Seasonal variability in the lower trophic level environments of the western subtropical Pacific and Oyashio Waters - a retrospective study

The seasonal cycles of phytoplankton and mesozooplankton and their environments in the western subtropical Pacific and Oyashio Waters have been compared and described, using all available historical oceanographic and atmospheric data from the regions. Remarkable differences exist in the seasonal cycles of water properties in the upper layers of these two regimes, primarily due to fundamental dissimilarities in the hydrographic and nutrient structures, and meridional differences in the amplitudes of the annual cycles of wind stress and solar irradiance in the two regions. In the Subtropical Water, seasonal variations in phytoplankton and mesozooplankton biomass are constrained within narrow bounds, because of semi-permanent stratification and downwelling typically associated with the subtropical anticyclonic gyres. A prominent feature here is that near-surface chlorophyll a concentrations increase during the deepening of the winter mixed layer when nutrient supply is greatest. This indicates that phytoplankton populations rapidly respond to higher nutrient flux during this time, since light levels are high enough for their growth. In contrast, in the Oyashio Water, large winter nutrient replenishment is followed by marked spring increases in phytoplankton and mesozooplankton biomass, initiated primarily by water-column stabilization following an increase in solar irradiance and shallowing of the mixed layer. An outstanding characteristic of the Oyashio Water is that the remarkable coincidence of the spring peak of mesozooplankton biomass with that of phytoplankton. From this it follows that newly recruited young copepodites arriving at the surface layer are capable of taking advantage of an abundant supply of food during the spring phytoplankton bloom to grow and build up a large standing stock. Another conspicuous difference observed in the two water masses is that the N:P ratios in the Subtropical Water exceed the Redfield value in all seasons, with highest N:P centered around 24.5-25.5sigma(1) in the core of the North Pacific Subtropical Mode Water. We speculate that N-2 fixation may be an important source of new nitrogen in the Subtropical Water. (C) 2002 Elsevier Science Ltd. All rights reserved.