Unusual abundance of macrobenthos and biological invasions in the Chukchi Sea

The data from the expedition of the program RUSALCA conducted in 2004 showed unexpectedly high quantitative indices of macrobenthos in the southeastern Chukchi Sea. Extensive areas of the bottom north- west of the Bering Strait were dominated by the bivalve Macoma calcarea . The greatest biomass of benthos in Macoma -dominated areas was 4232 g/m 2 with an average of 1382 g/m 2 for the investigated region. Such a high biomass of soft-bottom communities, which is extremely uncommon even in the temperate regions of the oceans, is reported for the Arctic for the first time. The long-term existence (more than 70 years) of highly pro- ductive benthic communities dominated by Macoma calcarea in one and the same area of the Chukchi Sea can most likely be attributed to gyres, which constantly arise in the region northwest of the Bering Strait. These cyclonic gyres carry nutrient-rich bottom water to the surface and hinder larval transport away from mother populations. They also keep and concentrate major food sources of benthos (live and dead phyto- and zooplank- ton and fecal pellets) over the benthic community locations. Most likely, a significant proportion of the primary production in the southeastern Chukchi Sea is used by benthos within the investigated Macoma community. Findings of three relatively large warm-water Pacific species near Point Hope in the Chukchi Sea are probably indicative of the progressive climate warming during the last century.