Deep-Sea Research II 49 (2002) 5487–5512 Seasonal variability in the lower trophic level environments of the western subtropical Pacific and Oyashio Waters—a retrospective study Atsamon Limsakul a, *, Toshiro Saino a,b , Joaquim I. Goes c , Takashi Midorikawa d a Hydrospheric Atmospheric Research Center, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan b Frontier Research System for Global Change, 3173-25 Showa-cho, Kanazawa-ku, Yokohama 236-0001, Japan c Bigelow Laboratory for Ocean Sciences, P.O. Box 475, West Boothbay Harbor, ME 04575, USA d Japan Meteorological Agency, 1-3-4 Olemachi, Chiyoda-ku, Tokyo 100-8122, Japan Received 31 January 2001; received in revised form 11 June 2001; accepted 20 June 2001 Abstract 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, andmeridionaldifferencesintheamplitudesoftheannualcyclesofwindstressandsolarirradianceinthetworegions. 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, intheOyashioWater,largewinternutrientreplenishmentisfollowedbymarkedspringincreasesinphytoplanktonand 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 differenceobservedinthetwowatermassesisthattheN:PratiosintheSubtropicalWaterexceedtheRedfieldvaluein allseasons,withhighestN:Pcenteredaround24.5–25.5s t inthecoreoftheNorthPacificSubtropicalModeWater.We speculate that N 2 fixation may be an important source of new nitrogen in the Subtropical Water. r 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction Ocean circulation, driven initially by the differ- ence in solar heating between low and high *Corresponding author. Tel.: +81-52-789-3443; fax: +81- 52-789-3436. E-mail address: atsa@ihas.nagoya-u.ac.jp (A. Limsakul). 0967-0645/02/$-see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S0967-0645(02)00208-4