Winter phytoplankton blooms in the shallow mixed layer of the South China Sea enhanced by upwelling

Chung Chi Chen, Fuh Kwo Shiah, Shi Wei Chung, Kon Kee Liu

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Abstract

The South China Sea (SCS) is the largest marginal sea in the world. Previous studies, including recent intensive paleo-oceanographic studies, suggest that the SCS is sensitive to many types of physical forcing on the short-term (e.g., internal waves and tides, mesoscale eddies, typhoons, etc.), annual (e.g., monsoon), inter-annual (e.g., El Niño), and very long-term (e.g., climate change) time scales. To better understand how various types of physical forcing influence biogeochemical cycles in the water column, a time-series study was initiated. Bimonthly hydrographic surveys occupied stations in the subtropical-tropic SCS at 19°N, 118.5°E. Results suggest that the Southeast Asian monsoons, northeasterly from October to April and southwesterly from May to September, have important effects on biogeochemical cycles in the upper water column. Hydrographic data showed that the mixed layer depth was much shallower in winter than in other seasons. During the winter monsoon period, the nitricline became shallower and upwelling sustained an elevated phytoplankton standing stock. Mean chlorophyll concentrations (0.65 mg Chl m- 3) in winter were 8 times higher than in summer, and the integrated primary productivity over the euphotic zone reached as high as ca. 684 mg C m- 2 day- 1 in winter. The upwelling is produced by convergence of currents in the cyclonic gyre near the Luzon Strait, where the Kuroshio intrudes. In summer the current reverses following the wind change. The nitricline is depressed as downwelling occurs off northwest Luzon, resulting in strong nutrient limitation and very low chlorophyll concentrations.

Original languageEnglish
Pages (from-to)97-110
Number of pages14
JournalJournal of Marine Systems
Volume59
Issue number1-2
DOIs
Publication statusPublished - 2006 Jan 1

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Keywords

  • Dissolved inorganic nutrient
  • Phytoplankton bloom
  • South China Sea
  • Time-series study
  • Upwelling

ASJC Scopus subject areas

  • Oceanography
  • Ecology, Evolution, Behavior and Systematics
  • Aquatic Science

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