Alexander, M., A. Capotondi, A. Miller, F. Chai, R. Brodeur and C. Deser, 2008:
Decadal variability in the Northeast Pacific in a physical-ecosystem model:
Role of mixed layer depth and trophic interactions
Journal of Geophysical Research, 113, C02017, doi:10.1029/2007JC004359.
A basin-wide interdecadal change in both the physical state and the ecology
of the North Pacific occurred near the end of 1976.
Here we use a physical-ecosystem model to examine whether changes in the physical
environment associated with the 1976-1977 transition influenced the lower
trophic levels of the food web and if so by what means.
The physical component is an ocean general circulation model, while
the biological component contains 10 compartments: two phytoplankton, two zooplankton,
two detritus pools, nitrate, ammonium, silicate, and carbon dioxide.
The model is forced with observed atmospheric fields during 1960-1999.
During spring, there is a ~40% reduction in plankton biomass in all four plankton
groups during 1977-1988 relative to 1970-1976 in the central Gulf of Alaska (GOA).
The epoch difference in plankton appears to be controlled by the mixed layer
depth. Enhanced Ekman pumping after 1976 caused the halocline to shoal, and
thus the mixed layer depth, which extends to the top of the
halocline in late winter, did not penetrate as deep in the central GOA. As a
result, more phytoplankton remained in the euphotic zone, and phytoplankton
biomass began to increase earlier in the year after the 1976 transition.
Zooplankton biomass also increased, but then grazing pressure led to a
strong decrease in phytoplankton by April followed by a drop in zooplankton
by May: Essentially, the mean seasonal cycle of plankton biomass
was shifted earlier in the year. As the seasonal
cycle progressed, the difference in plankton concentrations between
epochs reversed sign again, leading to slightly
greater zooplankton biomass during summer in the later epoch.