Year-to-year changes of the mesozooplankton community in the Chukchi Sea during summers of 1991, 1992 and 2007, 2008

A recent drastic decrease in sea ice cover area was observed in the western Arctic Ocean during summer, yet little information is available for its effect on zooplankton community. To evaluate the effect of sea ice reduction on zooplankton, we studied year-to-year changes of zooplankton community structure in the Chukchi Sea during summers of 1991, 1992 (when sea ice extended), 2007, and 2008 (when sea ice reduced). Zooplankton abundance ranged from 4,000 to 316,000 ind. m⁻² (mean: 70,000) and was greater north of Lisburne Peninsula in 2008. Zooplankton biomass ranged from 0.07 to 286 g wet mass m⁻² (mean: 36) and was greater south of Lisburne Peninsula in 2007. Cluster analysis based on zooplankton abundance showed a division of the zooplankton community into four groups. Occurrence of each group was separated geographically and interannually, and geographic distributions of each group in 1991 and 1992 were similar but those in 2007 and 2008 were shifted northward. Abundance and biomass in 2007/2008 were higher than in 1991/1992, indicating that further sea ice reduction would have a positive effect on zooplankton production (e.g. invasion of large Pacific species and temperature effects on their growth rate). The northern shift in geographic distribution of the zooplankton community in 2007/2008 indicates that sea ice reduction would have a negative effect on the zooplankton community (loss of characteristic Arctic species) in part of the Chukchi Sea. These apparently contradictory effects of sea ice reduction on zooplankton community emphasize the critical need for continued monitoring in this area.