Inter-annual variability of summer mesozooplankton communities of the western Chukchi Sea: 2004-2012

The Chukchi Sea shelf is a complex transition zone between the Pacific and Arctic Oceans, on which climate variation may have a profound impact. We examined summer zooplankton community structure of the western Chukchi Sea in Alaskan and Russian waters during 2004, 2009, 2010 and 2012 within the ongoing Russian-American Long-term Census of the Arctic program. The four study years were very different both in water mass properties and in zooplankton community structure. A "warm" year with an early ice retreat and highest water temperatures occurred in 2004, whereas the years 2009-2012 were "cold" with a later-than-average ice retreat and colder average water temperatures during the sampling period. The extent and prominence of different water masses (Bering Sea-Anadyr Water, Alaska Coastal Current, Siberian Coastal Current, Resident Chukchi Water) within the Chukchi Sea varied between years, which was in turn reflected within the zooplankton communities. Community structure was highly correlated with water mass properties, with bottom temperature being the most significant factor influencing communities. The "cold" summers of 2009-2012 had nearly twice the biomass and abundance of zooplankton compared with the "warm" summer of 2004. Biomass was dominated by the large copepod Calanus glacialis believed to originate from the Bering Sea, and abundance was dominated by small shelf species of copepods, such as Pseudocalanus spp., Acartia spp. and Oithona similis. We discuss the implications of the inter-annual variability of planktonic communities within the Chukchi Sea and the possible effects of longer-term climate change.