South China Sea Annual Meeting 2023 (SCSAM)

44 / 2023-07-24 15:08:04

Typhoon and storm-induced phytoplankton responses revealed by BGC-Argo

Typhoon, Phytoplankton, BGC-Argo

Understanding the responses of phytoplankton to extreme weather events is crucial for comprehending marine biogeochemical cycles and biological pump processes. While traditional studies have primarily relied on sea-surface Chlorophyll-a concentration observed by satellites, this approach has limitations. Satellites can only capture information at the sea surface, missing vital subsurface phytoplankton data. Moreover, cloud cover during typhoons and storms hampers ocean-color satellite observations, leading to discontinuous and biased data on clear days. To address these shortcomings, the BGC-Argo float has emerged as a valuable tool. This technology allows for all-weather and depth-resolved continuous observation, enabling comprehensive recording of phytoplankton responses to typhoons and storms in both temporal and vertical dimensions. In several observations conducted in the South China Sea and Northwest Pacific, two previously neglected key processes have been identified. The first process is known as the particle-entrainment effect, wherein strong wind-driven mixing and upwelling lead to a vertical redistribution of particles, particularly the entrainment of subsurface particles into the mixed layer. This effect can increase sea-surface chlorophyll-a concentrations without altering the depth-integrated phytoplankton biomass and pigment, resulting in an overestimation of surface-based observations. The second process is referred to as the photoacclimation effect, characterized by rapid pigment density changes in phytoplankton cells, particularly a fast increase in chlorophyll-a levels with low or unchanged carbon biomass. The photoacclimation effect is primarily driven by strong mixing and extremely low sunlight conditions during typhoons and storms, and it further amplifies phytoplankton responses when analyzing data based on chlorophyll levels. To accurately assess phytoplankton responses to extreme weather events, it is essential to differentiate and account for the contributions of different effects. By doing so, we can re-evaluate and improve our understanding of how typhoons and storms impact marine ecosystems and biogeochemical processes.