Northeast Pacific mesopelagic zooplankton feed increasingly on small (1-51 µm) particles with depth at Station Papa (EXPORTS)

Connor Shea1, Victor Evrard2, Natalie Wallsgrove3, Tamara Allen2, Joseph S Cope4, Deborah K Steinberg4, Dr. Amy E Maas, PhD5, Karen Stamieszkin4,6, Hilary G Close7 and Brian N Popp8, (1)University of Hawaii at Manoa, Department of Oceanography, Honolulu, HI, United States, (2)University of Hawaii at Manoa, Department of Earth Sciences, Honolulu, HI, United States, (3)University of Hawaii at Manoa, Department of Earth Sciences, Honolulu, United States, (4)Virginia Institute of Marine Science, Department of Biological Sciences, Gloucester Point, VA, United States, (5)Bermuda Institute of Ocean Sciences - Arizona State University, Julie Ann Wrigley Global Futures Laboratory, St. George's, Bermuda, (6)Bigelow Lab for Ocean Sciences, East Boothbay, United States, (7)University of Miami, Rosenstiel School of Marine, Atmospheric, and Earth Science, Miami, United States, (8)University of Hawaiʻi at Mānoa, Department of Earth Sciences, Honolulu, United States
Zooplankton inhabiting the ocean’s “twilight zone” consume, repackage, and remineralize sinking particles from the surface waters, thus changing the efficiency by which carbon is transferred to the deep sea via the biological pump. Here, the size spectrum of particles consumed by size fractionated zooplankton collected throughout the mesopelagic zone during the EXPORTS campaign at Station Papa in the northeast subarctic Pacific Ocean is determined using amino acid compound specific stable isotope analysis. The nitrogen isotopic composition of bulk zooplankton collected during a nighttime MOCNESS tow increase by 4-5‰ between the mixed layer and 1000 m suggesting an increase in average zooplankton trophic position and/or a change in the δ15N value at the base of the food web. However, differences in δ15N values of ‘trophic’ and ‘source’ amino acids indicate that trophic position remains constant throughout the depth range sampled. This indicates that indiscriminate 15N enrichment is occurring at the base of the zooplanktonic food web and suggests that small particles (1-51 µm) are a significant food resource for zooplankton throughout the mesopelagic zone in the subarctic Pacific. The implications of these food web-particle interactions for the efficiency of vertical transfer of organic matter below the well-lit surface ocean will be discussed.