Revised 01/10/00 Regime shift theory: A review of changing environmental conditions in the Bering Sea and Eastern North Pacific Ocean by

Multi-decadal changes in patterns of physical phenomena and populations of marine biota occur in the Bering Sea and Eastern North Pacific Ocean. The most marked physical manifestation of these changes is a variation in modes of atmospheric pressure patterns known as a " regime shift ". The physical mechanisms that generate regime shifts are hypothesized to include such primary driving forces as the lunar-nodal tide, variations in solar radiation and changes in the North Pacific circulation that affect air-sea exchange of heat and momentum with the Aleutian Low atmospheric pressure pattern. Secondary effects include interactions among upper atmosphere pressure patterns. The cause for the variations in these pressure patterns, however, is not known. The changes in atmospheric pressure fields together with their attendant winds, moisture and momentum, transfer energy throughout the oceanic environment. These fluctuations greatly influence such phenomena as variations in the extent of sea ice, transport of nutrients into the euphotic zone, water column temperature and turbulence. Changes in these aspects of the ecosystem, in turn, influence biota both through the nutrient-phytoplankton-zooplankton feeding sequence and by directly affecting biological rates and behavior. Changes in the physical environment, however, do not explain all the variations in populations. Clearly, biological interactions are also important, as are the impacts of human intervention. While our understanding of natural changes and their impact on the ecosystem is growing, many of the mechanisms that link physical to biological processes and the life histories of many species are not yet known. As our understanding of how marine populations respond to regime shifts grows, we will be able to use such knowledge toward sustaining the health and productivity of the ecosystem.