Seasonal and interannual variation in the planktonic communities of the northeastern Chukchi Sea during the summer and early fall

We analyzed the seasonal and interannual variability of the planktonic communities in a densely sampled region of the northeastern Chukchi Sea as part of a multidisciplinary ecosystem study from 2008 to 2010. Observations of chlorophyll-a, inorganic macronutrients, and zooplankton (using both 150-μm and 505-μm mesh nets) were made within two 900-NM 2 grids (Klondike and Burger) at high spatial resolution three times each in 2008 and 2009, with a third grid (Statoil) sampled twice in 2010. Sea-ice conditions prior to sampling varied notably during the study: seasonal sea ice retreat was earlier and sea-surface temperatures (SSTs) were warmer in 2009 than in 2008, whereas SSTs for 2010 were intermediate between the 2008 and 2009 values. Eighty taxonomic categories of zooplankton, including 11 meroplanktonic categories, were recorded, with the greatest diversity found within the copepods (25 species), followed by the cnidarians (11 species). All species are typical for the region and most are seeded from the Bering Sea. A seasonal progression of the community structure was apparent over each survey area and was likely influenced by temperature. Cold oceanographic conditions in 2008 likely slowed growth and development of the zooplankton, such that holozooplankton abundance averaged 2389 and 106 individuals m–3 and biomass averaged 10.5 and 8.3 mg DW m–3 in the 150- and 505-μm nets, respectively. An early phytoplankton bloom in 2009 apparently supported a zooplankton community of greater abundance, but moderate biomass, averaging 6842 and 189 individuals m–3, and 16.3 and 7.0 mg DW m–3 in the 150- and 505-μm nets, respectively. Highest zooplankton abundance and biomass values among the three years occurred in 2010: 7396 and 198 individuals m–3 and 102.9 and 33.5 mg DW m–3 in the 150- and 505-μm nets, respectively. Holozooplankton biomass changes were driven by increases in large-bodied, lipid-rich copepods. The contribution of meroplankton was substantial in this shallow-water ecosystem: numerically, they contributed 28% in 2008, 8% in 2009 and 56% in 2010 to the total zooplankton community and 43%, 27%, and 11%, respectively, terms of biomass for the 150-μm nets. Interannual differences in ice-melt timing, water temperatures, northward transport of water masses, and nutrients and chlorophyll concentrations resulted in highly variable pelagic productivity.