Evaluation of flushing time, groundwater discharge and associated nutrient fluxes in Daya Bay, China

Abstract. Radium quartet have been widely used to quantify the flushing time of water body and submarine groundwater discharge (SGD) in coastal zones. However, previous apparent age model based on mass balance of radium isotopes usually neglected the effects of rivers, open sea water end-member, sedimentary input, atmospheric deposits and recirculated seawater (RSGD). To enhance accuracy in estimating flushing time and SGD, here we present an improved model and then apply in Daya Bay, China. The flushing time estimated by the improved model is 11.8–27.7 d in Daya Bay. It is found that the previous model overestimated the flushing time by 10.7 %–103 %. Considering the radium losses caused by RSGD, the SGD flux is estimated to be (3.87–5.09) × 107 m³ d⁻¹ based on the derived flushing time. The SGD associated nutrient fluxes are estimated to be (1.36–1.76) × 106 mol d⁻¹ and (2.53–3.26) × 104 mol d⁻¹ for DIN and DIP, respectively, about 20 times greater than those from local rivers. The primary production supported by all the external DIN inputs is determined to be 323–390 mg C m⁻² d⁻¹, in which SGD provide approximately 73.1 % of total primary production. Our results reveals that SGD plays an important role in nutrient balance and may be responsible for the frequent outburst of red tides in Daya Bay. The present study provides baseline data for evaluating environmental effects in Daya Bay and similar coastal bay systems elsewhere.