Resilience of submarine groundwater discharge to extreme drought in a large river-dominated estuary: A case study of the Changjiang Estuary.

Submarine groundwater discharge (SGD) is a major chemical source to coastal ecosystems due to its large, nutrient-rich fluxes. However, its response to extreme drought remains unclear. Herein we investigated SGD in the Changjiang Estuary (CJE) during the 2022 extreme drought using radium mass balance (226Ra, 228Ra) and three end-member mixing models. These two different models yielded consistent estimates, with an average SGD flux of (1.5 ± 0.7) × 109 m3 d-1 (13.6 ± 6.4 cm d-1) for the CJE during the extreme drought. This flux was comparable to the range observed during normal dry season levels (18.4-44.0 cm d-1) but 4.4-22.7 times higher than the wet-season fluxes (0.6-3.1 cm d-1), demonstrating substantial resilience under drought conditions. During the extreme drought, SGD and riverine input were the dominant source of nutrients in the CJE, contributing approximately 96% of the total DIN and DIP fluxes. The large amount of dissolved DIN ((2.5 ± 1.9) × 108 mol d-1) and DIP ((2.0 ± 1.4) × 106 mol d-1) fluxes together with high N: P ratios (178:1) into the CJE would potentially exacerbate eutrophication, hypoxia, and harmful algal blooms.