Impact of upwelling on phytoplankton blooms and hypoxia along the Chinese coast in the East China Sea

This study evaluates the rarely observed phenomenon of the simultaneous
occurrences of phytoplankton blooms, hypoxia, and upwelling along the
Zhejiang coast in the East China Sea. Results show that the upwelling
uplifted bottom water to 5–10 m below the surface. In the upwelling
region, phytoplankton blooms (Chl a = 10.9 μg L−1) occurred and hypoxia or
low-oxygen appeared below the surface water. High concentrations of
nitrate and phosphate were regenerated in the hypoxic regions,
corresponding with mean values (± SD) of 16.9 (± 1.5) and 0.90 (± 0.14)
μM, respectively. The upwelling expanded the region of hypoxic water,
which nearly reached the surface, thereby increasing the threat to marine
life. In addition to fluvial nutrients, the upwelling of water with high
nutrient levels, especially phosphates, can enhance phytoplankton blooms.
The results suggest that hypoxia can become more severe due to further
decomposition of bloom-derived organic matter after blooms crash.