Deep-Sea Research II 50 (2003) 2567–2582 The biological response to the 1977 regime shift in the California Current John A. McGowan a, *, Steven J. Bograd b , Ronald J. Lynn c , Arthur J. Miller a a Scripps Institution of Oceanography, University of California-San Diego, La Jolla, CA, USA b NOAA/NMFS, Pacific Fisheries Environmental Laboratory, 1352 Lighthouse Avenue, Pacific Grove, CA 93950-2097, USA c NOAA/NMFS, Southwest Fisheries Science Center, La Jolla, CA, USA Received 16 December 2002; received in revised form 15 March 2003; accepted 6 April 2003 Abstract Among the least understood interactions between physics and biology in the oceans are those that take place on the decadal scale. But this temporal scale is important because some of the greatest ecological events take place on this time scale. More than 50 years of measurement in the California Current System have revealed significant ecosystem changes, including a large, decadal decline in zooplankton biomass, along with a rise in upper-ocean temperature. The temperature change was a relatively abrupt shift around 1976–77, concurrent with other basin-wide changes associated with an intensification of the Aleutian Low-pressure system. This intensification generates temperature anomalies in the ocean by altering the patterns of net surface-heat fluxes, turbulent mixing, and horizontal transport. Changes in the mean abundance of zooplankton in the southern California Current have been attributed to variations in the strength of coastal upwelling, variations in the horizontal transport of nutrient-rich water from the north, or increased stratification due to warming, all of which could be affected by fluctuations in the Aleutian Low. Here we show that a deepening of the thermocline accompanied the warming and increased the stratification of the water column, leading to a decrease in the supply of plant nutrients to the upper layers. This is the most likely mechanism for the observed plankton decline, and subsequent ecosystem changes. A global change in upper-ocean heat content, accompanied by an increase in stratification and mixed-layer deepening relative to the critical depth for net production, could lead to a widespread decline in plankton abundance. r 2003 Elsevier Ltd. All rights reserved. 1. Introduction There is ample evidence of the ecological consequences of recent climate change (Gian-Reto et al., 2002; Walther et al., 2002). Much of this evidence comes from the terrestrial realm and lakes. Although the mechanisms linking terrestrial populations and ecosystems with climate change are thought to be understood, the trajectories are not. The population changes are generally thought to be forced by climate warming. The rate of this warming, since 1977, has increased (Climate Change, 2001) and more such change is expected. Climate-related ecosystem changes also have been reported in the world’s oceans, but, as opposed to the land, the intermediary mechanisms linking the two are, for the most part, speculative ARTICLE IN PRESS *Corresponding author. Tel.: +1-858-534-2074. E-mail address: jmcgowan@ucsd.edu (J.A. McGowan). 0967-0645/$-see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0967-0645(03)00135-8