Carbon, alkalinity and nutrient budgets on the East China Sea continental shelf

Abstract

The western part of the Kuroshio Intermediate Water originates from the nutrient‐rich South China Sea intermediate water which upwells onto the East China Sea (ECS) continental shelf and contributes a large amount of nutrients to the ECS. This contribution, especially for phosphorus, is far more than the inputs from the phosphorus‐deficient Yangtze and Yellow Rivers. Mass balance calculations reveal that the net denitrification rate in the ECS shelf is 95 ± 50 × 109 mol N yr−1, which is more than the total riverine supply of nitrogen. The nutrient supply allows the ECS to produce 1030 ± 310 × 109 moles dissolved organic carbon and 709 ± 280 × 109 mol particulate organic carbon for a total of 1739 ± 870 × 109 mol organic carbon per year, which translates to a new production rate of phytoplankton as 64 ± 32 mg C m−2 d−1 or 15% of the average primary production rate. The downslope transport of modern particulate carbon is 2547 ± 1250 × 109 mol yr−1, of which 695 ± 350 × 109 mol yr−1 is organic. This value is only 12% of the offshore dissolved organic carbon transport of 5563 ± 1700 × 109 mol yr−1. The alkalinity budget, never attempted before, indicates that there is a substantial amount of alkalinity generation in the sediments (3.9 ± 3.9 mmol m−2 d−1) due to anaerobic respiration such as by iron and sulfate reductions.