Award: OCE-1130712

The hadal zone, comprised primarily of ocean trenches, represents the deepest marine habitat on Earth (6000-11,000m), accounting 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 to permit intra- or inter-trench comparisons 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 (Fig. 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 University of Hawaii, namely efforts to evaluate patterns of food supply and energetic demand. Our work took place in the Kermadec trench at depths between 4000 and 10000 meters (Fig. 2). We also leveraged NSF funding and conducted another expedition (shiptime funded by SOI) to the Mariana trench where we conducted similar sampling but without the Nereus ROV. Intellectual Merit A key outcome of our research has been to support the detrital or food accumulation hypothesis. The hypothesis suggests that food, often as small detrital particles, accumulates with increasing depth in the trench causing increases in animal densities, activities and changes in communities. Accumulation may occur through funneling of sinking material into the narrowing trench, downslope accumulation and/or earthquake induced slumps or turbidity currents. Our results provide considerable evidence for detrital accumulation with depth. 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. We find extremely high sediment community (Mariana trench, Fig. 3) and megafaunal community (Kermadec trench) respiration at great depths suggesting high food demand. Further, increases in nutritional condition of hadal snailfishes from ~6000 to 8000 m suggest better feeding at depth (Fig. 4). Preliminary results from collaborators also show that densities of animals increase with depth. Together these results show that trenches can be locations of high animal and/or microbial biomass and activity in the deep ocean. It is also interesting to speculate on whether subduction of detrital rich sediments could lead to carbon sequestration in trenches. In the rest of the deep-sea food supply declines predictably with depth due to water column consumption of sinking detritus. Identifying the factors leading to changes in marine communities with depth has been difficult because temperature, pressure, and food supply all covary with depth. Our finding of a reverse pattern in food supply with depth in the lower half of the ocean presents an unusual and intriguing ecological situation and provides an interesting opportunity to disentangle the relative importance of these environmental parameters on communities. Broader Impacts During a 10000m dive in the Kermadec trench, the Nereus vehicle imploded, attesting to the great technical challenges of working in the EarthÆs deepest places. Several new instruments were developed as part of this project including systems to measure respiration of hadal animals and sediments for the very first time. In addition, the use of landers, or vehicles which ...