Abstract 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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北太平洋亚热带环流中水浮游动物的季节性动态及其与粒子循环的关系

摘要 中水浮游动物是公海生物地球化学转化的主要媒介；然而，它们的特征和活动仍然鲜为人知。在这里，我们使用氨基酸化合物特异性同位素分析 (CSIA-AA)、营养位置和北太平洋亚热带环流 (NPSG) 中的元素组成来评估中层浮游动物生物量、氨基酸 (AA) 特异性稳定同位素组成（δ15N 值） ）。我们专注于在冬季、春季和夏季收集的浮游动物，以评估季节性周期中的中层营养动态。我们第一次发现中层浮游动物对 NPSG 生产和出口的季节性变化反应强烈。在夏季，当从透光区的出口升高并且这种“夏季脉冲”物质被迅速输送到深处时，CSIA-AA 表明大颗粒 (> 53 μm）在整个中层水域中占主导地位的浮游动物的食物网基础，在很大程度上甚至进入深海上游区。在冬季，当出口量较低时，中层中层浮游动物继续依赖大颗粒基础资源，但中层下层和深上层带的常驻浮游动物转向在其食物网中包含较小颗粒（0.7-53 μm）基，甚至是小粒子池的一个子集。中水浮游动物的迁徙模式也因季节而异，与冬季在上层混合层中被压缩的情况相比，迁徙者在夜间通过透光区分布更均匀。中深层浮游动物更深的迁徙在夏末也减少了，可能是由于本季节在深度处可用的大颗粒物质的数量增加。我们观察到的活动和营养动态的季节性变化在夏季通过中层海区驱动比冬季略高的生物量。相比之下，中层浮游动物的碳（C）、氮（N）和磷（P）组成不随季节变化。相反，我们发现 C:N、C:P 和 N:P 比率随着深度的增加而增加，这可能是由于蛋白质结构和有机 P 化合物的减少以及储存脂质随着深度的增加而增加。我们的研究强调了 NPSG 中水域小型浮游动物饲养策略的重要性和多样性。许多小型浮游动物，如oncaeid 和oithonid 桡足类，能够在深处获取小颗粒资源，并且可能是微生物环和深居 micronekton 物种之间的重要营养联系，这些 micronekton 物种也依赖于基于小颗粒的食物网。我们观察到的中水浮游动物对深度粒子场输出驱动变化的营养反应对中水代谢和 C 向深海的输出具有重要意义。