Seasonal variations in nutrients and biogenic particles in the upper and lower layers of East China Sea Shelf and their export to adjacent seas

Based on a three dimensional low-trophic ecosystem model, we revaluated budgets of nutrients and biogenic particles (phytoplankton and detritus) in the East China Sea (ECS), a continental shelf sea with high productivity that is affected by a large river (Changjiang River) and a western boundary current (Kuroshio). Following careful comparison of model results with available observation data, we calculated the monthly inventories of the nutrients and biogenic particles in the ECS and the fluxes of the nutrients and biogenic particles through the lateral and vertical interfaces of the ECS. As an improvement over previous nutrient budget calculations that treated the ECS as one box, we divided the water column into two layers: upper and lower layers corresponding to different light condition. Seasonal variations in inventories of nutrients and biogenic particles in both upper and lower layers are apparent but present different features, which are related to opposite contributions of biological processes and physical processes in the two layers. Our calculation shows the necessity of evaluating not only the horizontal fluxes of nutrients and biogenic particles into and out of the ECS through lateral boundaries with adjacent seas but also the exchange fluxes of nutrients and biogenic particles between the upper and lower layers because the latter is larger than the former. Our calculation also reveals two major export pathways of biogenic particles generated locally in the ECS: from the ECS to the Japan/East Sea through the Tsushima Strait and from the ECS to the Kuroshio region through the shelf break (200 in isobath). The export amount of biogenic particles is larger through the Tsushima Strait than through the shelf break in the water column. The depth for the biogenic particles exporting through the shelf break of the ECS is the middle layer (from -60 m to -160 m).