Inter-annual variation of the summer zooplankton community in the Chukchi Sea: spatial heterogeneity during a decade of rapid ice decline

Ice decline is believed to benefit the pelagic food chain in Arctic shelf ecosystems, but the impacts of ice decline are usually difficult to detect owing to the overlap of ice decline with natural variability. To evaluate the responses of zooplankton communities to sea ice reduction in the Chukchi Sea, we combined zooplankton samples collected in the early summers of 2003, 2008, 2010, and 2012 and compared the inter-annual changes in the subregions with various physical and biological characteristics. Three geographically separate communities were identified by a hierarchical cluster analysis. The Bering Sea water influenced the central (CCS) and south (SCS) Chukchi Sea communities, which had total abundances that were approximately ten times higher than that of the north (NCS) Chukchi Sea community, and the inter-annual variability was dramatic. The SCS and CCS shared dominant taxa of Calanus glacialis, Pseudocalanus spp., barnacle larvae (nauplii and cypris), and Oikopleura vanhoffeni, while Pacific and neritic species were recognized as the dominant species in only the SCS. The inter-annual variations in the dominant species assemblages can be explained by the variability in oceanic circulation and the counteractions between copepods and barnacle larvae. Despite the numerical fluctuations, an increase in the average abundances in the Pacific-influenced region and the summer establishment of the C. glacialis population are proposed to be the most pronounced responses to ice decline. The NCS, which is governed by cold Arctic water, was characterized by low abundances and a constant dominant taxa assemblage. This area was also characterized by the presence of the high Arctic species Calanus hyperboreus and a lack of barnacle larvae. The total abundance of the NCS doubled from 2003 to 2008, while the community structure remained consistent. These results indicate that sea ice reduction has a positive effect on the zooplankton community, but heterogeneity is the main obstacle to the detection of the zooplankton community in the Western Arctic Ocean.