and 80% of pelagic species found in the shallow zone extend into mesophotic depths. After accounting for confounding effects using logistic regression, pelagic species have significantly higher odds of extending between depth zones than benthic species. Analyses of species in adjacent 20 m depth bands reveal taxonomically distinct communities of benthic species from 0 to 100 m and pelagic species from 0 to 200 m. Peaks in probability distributions show that pelagic species generally have broader depth ranges than benthic species. Depth ranges that overlap between deeper and shallower zones show that movement of individuals or propagules in one or both directions between these habitats is either occurring now or has occurred in the past. Over the long term, deeper habitats can allow species to escape regional extirpation, and can contribute to the resilience and persistence of coastal ecosystems in the face of increased disturbance in shallow water. This is particularly true when pelagic species are considered.