Unraveling the mystery of cold regime ecosystem variability in the Bering Sea through acoustics

Abstract

The Bering Sea oscillates between warm and cold climatic regimes on 3–7 year cycles. During cold regimes, the bottom water layer, or cold pool, can remain below 2 °C the entire summer. When the cold pool forms, it acts as a cross-shelf barrier separating species of the outer shelf from others of the middle shelf and coastal areas. Backscatter recorded throughout the winter from echosounders on sub-surface moorings in the southeastern Bering Sea provide a first glimpse on how dynamic the cold pool influence can be within cold regime years. The fall/winter scattering community composition was predictive of the structure during the first blooms the following spring. Seasonal environmental conditions were also observed to play a dominant role in summer lower trophic level dynamics. Delayed ice retreat in the summer of the coldest years was associated with increased abundance of large zooplankton; yet relatively warmer years during the same cold climatic regime yielded a shift to smaller zooplankton scatterers during summer. Chlorophyll concentrations showed varying levels of correlation to zooplankton patterns, and sparse cruise-based data suggested differences in phytoplankton community composition likely influenced these relationships. Data on phytoplankton community structure remains a desperately needed dataset to fully understand the ecosystem dynamics.