Award: OCE-1131620

The hadal zone, comprised primarily of ocean trenches, represents the deepest marine habitat on Earth (6000-11,000m), accounts for the deepest 41% of the global ocean. Much of our knowledge of hadal biology is derived from two sampling campaigns in the 1950s (the Danish Galathea and the Soviet Vitjaz expeditions). These exploratory campaigns culminated in an initial catalogue of hadal species but they did not strategically sample at comparable depths or with sufficient replication upon which to draw ecological conclusions. As a result of historical factors and severe technical challenges associated with the extremes of hydrostatic pressure and distance from the sea surface, hadal systems remain among the most poorly investigated habitats on Earth. The successful development of the Hybrid Remotely-Operated Vehicle Nereus (Figure 1) prompted our international team of scientists to come together to address the foremost standing questions in hadal science. In short, we worked to determine the composition and distribution of hadal communities and how depth/pressure, food supply, metabolic rates/energy demand, and topography affect these communities. This report covers the research conducted by the Woods Hole Oceanographic Institution, namely efforts to delineate faunal ecosystems living in the hadal zone and evaluate patterns of community structure with increasing depth with corresponding changes in organic carbon. Our work took place in the Kermadec trench at depths between 4000 and 10000 meters (Figure 2). Intellectual Merit A key outcome of our research came from determining the composition, abundance, and diversity (i.e., community structure) of megafauna from 6000m to 10,000 meters in the Kermadec trench for the first time. This was achieved through the ability to move across the seafloor with Nereus (as apposed to static lander-type observations). In just four complete dives, Nereus visited places that have never been seen or surveyed and recorded more than 30 hours of ultra-high-definition video, including two seafloor imaging transects at 10,000 meters, the deepest habitats in the Kermadec Trench. Live and recorded imagery from the vehicle revealed a diversity of habitats and seafloor types that had not been previously documented, greatly increasing the potential biological diversity of organisms and reinforcing one of the central hypotheses of the HADES program—that the hadal seafloor is far more varied than previously thought and encompasses a high diversity of habitats, differing rock types, submerged wood, and even what may have been an extinct chemosyntethic tubeworm community at 8,100 meters (over 5 miles) below the surface. Nereus also collected a wealth of undescribed animals, including an organism resembling an octocoral that has so far defied description or categorization, the first physical evidence that organic material can be transported to the depths when biofouled pumice sinks to the seafloor, and the first evidence that metabolic activity in animals increases dramatically in depth below 4,000 meters. High-definition (4K) video imaging surveys imaged megafuana included: polychaetes (9 types), ophiuroids (3 types), holothurian sea cucumbers (6 types), crabs (3 types), shrimp (3 types), nudibranchs (1 types), stalked crinoids (4 types), anemones (3 types), asteroid sea stars (4 types), anemones (3 types), and isopods (3 types). The distribution of these species was not random. Specifically, we identified megafaunal communities that differed at each ~1,000 m depth intervals during 9 imaging surveys (>200m long) at ~6,000, 7,000, 8,000, and 10,000m depth (Figure 3). We found that the hadal seafloor hosts a high diversity of habitats with depth, including wood, plant material (e.g., coconuts), rock types, and sediment types that corresponded with megafaunal species composition and dominance with depth: specifically, 7000m surveys were dominated by xenophyophores and anemones, 8000m by stalked crinoids on rocks, 9000m by holothurian sea cucumbers, 10,000m by polychaetes correlating with observed organic carbon and lipid concentrations increasing with depth to 9000m. To directly address these observations, 45 sediment cores were collected for co-located determination of microbial, meiofaunal, and macrofaunal content as well as the amount, age, origin, and quality of organic matter. To date, our collaborator Dan Mayor (U. of Aberdeen) has found increases in total sediment organic carbon content and microbial organic content (phospholipid fatty acids) with depth in the Kermadec trench. In the rest of the deep-sea, food supply declines predictably with depth due to water column consumption of sinking detritus. Our finding of a reverse pattern in food supply with hadal depth presents an unusual and intriguing ecological situation and provides an interesting opportunity to disentangle the relative importance of these environmental parameters on communities. A hadal science web portal (http://web.whoi.edu/hades/) was created to provide a public interface for outreach education (Figure 4). This sites features tools and technology, images and video, a discovery timeline for hadal research, and lists of expeditions. The goal of this site is to advance knowledge of trench and hadal ecosystems, relate recent discoveries, and inform stewardship of the deep ocean. Last Modified: 02/13/2017 Submitted by: Timothy M Shank