Characteristics and Mechanisms of Typhoon-Induced Decomposition of Organic Matter and Its Implication for Climate Change

We recently reported that decomposition (as a carbon source) of organic matter (OM) is the dominant process in coastal waters after typhoons, which is contrary to phytoplankton blooms (as a carbon sink) in previous studies. However, the characteristics and mechanisms of typhoon-induced decomposition and the question whether the decomposition mainly decompose particulate OM (POM) or dissolved OM (DOM) are still unclear. To address these issues, physicochemical parameters and multiple isotopes in the northern South China Sea were investigated before and after Typhoon Merbok (2017). After the typhoon, the chlorophyll-a (Chl-a) level decreased (25%), whereas the upwelled bottom/sedimentary OM (SOM) increased in the nearshore due to the severe disturbance caused by the typhoon. In contrast, the Chl-a level increased (over 3 times) offshore. Nevertheless, a positive apparent oxygen utilization value occurred in both nearshore and offshore, suggesting that decomposition of OM was the dominant biogeochemical process regardless of whether phytoplankton blooms occurred after the typhoon. The decomposed POM in the nearshore was mainly from the SOM, while the offshore was primarily from locally produced fresh phytoplankton. The decomposition of OM contributed to 66% of the total oxygen consumption in the nearshore, while it contributed to 36% (dominated by the decomposition of POM, accounting for 25%) in the offshore. This study suggests that typhoon-induced decomposition might be dominated by POM, which is not conducive to the storage of OM in sediments. It means that the capacity of sediments as a carbon sink will be weakened under global warming (increasing typhoon events).