A subsurface chlorophyll a bloom induced by typhoon in the South China Sea

• Typhoon can induce Chl-a bloom in the subsurface in the ocean.
• The Chl-a concentration is higher in the subsurface than that on the surface.
• The subsurface blooms last longer time than the surface Chl-a bloom.
• Typhoon causes upwelling to transport nutrients to the subsurface.
• The upwelled nutrients support the subsurface Chl-a bloom.

Previous studies showed that typhoons often induce chlorophyll a (Chl-a) blooms in the surface waters. This paper shows that Chl-a blooms can occur not only on the surface but also in the interior just above the thermocline after the passage of a typhoon. We used satellite and cruise survey data to analyze physical and biological characteristics in the South China Sea after the passage of the typhoon Nuri in August 2008. This paper shows that a subsurface (20 to 100 m depth) Chl-a bloom (1.31 ± 0.47 mg m− 3) occurred and lasted for three weeks, stronger and longer than the surface Chl-a bloom (0.48 ± 0.23 mg m− 3). The maximum value of Chl-a of 2.10 mg m− 3 was detected at 50 m depth. This value was approximately 4–5 times higher than the background value of 0.48 mg m− 3 measured at non blooming areas at the same time and about 7.5 times higher than the mean Chl-a value of 0.28 ± 0.13 mg m− 3 measured over the period of five years. The mixed layer depth and the thickness of the Chl-a bloom increased after the typhoon. Our analysis clearly shows that a subsurface upwelling caused by the passage of the typhoon, transported nutrients to the euphotic zone and supported the Chl-a bloom. These observations provide some insight on the effect of typhoons on marine ecosystems, especially as related to the Integrated Primary Production.