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1. Ocean deoxygenation and zooplankton: Very small oxygen differences matter

   https://doi.org/10.1126/sciadv.aau5180  

   Wishner, K. F.; Seibel, B. A.; Roman, C.; Deutsch, C.; Outram, D.; Shaw, C. T.; Birk, M. A.; Mislan, K. A.; Adams, T. J.; Moore, D.; et al (December 2018, Science Advances)

   Oxygen minimum zones (OMZs), large midwater regions of very low oxygen, are expected to expand as a result of climate change. While oxygen is known to be important in structuring midwater ecosystems, a precise and mechanistic understanding of the effects of oxygen on zooplankton is lacking. Zooplankton are important components of midwater food webs and biogeochemical cycles. Here, we show that, in the eastern tropical North Pacific OMZ, previously undescribed submesoscale oxygen variability has a direct effect on the distribution of many major zooplankton groups. Despite extraordinary hypoxia tolerance, many zooplankton live near their physiological limits and respond to slight (≤1%) changes in oxygen. Ocean oxygen loss (deoxygenation) may, thus, elicit major unanticipated changes to midwater ecosystem structure and function. 

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2. Seasonal dynamics of midwater zooplankton and relation to particle cycling in the North Pacific Subtropical Gyre

   https://doi.org/https://doi.org/10.1016/j.pocean.2020.102266  

   Hannides, CCS; Popp, BN.; Close, HG; Benitez-Nelson, CR; Ka’apu-Lyons, CA; Gloeckler, K; Wallsgrove, N; Uhau, B; Palmer, E; Drazen, JC (January 2020, Progress in oceanography)

   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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3. Active Carbon Transport by Diel Vertical Migrating Zooplankton: Calculated and Modeled, but Never Measured

   https://doi.org/10.1146/annurev-marine-121422-015330  

   Clements, Daniel J; Stamieszkin, Karen; Bianchi, Daniele; Blanco-Bercial, Leocadio; Record, Nicholas R; Rodriguez-Perez, Rocio B; Maas, Amy E (July 2025, Annual Review of Marine Science)

   Zooplankton diel vertical migration (DVM) is a globally ubiquitous phenomenon and a critical component of the ocean's biological pump. During DVM, zooplankton metabolism leads to carbon and nutrient export to mesopelagic depths, where carbon can be sequestered for decades to millennia, while also introducing labile, energy-rich food sources to midwater ecosystems. Three pervasive metabolic pathways allow zooplankton to sequester carbon: fecal pellet egestion, dissolved organic matter excretion, and respiration. Additionally, there are several less well-parameterized sources of DVM transport associated with growth, feeding, reproduction, and mortality. These processes are challenging to measure in situ and difficult to extrapolate from laboratory experiments, making them some of the most poorly constrained factors in assessments and models of the biological pump. In this review, we evaluate and compare observational and modeling approaches to estimate zooplankton DVM and the resulting active carbon flux, highlighting major discrepancies and proposing directions for future research. 

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4. Divergent patterns of zooplankton connectivity in the epipelagic and mesopelagic zones of the eastern North Pacific

   https://doi.org/10.1002/ece3.10664  

   Matthews, Stephanie A.; Blanco‐Bercial, Leocadio (November 2023, Ecology and Evolution)

   Abstract Due to historical under‐sampling of the deep ocean, the distributional ranges of mesopelagic zooplankton are not well documented, leading to uncertainty about the mechanisms that shape midwater zooplankton community composition. Using a combination of DNA metabarcoding (18S‐V4 and mtCOI) and trait‐based analysis, we characterized zooplankton diversity and community composition in the upper 1000 m of the northeast Pacific Ocean. We tested whether the North Pacific Transition Zone is a biogeographic boundary region for mesopelagic zooplankton. We also tested whether zooplankton taxa occupying different vertical habitats and exhibiting different ecological traits differed in the ranges of temperature, Chl‐a, and dissolved oxygen conditions inhabited. The depth of the maximum taxonomic richness deepened with increasing latitude in the North Pacific. Community similarity in the mesopelagic zone also increased in comparison with the epipelagic zone, and no evidence was found for a biogeographic boundary between previously delineated mesopelagic biogeochemical provinces. Epipelagic zooplankton exhibited broader temperature and Chl‐aranges than mesopelagic taxa. Within the epipelagic, taxa with broader temperature and Chl‐aranges also had broader distributional ranges. However, mesopelagic taxa were distributed across wider dissolved oxygen ranges, and within the mesopelagic, only oxygen ranges covaried with distributional ranges. Environmental and distributional ranges also varied among traits, both for epipelagic taxa and mesopelagic taxa. The strongest differences in both environmental and distributional ranges were observed for taxa with or without diel vertical migration behavior. Our results suggest that species traits can influence the differential effects of physical dispersal and environmental selection in shaping biogeographic distributions. 

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5. Deep zooplankton rely on small particles when particle fluxes are low

   https://doi.org/doi: 10.1002/lol2.10163  

   Romero-Romero, Sonia; Ka’apu-Lyons, Cassie A.; Umhau, Blaire P.; Benitez-Nelson, Claudia R.; Hannides, Cecelia C.; Close, Hilary G.; Drazen, Jeffrey C.; Popp, Brian N. (June 2020, Limnology and oceanography letters)

   The fate of organic matter (OM) in the deep ocean remains enigmatic, with little understood regarding the flux and its utilization by deep food webs. We used compound-specific nitrogen stable isotope ratios of source amino acids measured in particle size classes and deep zooplankton (700–1500 m) to determine the contribution of small (0.7–53 μm) vs. large particles (> 53 μm) to their diet at four sites in the North Pacific. Our results show that small particles constitute between 9% and 98% of zooplankton diets, being the contribution higher at sites with lower flux regimes. The contribution of small particles to the diet of deep zooplankton was also higher when biomass of vertical migrators, and therefore actively transported OM, was lower. Climate-driven changes in primary production and export are expected to shift particle fluxes to smaller size classes, and thus their importance in midwater food webs may become more widespread. 

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