Copepod community structure in the transition region of the North Pacific Ocean: Water mixing as a key driver of secondary production enhancement in subarctic and subtropical waters

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 degrees E - 165 degrees 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 avail-ability for small pelagic fish that exploit the region as a key nursery and feeding ground.