Summer nutrient dynamics and biological carbon uptake rate in the Changjiang River plume inferred using a three end-member mixing model

• Nutrients distributions indicate three end-member mixing in the estuary.
• High Chl a area is featured with high DIN/P ratio and excess O2.
• A box model is used to evaluate nutrients deviation from conservative mixing.
• We calculate end-member composite and nutrient uptake ratio with the box model.
• We estimate biological carbon uptake rate within the depth of PZD10.

A three end-member (Changjiang River Plume, CRP; Outer-shelf Surface Water, OSW; and Outer-shelf Deep Water, ODW) mixing model based on quasi-conservative temperature and salinity was adopted to identify the relative contribution of different water masses to nutrient inventory and to estimate biological nutrient uptake in the plume-impacted area by considering the difference between the model-predicted and ambient concentration of nutrients up to a depth of PZD10 (photic zone depth to 10% of surface photosynthetically active radiance (PAR)). The end-member composite suggested that the mixing of CRP and OSW was the main process regulating nutrient dynamics and phytoplankton growth, while the correlation of ODW with ΔP indicated that the outcropped upwelling water contributed to the replenishment of P leading to Chl a accumulation to some extent. The ratio ΔDIN:ΔP:ΔSi:excess O2 in the euphotic zone where excess O2>10 μmol L−1 was estimated to be 28±9:1:33±13:145±71.A simple box model was used to evaluate biological carbon uptake rate in the euphotic zone based on nutrient deviation, Redfield ratio (6.6 C:1 DIN), and residence time of nutrients, assuming that the Changjiang River was the unique source of nutrients in the quasi-static box. The biological carbon uptake rates derived from the DIN, P and Si deviation were 465, 344, and 626  mg C m−2 d−1, respectively; these values were comparable to the POC flux (486±275 mg C m−2 d−1) derived from sediment trap. This finding suggested that the Changjiang River Plume was responsible for phytoplankton growth and subsequent high POC flux out of the euphotic zone. Furthermore, the community respiration rate was estimated to be 634 mg C m−2 d−1 based on the integrated 14C-based gross primary production of 1260 mg C m−2 d−1 and the net community production of 626 mg C m−2 d−1 in the euphotic zone of the region.