Distribution of Megabenthic Communities Under Contrasting Settings in Deep-Sea Cold Seeps Near Northwest Atlantic Canyons

Cold seeps support fragile deep-sea communities of high biodiversity and are often
found in areas with high commercial interest. Protecting them from encroaching human
impacts (bottom trawling, oil and gas exploitation, climate change) requires an advanced
understanding of the drivers shaping their spatial distribution and biodiversity. Based
on the analysis of 2,075 high-quality images from six remotely operated vehicle dives,
we examined cold seep megabenthic community composition, richness, density, and
biodiversity at a relatively shallow (400 m water depth) site near Baltimore Canyon (BC)
and a much deeper site (1,500 m) near Norfolk Canyon (NC), in the northwest Atlantic.
We found sharp differences in the megabenthic composition between the sites, which
were driven mostly by bathymetric gradients. At both BC and NC there were significant
differences in megabenthic composition across habitats. Hard habitats in and around
cold seeps had significantly higher values of species richness, density, and biodiversity
than soft habitats. Depth and habitat complexity were the leading environmental
variables driving megabenthic variability. The presence of microbial mats and gas
bubbling sites had a statistically significant contribution to explaining megabenthic
variability mainly in the shallower BC and less in the deeper NC areas examined;
drivers behind this discrepancy could be related to differences between BC and NC
in terms of chemical compound fluxes and megafaunal life history characteristics. Our
surveys revealed marine litter, primarily from commercial fisheries. This study highlights
the importance of habitat complexity for the proliferation of highly diverse cold-seep
ecosystems and underscores the importance of discovery science to inform spatial
management of human activities in the deep and open ocean.