Nutrient-rich submarine groundwater discharge fuels the largest green tide in the world

One of the largest green tide (Ulva prolifera) outbreaks in the world has occurred every year from 2007 to present in the Southern Yellow Sea, China. Currently, the coastal area around Jiangsu Province (Subei Shoal region) is thought to be the origination point of these giant green tide blooms. The combination of high nutrient demand but low river discharge and other inputs suggests that there is a significant flux of submarine groundwater discharge (SGD) in this area. By using a radiummass balance model, we estimated the SGD flux in the area to be (0.7-1.4) x 10(9) m(3) d(-1) (6.1-12 cm d(-1)), at the high end of SGD fluxesworldwide. Geographically, Subei Shoal is less than 5% of the entire Southern Yellow Sea area, while our calculated SGD flux just for the shoal area is similar to 3 times larger than previously documented for the whole Southern Yellow Sea. Therefore, Subei Shoal may be considered a SGD hotspot that plays an important role in SGD associated material fluxes. Compared to inputs from local rivers, atmospheric deposition, and anthropogenic activities, SGD-derived nutrients are the main source term that can support the growth of macroalgae. We specifically highlight that this type of areas that are shallow, intensively mixed, anthropogenically polluted, sandy or muddy with heavy bio-irrigation, may have a higher risk of suffering harmful ecological problems, even with limited terrestrial runoff. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The research indicates that Subei Shoal in the Southern Yellow Sea of China may be the epicenter of the world's largest green tide outbreaks, which are linked to high submarine groundwater discharge in the area. Submarine groundwater discharge is a key feature of shallow coastal waters in this region. Nutrients derived from SGD are crucial for the growth of macroalgae and may outweigh other nutrient sources. Specific types of areas, such as sandy or muddy areas with pollution and heavy bio-irrigation, could face significant ecological challenges even with limited terrestrial runoff.