Seasonal variations in nutrients and chlorophyll-a concentrations in the Northern east China sea |
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Authors: | Dongseon Kim JeongHee Shim Sinjae Yoo |
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Institution: | (1) Marine Environment Research Department, KORDI, Ansan pbP.O. Box 29, 425-600 Seoul, Korea |
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Abstract: | Nutrients, chlorophyll-a, particulate organic carbon (POC), and environmental conditions were extensively investigated in
the northern East China Sea (ECS) near Cheju Island during three seasonal cruises from 2003 to 2005. In spring and autumn,
relatively high concentrations of nitrate (2.6~12.4 μmol kg-1) and phosphate (0.17~0.61 μmol kg-1) were observed in the surface waters in the western part of the study area because of the large supply of nutrients from
deep waters by vertical mixing. The surface concentrations of nitrate and phosphate in summer were much lower than those in
spring and autumn, which is ascribed to a reduced nutrient supply from the deep waters in summer because of surface layer
stratification. While previous studies indicate that upwellings of the Kuroshio Current and the Changjiang (Yangtze River)
are main sources of nutrients in the ECS, these two inputs seem not to have contributed significantly to the build-up of nutrients
in the northern ECS during the time of this study. The lower nitrate:phosphate (N:P) ratio in the surface waters and the positive
correlation between the surface N:P ratio and nitrate concentration indicate that nitrate acts as a main nutrient limiting
phytoplankton growth in the northern ECS, contrary to previous reports of phosphate-limited phytoplankton growth in the ECS.
This difference arises because most surface water nutrients are supplied by vertical mixing from deep waters with low N:P
ratios and are not directly influenced by the Changjiang, which has a high N:P ratio. Surface chlorophyll-a levels showed
large seasonal variation, with high concentrations (0.38~4.14 mg m-3) in spring and autumn and low concentrations (0.22~1.05 mg m-3) in summer. The surface distribution of chlorophyll-a coincided fairly well with that of nitrate in the northern ECS, implying
that nitrate is an important nutrient controlling phytoplankton biomass. The POC:chlorophyll-a ratio was 4~6 times higher
in summer than in spring and autumn, presumably because of the high summer phytoplankton death rate caused by nutrient depletion
in the surface waters. |
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Keywords: | nutrients chlorophyll-a POC seasonal variations East China Sea |
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