Decadal and basin-scale variation in mixed layer depth and the impact on biological production in the Central and North Pacific, 1960-88

Changes in winter and spring mixed Layer depths in the North Pacific on decadal and basin scales affect biological production. In the subtropical and transition zones these depths were 30-80% greater during 1977-88 than during 1960-76; in the subarctic zone they were 20-30% shallower. We attribute these changes to an intensification of the Aleutian Low Pressure System. A deeper mixed layer might increase phytoplankton production in nutrient-poor regions by supplying more deep nutrients; it might decrease production in light-poor regions by mixing cells into darker water. A plankton population dynamics model suggests that a deeper subtropical mixed layer and a shallower subarctic mixed layer both would increase primary and secondary production by about 50%, and these increases were found not to be very sensitive to model parameter values; in the transition zone, however, the predicted change in production was smaller and more sensitive to changes in model parameters. Increases in higher tropic levels have been observed in subtropical and subarctic zones during 1977-88. This is consistent with model results and the idea that the subtropical zone is nutrient-poor, the subarctic zone is light-poor, and the transition zone is not consistently limited by any one thing. Further, our results show changes in mixed layer depths occur on decadal and basin scales and may be an important mechanism Linking variation in the atmosphere and oceanic ecosystem productivity.