McGowan, J. A., S. J. Bograd, R. J. Lynn and A. J. Miller, 2003:
The biological response to the 1977 regime shift
in the California Current.
Deep-Sea Research, 50, 2567-2582.
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.