The North Pacific Ocean subtropical–subarctic gyre transition region is recognized as an important small pelagic fish nursery and feeding ground. To clarify the lower trophic level variability of the transition region pelagic ecosystem, we investigated the large-scale variation in copepod communities in relation to the oceanographic environment. We net sampled zooplankton in the upper 150 m from the coast to approximately 4,500 km offshore to the east (140° E − 165° W) during early summer in 2017 and 2018. Cluster analysis based on abundance and species composition at 233 sampling stations showed that the subarctic boundary, defined by a salinity of 34.0 at 150 m depth, separated subarctic and subtropical copepod communities, as a clear biogeographical border. The two communities were each further classified into four subgroups. Among the subgroups, a typical subarctic subgroup dominated by Neocalanus spp. was broadly distributed in the subarctic HNLC (high-nutrient and low-chlorophyll a concentrations) region, whereas a typical subtropical subgroup characterized by a higher overall species diversity occurred in the warm oligotrophic water. In addition, in both these subgroups, the composition of the chlorophyll a size fraction suggested that the food web was mainly based on small-sized phytoplankton. Further analysis of the other subgroups indicated that both the subtropical and subarctic communities showed distinctive succession patterns depending on the degree of water mixing. During succession, a consistent increase in copepod abundance, especially large-sized suspension feeders (e.g., Eucalanus and Calanus) associated with the high productivity of large diatoms was observed in both main communities. The present study revealed that water mixing in the transition region consistently enhances the grazing food chain in both subtropical and subarctic communities, showing the potential underlying mechanisms of control of prey availability for small pelagic fish that exploit the region as a key nursery and feeding ground.

北太平洋亚热带-亚北极环流过渡区被公认为重要的小型中上层鱼类育苗和觅食地。为了阐明过渡区远洋生态系统的低营养级变异性，我们调查了桡足类群落与海洋环境相关的大规模变异。我们在 2017 年和 2018 年初夏期间对从海岸到东部近海约 4,500 公里（140° E - 165° W）的上层 150 m 的浮游动物进行了净采样。基于 233 个采样站的丰度和物种组成的聚类分析显示亚北极边界由 150 m 深度处的 34.0 盐度定义，将亚北极和亚热带桡足类群落分开，作为一个明确的生物地理边界。这两个社区分别进一步分为四个亚组。Neocalanus spp。广泛分布在亚北极HNLC（高营养和低叶绿素a浓度）区域，而典型的亚热带亚群则以较高的整体物种多样性为特征，发生在温暖的贫营养水中。此外，在这两个亚群中，叶绿素a大小部分的组成表明食物网主要以小型浮游植物为基础。对其他亚群的进一步分析表明，亚热带和亚北极群落都表现出独特的演替模式，具体取决于水混合的程度。在演替过程中，桡足类丰度持续增加，尤其是大型悬浮饲养者（例如，桉树和卡拉努斯）) 在两个主要群落中都观察到与大型硅藻的高生产力相关。本研究表明，过渡地区的水混合持续增强了亚热带和亚北极社区的放牧食物链，显示了控制利用该地区作为主要育苗场和觅食地的小型中上层鱼类猎物可用性的潜在潜在机制。