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 (522KB)
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36,753,000 Yen (FY1998; 1 1,265,000 Yen)
To understand effects of pollutant loading from the Changjiang River on the estuarine plankton ecosystem, phosphorous- and oil-enrichment mesocosm experiments were carried out in the Changjiang estuary. A floating mesoeosm system was developed that could endure the strong tidal currents and waves of over I m in the Changjiang estuary. The mixing conditions in the mesocosm water mass were determined using a dye (Rhodamine WT) . Wave heights from 30 to 60 cm resulted in a well-mixed water mass (vertical diffusion coefficient ca. 11cm2 sec-1) in the mesocosm. The phosphorous enrichment experiments in the Changjiang estuary in autumn and spring revealed that phytoplankton blooms could be easily raised by the addition of only phosphorous to the estuary. The phytoplankton that formed the blooms were diatoms (autumn) and dinoflagellates (spring). Since silica was rich in both seasons, there seems to be a factor other than silica concentration in the selection of the dominant phytoplankton species. It was also shown that control of phytoplankton blooms by zooplankton grazing was more effective for diatoms than for dinoflagellates. The oil enrichment experiment using water-soluble parts of fuel oil showed that photosynthetic activity was clearly affected by the oil. Ciliates and noctiluca drastically decreased after the addition of oil, showing that they were more sensitive to the oil than the other organisms in the mesocosm. This experiment suggests that oil pollution may have a direct impact on primary productivity in the Changjiang Estuary.
Changjiang Estuary, mesocosm, marine ecosystem, nutrients, oil pollution