Copyright © The Oceanographic Society of Japan. Spatiotemporal Decreases of Nutrients and Chlorophyll-a in the Surface Mixed Layer of the Western North Pacific

Using time series of hydrographic data in the wintertime and summertime obtained along 137°E from 1971 to 2000, we found that the average contents of nutrients in the surface mixed layer showed linear decreasing trends of 0.001~0.004 µmol-PO4 l–1 yr–1 and 0.01~0.04 µmol-NO3 l–1 yr–1 with the decrease of density. The water column Chl-a (CHL) and the net community production (NCP) had also declined by 0.27~0.48 mg-Chl m–2 yr–1 and 0.08~0.47 g-C-NCP m–2 yr–1 with a clear oscillation of 20.8 ± 0.8 years. These changes showed a strong negative correlation with the Pacific Decadal Oscillation Index (PDO) with a time lag of 2 years (R = 0.89 ± 0.02). Considering the recent significant decrease of O2 over the North Pacific subsurface water, these find-ings suggest that the long-term decreasing trend of surface-deep water mixing has caused the decrease of marine biological activity in the surface mixed layer with a bidecadal oscillation over the western North Pacific. have suggested the variation patterns in biological com-ponents in the North Pacific due to the changing physical conditions associated with climatic regime shifts in 1976