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Jan 26, 2004 |
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Phytoplankton bloom in spring in seas around Japan |
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These images show the monthly mean chlorophyll a (*1) concentration contained in ocean phytoplankton from March to July 2003 in seas around Japan as observed by GLI. Red indicates a high concentration, and blue, a low concentration. GLI's continuous chlorophyll a observations capture phytoplankton bloom phenomenon clearly, from beginning to end. Ocean phytoplankton photosynthesis is active and increases explosively in the spring by taking up nutrients (*2) carried into the sea-surface layer during winter. High chlorophyll a concentration regions (red or yellow in images) are seen in the western part of Japan Sea, East China Sea and Yellow Sea in April. We can also see that they shift northwards to offshore Sanriku, the northern part of the Japan Sea, the sea around Hokkaido, and the Okhotsk Sea from May to June, coincident with the increase of solar radiation during spring and summer. In subtropical regions in the southern part of Japan, low chlorophyll a concentrations persist, and no phytoplankton bloom is seen (blue in images).
The phytoplankton bloom occurs is believed to be caused by the increase of solar radiation during spring and summer and by nutrients supplied to the sea surface, such as nitrates and phosphates. Ocean phytoplankton photosynthesis absorbs this sunlight and these nutrients. In lower latitudes, solar radiation increases earlier, so spring bloom occurs earlier and ends earlier because of the consumption of nutrients by photosynthesis. In contrast, in higher latitudes, solar radiation increases later than that in low latitudes, and spring blooms occur later. Mid-latitude regions around Japan known for their nutrient-rich oceans, and large spring blooms are seen in offshore Sanriku areas and the Okhotsk Sea. The subtropical region is also called an oligotrophic ocean because rapid increases in phytoplankton concentrations are not seen in spite of adequate solar radiations due to the lower nutrient levels and phytoplankton concentrations throughout the year.
Phytoplankton blooms greatly influence the annual primary production (the amount of carbon per unit time and area that phytoplankton takes up and fixes as carbon dioxide in sea water by photosynthesis). Monitoring the distributions of chlorophyll a concentrations is thus useful for clarifying carbon cycles in the ocean relevant to global warming. Monitoring is also useful for protecting and managing marine resources since phytoplankton plays a significant role in feeding marine commercial resources such as sardine and tuna through a marine food chain.
(*1) Chlorophyll-a exists in plants and plays an important role in photosynthesis. Chlorophyll includes types a, b, c, d, and e; of these, chlorophyll-a can be a good indicator of the amount of ocean phytoplankton because it exists in all phytoplankton in common, as well as in all algae and land plants.
(*2) These nutrients are supplied from deeper layers to the sea-surface layer by the influence of sea water mixing caused by the strong monsoonal winds in winter, upwelling caused by ocean-current along coastal regions, and river-flow.
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