Award: OCE-1829519

The abyssal plains of the oceans cover roughly half of the earth’s surface, host enormous reservoirs of biodiversity and mineral resources, and play important roles in nutrient regeneration and carbon sequestration. The most important process controlling the structure and function of these ecosystems is the quantity and quality of food (mostly sinking organic particles) that reaches the deep-sea floor. However, we do not fully understand the processes provisioning this vast ecosystem. The goals of this research were to evaluate the relative importance of small and larger "marine snow" particles that sink to deep-sea benthic communities by using a combination of stable and naturally occurring radioactive tracers that tracked food sources and followed the consumption of these sinking food particles by fauna on the seafloor. Our work compared ecosystems from the productive waters off of California with the nutrient poor central Pacific, north of Hawaii. Our results indicate that both small and large particles serve as a food source to mesopelagic and benthic communities with their importance varying in response to overlying surface production. When production and subsequent particle fluxes are low, such as during the winter months in Hawaii, more than 90% of animal (zooplankton) diets are comprised of small particles less than 53 mm in size. When surface production is high and sinking particles are abundant, such as off the coast of California, larger particles dominate the food supply. Regardless of the season, however, small particles, often overlooked as a food source to the deep ocean, are critical sources of food at depth and are able to sustain deep zooplankton communities when surface production is low. These results argue that more traditional methods for capturing sinking particles, such as deep moored sediment traps, may miss smaller particle size classes in their collections. In other words, sediment trap collected detritus may not fully represent food sources of the abyssal fauna. Processes modifying small and large composition vary with depth and location. Below about 1500 m, larger animal (metazoan) grazing and repackaging moderate particle flux and food supply to the abyssal seafloor regardless of the season. In contrast in highly productive surface waters and the upper 500 m, animal feeding and grazing dominates particle signatures in the. When surface waters are less productive, microbial activity dominates. As such, while smaller particles may contribute to deep ocean benthic sustenance, the nutritional quality of these particles may be lower. Once particles reach the sediments and in times of higher particle fluxes, abyssal deposit feeders obtain their food directly from the sediments/detritus itself. During periods of food scarcity, the microbial community within deposit feeder guts may play an important role in synthesizing essential amino acids for their nutritional needs. Combined, our results highlight the importance of small particles as a food source to deep foodwebs. Climate driven changes in ocean biogeochemistry include an expansion of low productivity waters. As such, the importance of small particles to the nutrition of deep mesopelagic zooplankton is likely to become more widespread. At the same time, the increasing reliance on smaller particles may maintain the role of the oceans as an atmospheric carbon sink. It has long been assumed that carbon sequestration in the deep ocean, an important removal menchainsm of carbon dioxide from the atmosphere, only occurs via large rapidly sinking particles because small particles tend to be more easily remineralized in the water column However, if organisms feed on smaller particles and tranport this material to depth via vertical migration and repackaging, it provides an additional pathway of carbon removal to the deep sea. This project supported the education and training of numerous undergraduate and graduate students, as well as several postdoctoral fellows. During our time at sea, we live-streamed feeds to shore-based students of our particle collection pump operations as well as our remotely operated vehicle dives to the seafloor. We further communicated with Hawai‘i high school students (and teachers) participating in the NSF-GEOPATHS funded EP‘IK Summer camp— a high school summer educational program that seeks to diversify and broaden participation in the geosciences, with a particular emphasis on Native Hawaiian students. Additionally, we communicated with university students participating in the Earth Science on Volcanic Islands (ESVI) REU hosted by UH Mānoa. To communicate broadly to the public, we shared our research 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 people’s living rooms, enabling viewers to virtually travel to the abyssal plains of the deep sea. The episode was broadcast on TV to audiences in December 2020 in Hawai‘i and throughout the Pacific Basin. This was focused on our August 2020 cruise. This episode won a Bronze International Telly Award for educational documentary TV. Last Modified: 01/05/2022 Submitted by: Claudia R Benitez-Nelson