Summer nutrient dynamics and biological carbon uptake rate and their indications to hypoxia in the Changjiang River Estuary

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. As inferred from the similar model from another cruise in August 2009, the contributions of ODW to DIN and silicate inventories in the euphotic zone were 18.2% and 28.1%, respectively, much lower than that of CDW. However, 37.6% and 39.1% of the phosphate inventory were from ODW and CDW, respectively, indicating that ODW phosphate contribution was comparable with that of CDW in the euphotic zone, while DIN and silicate brought by ODW were minor. The nutrients uptake ratio was estimated. Biological carbon uptake rate was evaluated using a simple box model in the euphotic zone based on nutrient deviation, Redfield ratio, 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; 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, P/R ratio of 1.98 indicated the Changjiang Estuary within euphotic zone would be ~50% autotrophic. It would take ~8 d to drive O₂ concentration from 7 to 2 mg/L (Hypoxia)　within 1 m thickness aphotic water layer when primary produced carbon was transported to aphotic layer and all respired under stable hydrodynamic conditions.