Temperature, food and the seasonal vertical migration of key arctic copepods in the thermally stratified Amundsen Gulf (Beaufort Sea, Arctic Ocean)

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We investigated the seasonal vertical migration of seven dominant arctic copepod species from October 2007 to July 2008 in Amundsen Gulf. The large herbivore Calanus hyperboreus resided in the deep Atlantic Layer from December to mid-April, rapidly invaded the surface layer at the onset of the phytoplankton bloom in early May, and started its descent to overwintering depth in July. C. glacialis overwintered at shallower depths than C. hyperboreus, moved into the surface layer in early April as ice algae bloomed, and remained in the subsurface chlorophyll maximum (SCM) until late July. The small omnivore Oithona similis slowly rose into the Polar-Mixed Layer from February to April, reaching the ice–water interface in early April as ice algae developed. The very small Triconia borealis associated with C. hyperboreus in the Atlantic Layer from October to April. The mesopelagic omnivores Metridia longa and Microcalanus pygmaeus could be characterized as mid-depth interceptors, feeding on microzooplankton in winter, the upward flux of C. hyperboreus eggs in late winter-early spring, and on the rain of microalgae in spring-summer. The epipelagic Pseudocalanus spp. distributed in a narrow band centered around 50 m from October to April, then descended under the SCM from May to July. Copepods generally avoided the lens of warm (2–8°C) surface water in summer, but readily migrated across the −1.7°C to +0.3°C gradient from the Polar-Mixed Layer to the Atlantic Layer. The different patterns of seasonal migrations dictated the different temperature regimes experienced over the year by each species and copepodite stage.

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