Response of lower trophic level production to long-term climate change in the southeastern Bering Sea

The Bering Sea ecosystem has undergone profound changes in response to climate regime shifts in the past decades. Here, lower trophic level production is assessed with a vertically one-dimensional (1-D) coupled ice-ocean ecosystem model, which was applied to data collected by a National Oceanic and Atmospheric Administration (NOAA)/Pacific Marine Environmental Laboratory (PMEL) mooring from 1995 to 2005. The physical model is forced by sea surface winds, heat and salt fluxes, tides, and sea ice. The biological model includes coupled pelagic and ice algae components. Model results are validated with daily mooring temperature, fluorometer, and daily Sea-viewing Wide Field-of-view Sensor (SeaWiFS) chlorophyll data. Two distinct ocean conditions and phytoplankton bloom patterns are related to the Pacific Decadal Oscillation (PDO) Index regimes: warmer temperature and later warm-water phytoplankton species bloom in PDO > 1 year; colder temperature and earlier cold-water phytoplankton species bloom in PDO < -1 year. Productivity of different phytoplankton species changed dramatically after the 1976 climate shift, but the total annual net primary production (NPP) remained flat over the past four decades under similar nutrient regulation. Climate shift also affected the vertical distribution of lower trophic level production and energy flow to the upper ocean pelagic ecosystem or the benthic community. A long-term PDO regime shift occurred in 1976, and a short-term PDO reversal occurred in 1998. Phytoplankton biomass responded promptly to both short- and long-term climate changes. Zooplankton biomass responded more to the long-term than to the short- term climate shift. The model results captured observed trends of zooplankton abundance changes from the 1990s to 2004.