Marine phytoplankton biomass responses to typhoon events in the South China Sea based on physical-biogeochemical model

- The biogeochemical phytoplankton responses to all 79 typhoon events that affected the SCS during 2000-2009 was investigated.
- An increased nitrate concentration is the basic and key precondition for phytoplankton blooming in the oligotrophic SCS.
- Typhoon intensity, and translation speed control the upward flux of nitrates together, and the latter is more effect.

Most previous studies confirmed that marine phytoplankton biomass can increased dramatically and then formed blooms along the typhoon track after typhoon passage. However, many events of no significant responses to typhoons were neglected. In order to figure out the most important factor for bloom formation, a coupled physical-biogeochemical model was used to study the biogeochemical phytoplankton responses to all 79 typhoon events that affected the South China Sea (SCS) during 2000-2009. The major factors investigated included typhoon intensity and translation speed, Chlorophyll a concentration (Chl-a) and vertical nitrate transport in the euphotic zone before and after typhoon passage. The results revealed that phytoplankton blooms were triggered after 43 typhoon events, but no significant blooms were found after 36 other typhoons. Of the 43 typhoon events that triggered blooms, 24 were in the open ocean and 19 were on the coast. Subsurface blooms were detected after five typhoon events that did not trigger surface blooms. Over half of the typhoons that affect the oligotrophic SCS can trigger phytoplankton blooms, and contribute to the marine primary productivity. The mechanism of the above results were surveyed, we found that (1) an increased nitrate concentration is the basic and key precondition for phytoplankton blooming in the oligotrophic SCS; (2) typhoon intensity, and translation speed control the upward flux of nitrates together, and translation speed has more effect than intensity; (3) uplifted nitrates could trigger phytoplankton bloom, and Chl-a levels reached a peak 3 days later than nitrate levels; (4) mesoscale eddies and the nutricline depth before a typhoon's arrival also affects bloom genesis; and (5) the composition of phytoplankton functional groups in the coast was adjusted by typhoon, which have more complex mechanism of bloom formation than that in the open ocean. In summary, the physical driving force that modulates blooms is vertical nutrient transportation in the SCS.