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.

Seawater temperature, conductivity, and pressure were recorded throughout the water column with a CTD (SBE 9 plus, Seabird Electronics Inc., USA). Photosynthetically active radiation (PAR) was measured throughout the water column using an irradiance sensor (4π; QSP-200 L).

Water samples were collected using Teflon-coated Go-Flo bottles (20 L, General Oceanics Inc., USA) mounted on a General Oceanic Rosette® assembly (Model 10151212, General Oceanics Inc.). Seawater at each station was taken at six depths at intervals of 3 to 25 m depending on the water column depth. Subsamples were taken immediately for further analysis [i.e., dissolved oxygen (DO), nutrients, particulate organic carbon (POC), and chlorophyll a (Chl a)]. The analyzed methods for each variable can be found from the paper. The data has been processed by a linear regression analysis using “SigmaStat” to produce a ms accepted for publication in Marine Pollution Bulletin.

The readme file contains an explanation of each of the variables in the dataset, its measurement units. Information on how the measurements were done can be found in the associated manuscript referenced above.