DEEPEND: First Evidence for Vertical Migration by the Gulf Bathypelagic Fauna and its Relationship to the DWHOS Deep Hydrocarbon-Dispersant Plume

The classical paradigm of the vertical ecology of the pelagic ocean is that the bathypelagic fauna (below 1000 m daytime depth of residence) do not vertically migrate on a daily basis, unlike most of the mesopelagic fauna (200-1000 m daytime depth). This paradigm is supported by several factors: the primary driver of daily vertical migration is thought to be sunlight, which is absent at bathypelagic depths; there is a fundamental change in assemblage composition below 1000 m; and the energetic costs of bathypelagic migration are thought to be prohibitive. Nonetheless, there remains a huge gap in our understanding of the bathypelagic realm since the vast majority of deep-pelagic studies do not sample below 1000 m. Pursuant to the Natural Resource Damage Assessment (2010-11) and the current GoMRI DEEPEND Consortium (2015-17), two large-scale, discrete-depth sampling programs were implemented, both having a goal of sampling above, within, and below the depth of the trapped plume of dissolved DWHOS contaminants and fine oil droplets, which was centered at c. 1100 m. These data provide an unprecedented amount of information about bathypelagic ecology, and reveal that bathypelagic fishes do undertake active daily migrations. Potential cues for this migration in the absence of solar light will be discussed. Vertical migration from the bathypelagic realm, combined with bathypelagic foraging of some marine mammals, represents a potential vector of contamination from the deep plume to the meso- and epipelagic (above 200 m depth) food webs. These findings suggest that previous paradigms about deep pelagic fauna may need to be reconsidered and underscores the urgent need to better understand connectivity through the water column when assessing injuries to large marine ecosystems such as the Gulf of Mexico.