Temporal variations in lower trophic level biological environments in the northwestern North Pacific Subtropical Gyre from 1950 to 1997

An examination of large archives (1950-1997) of the oceanographic and atmos pheric data from the northwestern North Pacific Subtropical Gyre has reveal ed clear linkages between atmospheric forcing factors, physical processes a nd biological events. Large changes in the winter and spring biomass of phy toplankton and macroplankton observed over annual, decadal and inter-decada l time scales could clearly be attributed to climate-related changes in oce anographic processes. Interannual changes in the intensity of the winter-ti me East Asian Monsoon had a significant impact on the extent of convective overturning, on nitrate inputs into the euphotic zone and the concentration s of chlorophyll a in winter and during the following spring. A prolonged p eriod of deeper winter mixed layers observed from the mid-1970s to the mid- 1980s led to a sizeable increase in winter mixed-layer nitrate concentratio ns. This change resulted in a decrease in winter-time phytoplankton biomass . Spring-time chlorophyll a, in contrast, showed a steady increase during t his period. The decline in winter phytoplankton biomass could be attributed to the depths of mixed layer. A deeper mixed layer prevents phytoplankton from remaining in the euphotic zone for long enough to photosynthesize and grow, leaving substantial amounts of nutrients unutilised. However, as a re sult of stratification of the water column in spring following each of thes e winters, phytoplankton could take advantage of the enhanced ambient conce ntrations of nutrients and increase its biomass. Another noteworthy observa tion for the period from the mid- 1970s to the early 1980s is that the west ern subtropical gyre progressively became phosphate limited. The period of diminishing mixed-layer phosphate concentrations was observed in our study area from the early 1990s onwards was consistent with recent observations a t Station ALOHA in the eastern subtropical gyre. (C) 2001 Elsevier Science Ltd. All rights reserved.