Impact of Submesoscale Vertical Advection on Primary Productivity in the Southern East China Sea

This study aims to investigate the impact of submesoscale vertical advection (SVA) on the primary productivity in the southern East China Sea. The analysis is based on a comparison between two numerical simulations by using a three-dimensional coupled physical-biogeochemical model. One simulation directly resolves SVA on a high-resolution mesh, and the other leaves SVA unresolved on a low-resolution mesh. The high-resolution simulation outperforms the low-resolution simulation in reproducing the observed chlorophyll distribution, particularly in summer. Resolving SVA results in an approximately 40% increase in primary productivity during the summer, though SVA activity is relatively weak in this season than in other seasons. Among multiscale physical processes, SVA, rather than mixing, is found to be the most important vertical nutrient supply pathway from the nutrient-rich bottom water to the nutrient-depleted surface water in summer, particularly on the middle and outer shelves. The impact of SVA on the shelf is unique compared to the open ocean in that it efficiently enhances vertical supply of nutrient-rich subsurface waters to the nutrient-depleted surface layer. This study highlights the importance of SVA in promoting primary productivity in stratified shelf seas. Plain Language Summary The southern East China Sea, located on a wide shelf, holds several world-famous fishing grounds. Algal growth plays a fundamental role in the marine food web. Nutrient elements such as nitrogen and phosphorus are crucial for algal growth. In summer, the stable water column obstructs nutrient-rich deep water from mixing with the nutrient-poor surface water. However, scientists found that algal growth in summer remains high. A question arises: How are nutrients supplied to the surface water? This study focuses on how small-scale (1-10 km) and fast-changing (days to weeks) ocean currents affect algal growth. Computer-based models can simulate the interaction between multiple-scale ocean currents and nutrients, algae, and small animals. In our model, the small-scale vertical transport forms a significant vertical nutrient supply pathway in the southern East China Sea, particularly in summer. Therefore, in layered shelf seas, the small-scale vertical transport may become the predominant vertical nutrient transport pathway, thus supporting strong algal growth. This finding contributes to better understanding the impact small-scale physical processes have on the marine ecosystem. Key Points Accuracy in modeling chlorophyll is improved by resolving submesoscale vertical advection (SVA) Relatively weak SVA in summer exerts a remarkably positive impact on primary productivity Transport of nutrients to the euphotic layer by SVA is crucial for high primary productivity in a stratified shelf sea