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

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.