Award: OCE-1333734

The ocean?s deep midwaters (500-1500 m) are the largest living space on the planet. The deep midwater food web plays key roles in transferring carbon to the deep ocean and small organisms living at these depths are the food for commercially harvested species like tunas. Despite its importance, our understanding of this food web is poor as evidenced by recent calculations that cannot identify adequate food resources to meet estimated food demand. Some food inputs to the mesopelagic food web have been underestimated or missed. Small and slowly settling suspended particles frequently are not sampled effectively with conventional methods and may be an overlooked food source. Because identifying the principal inputs of food to the deep-sea food web is critical to understanding its function, we sought to evaluate the importance of suspended particles relative to conventionally sampled and larger sinking particles to the deep midwaters in the central North Pacific. Our approach was to use stable isotope food web tracers to characterize the different particles and trace them into zooplankton and micronekton (small fishes, squids and shrimps). To do this we sampled particles, zooplankton and micronekton from the surface to 1500 meters depth on three research expeditions. Intellectual Merit We identified distinct isotope values that distinguish sinking from suspended particles and that change within each particle type with depth. These distinct nitrogen (and possibly carbon) isotope values of individual amino acids in sinking and suspended particles can be traced into zooplankton and micronekton. Several of our analyses suggest that suspended particles contain a high proportion of amino acids that either derive from heterotrophic bacterial cells or have otherwise been affected by microbial reworking – and that this proportion increases with depth. Thus we have developed a tool to trace products of the microbial loop into higher trophic levels. It is clear that suspended particles comprise a substantial fraction of the fuel for food webs leading to higher trophic levels in deep midwaters. Though many zooplankton and micronekton studied are unlikely to eat the particles directly it is clear that these animals are increasingly reliant on suspended particles and the grazers eating those particles with increasing depth. In addition, suspended particles are more important as a food source to zooplankton in the winter when sinking particle fluxes are low. However, this pattern is not apparent in the longer lived micronekton, which likely integrate the isotopic values of prey across seasons. We estimate that about 20% of the zooplankton biomass deeper than 150m and 30% of the micronekton biomass are ultimately derived from a suspended particle food web. This new information tells us that suspended particles are a previously overlooked food source to mesopelagic communities. These results substantially improve our understanding of mesopelagic food webs, which are important in sequestering carbon to the deep ocean and in providing a forage base for commercially important fishes. Estimates of mesopelagic carbon supply are far lower than estimates of carbon demand and our results suggest that incorporating suspended particles into models will help close that gap. Suspended particles are also expected to become more important as a food source in the future; our results show that as larger sinking particle flux is reduced, as expected with future gyre expansion and reductions in ocean productivity, there will be increasing reliance on the suspended particle pool. Broader Impacts CoPI Duncan has generated substantial public outreach for the project. Six television episodes of the Telly Award winning series Voice of the Sea have been produced (e.g https://youtu.be/iM6Tmz4cwGw; https://www.youtube.com/watch?v=MG2jzalEqhQ). These episodes aired widely in Hawaii and in U.S. Pacific territories and affiliated Pacific regions. In Hawaii alone, our viewership averages 8,000–10,000 viewers per episode (Nielsen Ratings and Marshall Marketing). Audience feedback shows that these episodes have been effective at increasing viewer knowledge and interest as well as viewer connection to scientists across age groups, from middle-school, to college, to adult populations. This project provided training opportunities for two graduate students and one undergraduate student, all women and one of native Hawaiian ancestry. Further we provided at sea experiences for a further 6 undergraduates and 7 additional graduate students exposing them to a diversity of sampling techniques and learning opportunities. This proposal also provided early career opportunities to coPIs Hannides and Close. Close is now on the faculty at RSMAS, University of Miami. We have presented our results at numerous seminars and conference presentations and continue to do so. Results of this research have been published in 2 peer reviewed publications. One manuscript has been submitted and at least three additional publications are in preparation for submission in 2017. Our data is publicly available through the Biological and Chemical Oceanography Data Management Office (http://www.bco-dmo.org/project/537123). Last Modified: 03/31/2017 Submitted by: Jeffrey C Drazen