Community and trophic structures of pelagic copepods down to greater depths in the western subarctic Pacific (WEST-COSMIC)

Abstract

As part of the research program Western Pacific Environment Study on CO 2 Ocean Sequestration for Mitigation of Climate Change (WEST-COSMIC), vertical distribution and community structure of copepods were studied at Station Knot (44°N, 155°E) down to 4000 m depth in the western subarctic Pacific. Vertical carbon flux mediated by copepod communities was also estimated. Both abundance and biomass of copepods were greatest in the near surface layer and decreased with increasing depth. Decrease of abundance with depth was best fitted to a power regression model, while that of biomass was best described by an exponential regression model. Copepod carcasses occurred throughout the layer, and carcasses/living specimens ratios were greatest in the deepest layer (the ratio was 9.3 at 3000–4000 m depth). A total of 98 calanoid copepod species belonging to 38 genera and 15 families occurred in the 0–4000 m water column (Cyclopoida, Harpacticoida and Poecilostomatoida were not identified to species). The number of genera and species showed bimodal vertical distributions with peaks at 500–1000 m and at 2000–3000 m both during day and night. Based on the species similarity indices, copepod community could be classified into epipelagic, mesopelagic and bathypelagic communities. Based on the feeding pattern, copepods were divided into four types: suspension feeders, suspension feeders in diapause, detritivores and carnivores. In terms of abundance, the most dominant group was suspension feeders (mainly the cyclopoid genus Oithona) in the epipelagic zone, and detritivores (mainly Poecilostomatoida genus Oncaea) were dominant in the meso- and bathypelagic zones. In terms of biomass, suspension feeders in diapause (calanoid genera Neocalanus and Eucalanus) were the major component (ca. 70%), especially at 200–2000 m depth. Comparison of vertical flux of particulate carbon with estimated copepod ingestion/egestion rates suggests that the suspension feeding copepods receive sufficient food. For detritivorous copepods, copepod carcasses, a possible food source, are not abundant enough, so other food sources need to be considered. As a food source for carnivorous copepods, the abundance of suspension feeding and detritivorous copepods appears to be high enough to meet their demand. Our calculation showed that an average of 32% of the particulate carbon flux is consumed by copepods in the 0–4000 m water column.