Biological and Nutrient Responses to a Typhoon in the Yangtze Estuary and the Adjacent Sea

Typhoon Haikui passed through the Yangtze River estuary and the adjacent sea during the summer of 2012. Three field surveys were conducted before and after the passage of this typhoon. The effects of Haikui in this area on nutrient levels and biological production were studied by combining model data and several satellite observations. During the typhoon's passage, the water column was affected mainly by vertical mixing, which was caused by the strong wind. As the wind subsided, terrestrial runoff and offshore water intrusion became more important for changing the physical and chemical parameters. The Yangtze Diluted Water extended into the upper layer by 4 days after Haikui's landfall, largely because of the heavy typhoon-associated precipitation and considerably increased surface concentrations of dissolved inorganic nitrogen, silicate, and phosphate (by 53%, 27%, and 97%, respectively). Meanwhile, the strong onshore wind induced a strong intrusion of high-salinity offshore water in the bottom layer, and this process somewhat decreased the bottom nutrient concentrations (except for phosphate) after the typhoon. The changes in nutrient concentrations in the middle layer were quite complex because of the multiple effects of the processes described above. The increased nutrient concentrations, particularly those of phosphate, greatly promoted the growth of the upper phytoplankton in the offshore areas, although the phytoplankton biomass was found to decline rapidly a few days after the typhoon had passed. The results of this study suggest that Haikui had a profound influence on water circulation and on the chemical and biological responses in the Yangtze River estuary and its adjacent sea.