Temporal changes in benthic ostracode assemblages in the Northern Bering and Chukchi Seas from 1976 to 2010

We analyzed living ostracode assemblages from the northern Bering Sea, collected between 1976 and 2010, and from the Chukchi Sea, collected in 2009 and 2010, to examine how climatic and oceanographic changes are affecting modern ostracode species distributions. Totals of 21 and 28 ostracode species were identified, respectively, from Bering and Chukchi Sea surface sediment samples. The Bering Sea assemblage is largely transitional in species composition between those inhabiting western Arctic continental shelves and the subarctic Gulf of Alaska. Temporal changes in the Bering Sea assemblage provide evidence that decadal temperature changes have affected species composition. For example, the proportion of Normanicythere leioderma, a predominantly Arctic species, decreased from 70% of the total assemblage population in 1999 to 15% by 2006. This decrease coincided with a shift in the Arctic Oscillation toward a positive mode and warmer Bering sea-surface temperatures (SST) beginning in the early 2000s. In contrast, the more temperate species, Pectocythere janae (also known as Kotoracythere arctoborealis) made up less than 4% of the Bering assemblage prior to 2000 but increased in abundance to as much as 30% as Bering Sea temperatures rose from 2001 to 2006. This pattern has reversed since 2006 when cooler temperatures led to a decline in P. janae and return in the prominence of N. leioderma. Our results support the idea that recent ocean temperature changes and a reduced sea-ice season in the Bering–Chukchi Sea region are changing species composition in benthic ecosystems.