Since 1951, the biomass of macrozooplankton in waters off southern Cal
ifornia has decreased by 80 percent. During the same period, the surfa
ce layer warmed-by more than 1.5 degrees C in some places-and the temp
erature difference across the thermocline increased. Increased stratif
ication resulted in less lifting of the thermocline by wind-driven upw
elling. A shallower source of upwelled waters provided less inorganic
nutrient for new biological production and hence supported a smaller z
ooplankton population. Continued warming could lead to further decline
of zooplankton.