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Midwater zooplankton are major agents of biogeochemical transformation in the open ocean; however their characteristics and activity remain poorly known. Here we evaluate midwater zooplankton biomass, amino acid (AA)-specific stable isotope composition (δ15N values) using compound-specific isotope analysis of amino acids (CSIA-AA), trophic position, and elemental composition in the North Pacific Subtropical Gyre (NPSG). We focus on zooplankton collected in the winter, spring, and summer to evaluate midwater trophic dynamics over a seasonal cycle. For the first time we find that midwater zooplankton respond strongly to seasonal changes in production and export in the NPSG. In summer, when export from the euphotic zone is elevated and this ‘summer pulse’ material is transported rapidly to depth, CSIA-AA indicates that large particles (> 53 μm) dominate the food web base for zooplankton throughout the midwaters, and to a large extent even into the upper bathypelagic zone. In winter, when export is low, zooplankton in the mid-mesopelagic zone continue to rely on large particle basal resources, but resident zooplankton in the lower mesopelagic and upper bathypelagic zones switch to include smaller particles (0.7–53 μm) in their food web base, or even a subset of the small particle pool. Midwater zooplankton migration patterns also vary with season, with migrants distributed more evenly at night through the euphotic zone in summer as compared to being more compressed in the upper mixed layer in winter. Deeper zooplankton migration within the mesopelagic zone is also reduced in late summer, likely due to the increased magnitude of large particle material available at depth during this season. Our observed seasonal change in activity and trophic dynamics drives modestly greater biomass in summer than winter through the mesopelagic zone. In contrast midwater zooplankton carbon (C), nitrogen (N), and phosphorus (P) composition does not change with season. Instead we find increasing C:N, C:P, and N:P ratios with greater depths, likely due to decreases in proteinaceous structures and organic P compounds and increases in storage lipids with depth. Our study highlights the importance and diversity of feeding strategies for small zooplankton in NPSG midwaters. Many small zooplankton, such as oncaeid and oithonid copepods, are able to access small particle resources at depth and may be an important trophic link between the microbial loop and deep dwelling micronekton species that also rely on small particle-based food webs. Our observed midwater zooplankton trophic response to export-driven variation in the particle field at depth has important implications for midwater metabolism and the export of C to the deep sea.
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Small particles and heterotrophic protists support the mesopelagic zooplankton food web in the subarctic northeast Pacific Ocean
Abstract A quantitative understanding of the mesopelagic zooplankton food web is key to development of accurate carbon budgets and geochemical models in marine systems. Here we use compound specific nitrogen stable isotope analysis of amino acids to quantify the trophic structure of the microzooplankton and mesozooplankton community during summer in the subarctic northeast Pacific Ocean during the EXport Processes in the Ocean from Remote Sensing (EXPORTS) field campaign. Source amino acid values in particles and zooplankton provide strong evidence that basal resources for the mesopelagic zooplankton food web were primarily small (), suspended or slow‐sinking particles, but that surface organic matter delivered by vertically migrating zooplankton may have also been important. Comparisons of values of source and trophic amino acids provide estimates of food web length, which decrease significantly with depth and suggest that protistan microzooplankton are key components of the food web from the surface to at least 500. These results emphasize the importance of small particles as a source of carbon and nitrogen to mesopelagic communities in this region, support observations of an inverse relationship between zooplankton vertical migration and small particles as sources of carbon to deep‐sea food webs in low productivity environments, and document the role of heterotrophic protists as key trophic intermediaries in the mesopelagic zone at this location.
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- PAR ID:
- 10431573
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Limnology and Oceanography
- Volume:
- 68
- Issue:
- 8
- ISSN:
- 0024-3590
- Page Range / eLocation ID:
- p. 1949-1963
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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