Functional diversity of copepod assemblages along a basin-scale latitudinal gradient in the North Pacific Ocean

The northern Pacific Ocean is one of the most sensitive areas globally to climate change. Copepods typically account for between 60% and 90% of mesozooplankton in the open ocean. Because copepods are a key link in marine food webs, their response to environmental changes is an important topic in marine biodiversity and ecosystem functioning relationships. The relationship between copepod assemblages and the marine environment in the northern Pacific Ocean from a traits-based perspective has been largely unknown until now. In this study, we used the functional traits and geographic distribution of 177 copepod species along a latitudinal gradient, ranging from 4°S to 46°N in the northern Pacific Ocean, to evaluate the latitudinal variation of functional diversity and assembly rules of copepod assemblages. Based on a cluster analysis of four key functional traits, seven functional groups were identified. Redundancy analysis revealed environmental preferences for different functional groups. Large carnivores showed a stronger preference for higher temperatures than small carnivores, Omnivores and herbivores showed a stronger preference for higher chlorophyll a concentrations. The distribution of detritivores was nearly independent of temperature and chlorophyll a concentration. Most functional diversity indices showed non-linear decreasing trends along the latitudinal gradient. These trends remained stable in the tropic and subtropic regions (WARM and NPSG), but decreased sharply in the Kuroshio extension and Pacific subarctic gyres regions (KURO and PSAG). A null model revealed the assembly rules of copepod assemblage significantly varied with latitude: environmental filtering was dominant in the KURO and PSAG, whereas both environmental filtering and limited similarity played important roles in the WARM and NPSG, in addition to the neutral process. Our results suggested that with ocean warming, a northward shift in the distribution range of specific functional groups (such as large carnivores) might significantly alter the biodiversity and ecosystem function of zooplankton communities in the Kuroshio extension region. This study highlights the importance of a traits-based approach in the study of biodiversity and ecosystem function in pelagic ecosystems at large scales.