Changes in Population Dynamics of a Key Food Web Component, the Bivalve Macoma calcarea in the Northern Bering Sea from 1998-2015 and its Relation to Rapidly Changing Environmental Parameters

The northern Bering Sea is experiencing rapid change, with recent shifts to low sea ice extent and significantly higher surface and bottom water temperatures. For example, bottom water temperatures as high as +1.5°C were recorded in July 2018 at sites within the Distributed Biological Observatory (DBO) region 1, south of St. Lawrence Island, and well above a multi-decadel record of bottom water temperatures not exceeding 0°C year-round. Recent studies using dynamic factor analysis (DFA) of bivalve abundance and biomass, along with six covariates, including number of days with no sea ice cover and bottom water temperature, have identified benthic macrofaunal changes within the population of a key bivalve, Macoma calcarea, that is prey for higher trophic level predators such as spectacled eiders (Somateria fischeri) and the Pacific walrus (Odobenus rosmarus divergens). We examined five sites in DBO region 1 in the Northern Bering Sea hotspot using DFA and other time-series approaches in order to track and compare changes in the abundance, biomass, and dominant size class of M. calcarea in the northern Bering Sea as they relate to a suite of environmental indices such as annual days without sea ice cover, bottom water temperature, and sediment parameters including sediment chlorophyll-a and grain size. In addition to abundance and biomass, size of individuals and abundance of each clam size class provides information about recruitment within the population and the growth and sustainability of the population through time. The abundance and biomass of this species is now highest at the two most northern stations of DBO1 (SLIP5 and SLIP4) and has decreased over the past decade at sites further to the south. Because of the foraging site fidelity of spectacled eiders, this food supply decrease could impact eider feeding south of St. Lawrence Island where the world population of this diving duck annually forages between October and April, whereas walrus can move north with sea ice retreat to feed on other prey patches.