Springtime renewal of zooplankton populations in the Chukchi Sea

Although considerable work has been done in the Chukchi Sea during summer, much less has been done during other seasons. This has limited our ability to fully understand seasonal cycles and transformations of the Chukchi Sea zooplankton, particularly the key copepod species Calanus glacialis. Abundance and distributions of large zooplankton and of all life stages of C. glacialis in the northeastern Chukchi Sea during May-June 2014 are described. Three main zooplankton communities are identified; “arctic oceanic” along the Chukchi slope associated with off-shelf water masses; Chukchi Sea “overwintering” associated with cold winter water in the northern part of the study area; and Chukchi Sea “spring” associated with early season summer water in the southern portion of the study area. The overwintering and spring communities were distinguished by the near total absence of younger copepodid (CI-CIII) C. glacialis stages and meroplankton in the overwintering community while older (CV-adult) C. glacialis, amphipods, and chaetognaths were present in both. The distributions of the communities followed the major circulation pathways in the northeastern Chukchi Sea. Water and plankton flooding in from the northern Bering Sea was filling the Chukchi Sea and replenishing the zooplankton communities as the remnants of the overwintering community was being advected northwards and into the Canadian Basin. A conceptual model of the seasonal evolution of C. glacialis populations in the Chukchi Sea, based on the interaction of C. glacialis phenology and advective drivers, enfolds both these spring observations and summer observations from numerous previous studies. Because the flushing time of the Chukchi Sea is shorter than the C. glacialis generation time, the copepod is unable to establish an endemic population in the Chukchi Sea, hence the population there must be renewed annually from the northern Bering Sea.