The intermediate-sized midwater fauna (fishes, shrimps, and cephalopods; “micronekton” collectively) are dominant components of the pelagic ocean, which is by far the largest ecosystem type on Earth by several metrics (volume, organismal numbers, biomass, and productivity). Deep-pelagic micronekton, those animals residing in the water column below 200 m depth during the day, are the direct link between plankton and oceanic top predators, and through the linked processes of feeding and daily vertical migration facilitate one of Earth's most important ecosystem services to humans, carbon sequestration. Despite increasing recognition of this importance, a disconnect exists between stewardship and human impact; only a miniscule fraction of the deep-pelagic ocean has been studied in detail, while anthropogenic threats to that system are increasing rapidly. Perhaps nowhere on Earth is that dichotomy more demonstrable than the Gulf of Mexico (Gulf hereafter), a complex, high-diversity ecosystem under intense human usage and subjected to arguably the worst marine pollution event in human history.

Assessment of the impacts of the Deepwater Horizon disaster to the deep-pelagic biota was impeded from the start by the lack of pre-event information, both in terms of baselines (faunal composition, abundance, and distribution) and in terms of understanding natural variability, against which impacts of anthropogenic disturbance could be detected and quantified. In this Research Topic, we present a description of three interlinked research programs (ONSAP, DEEPEND, and DEEPEND|RESTORE, described below) that began in 2010 and continue as of this writing. These programs were designed to investigate key aspects of the Gulf pelagic ecosystem, including its faunal structure, biophysical drivers of that structure, organismal and community ecology, natural variability, and potential resilience to disturbance. The contributed papers are grouped below by major themes, indicated in the conceptual model (Figure 1) of DEEPEND (Deep Pelagic Nekton Dynamics; www.deependconsortium.org), the largest of the three aforementioned research programs.