Award: OCE-1829612

The deep-sea abyssal plains cover roughly half of the earths surface, host enormous reservoirs of biodiversity and mineral resources, and play important roles in nutrient regeneration and carbon sequestration. The most important process controlling these ecosystems is the quantity and quality of food that ultimately reaches the deep-sea floor, typically as small particles of organic detritus. Funnel-like sediment traps are used to collect these particles and estimate the food supply to the seafloor. Past studies have observed a large deficit between this food supply and the demand for food by the abyssal community. Recent evidence suggests that very small organic particles, that may not be captured in sediment traps, are a significant source of food leaving the upper ocean and important as nutrition to deep midwater animals. We evaluated the relative importance of these small particles in relation to larger marine snow particles, that are captured by sediment traps, to abyssal seafloor communities. To do this, we used a combination of particle flux measurements and state of the art compound specific stable isotope analysis of amino acids at a high flux site off California and a low flux site off Hawaii in both spring and fall. At each site, we collected particle food sources in sediment traps and filtered from the water column, as well as seafloor animals from small worms to large sea cucumbers and fishes. We compared their isotopic values to different sized particles to evaluate their sources of nutrition. Intellectual Merit We found that below 1500 meters depth distinct isotope values distinguish small and large particles from one another, as has been found in shallower waters. Near-seafloor sediment trap isotopic data from both stations do not match the source amino acid isotopic values and trophic positions of small particles collected at the same time, suggesting that sediment traps may capture only the larger particle sizes of sinking material. The larger particles are arguably fresher material from surface waters, that could include animal faecal pellets. However, by excluding the smaller particles, sediment traps are missing a significant source of nutrition to abyssal food webs. If trap-collected detritus was the main food source, we would expect that the abyssal animals will have isotopic values similar to it. In contrast, our results suggest most of the fauna have isotopic values intermediate between those of the traps and in situ small particles collected on filters. Thus, at both stations the abyssal animals rely to some extent on small particles, often for more than half of their nutrition. Very small detrital particles partially resolve the long-standing food deficit to the abyssal ecosystem noted in many areas around the globe. We also explored the nutrition of deposit feeders like sea cucumbers and sea urchins that predominantly feed on detritus consumed along with seafloor mud. Our results suggest that these animals have a rich and distinct flora of gut microbes that appear to modify food in the animals gut and provide nutrition to their animal hosts. These results substantially improve our understanding of abyssal food webs, which cover about half of our planet. Broader Impacts We shared our research with the public through production of a half-hour episode of the Voice of the Sea TV series designed to bring the excitement of scientific research at-sea to our student participants and to peoples living rooms, enabling viewers to virtually travel to the abyssal plains of the deep sea (https://seagrant.soest.hawaii.edu/virtually-exploring-the-abyssal-plains/.). These episodes have been effective at increasing viewer knowledge and interest from middle-school to adult populations. We held two workshops for grade 6-12 science teachers entitled Deep Sea Discovery: revealing Hawaiis biggest ecosystem to its students. Our goal was to develop the teachers knowledge of the deep ocean and help them develop curriculum and/or include deep ocean material in their existing curriculum. This is a great need as Hawaii is surrounded by the deep ocean yet so few students have any knowledge of it. The workshops were attended by 21 teachers and included lectures, hands-on activities, discussions, a field trip the UH Marine Center and lesson planning. The workshops were very successful with many teachers including deep-sea content in their classrooms and maintaining a relationship with our group to the present. This project provided training opportunities for four graduate students, two undergraduate students, and two postdoctoral scholars. Further we provided at sea experiences for a further two undergraduates and four additional graduate students exposing them to a diversity of sampling techniques and learning opportunities to further develop their oceanographic field skills. We have presented our results at several conferences and continue to do so. Results of this research have been published in 3 peer reviewed publications. At least three additional publications are in preparation for submission in the next year. Our data is publicly available through the Biological and Chemical Oceanography Data Management Office (https://www.bco-dmo.org/project/775278). Last Modified: 12/11/2023 Submitted by: JeffreyCDrazen