Radium isotope tracing reveals that submarine groundwater discharge is a key influencing factor for blue carbon balance and mangrove carbon storage in the southern waters of Jinhai Bay

Jinhai Bay, situated in the coastal zone of Guangxi, southern China (21°23′N–21°39′N, 109°17′E–109°29′E). The objective of this study is to evaluate how submarine groundwater discharge (SGD) impacts the blue carbon balance in Jinhai Bay. An innovative equation has been formulated to quantify dissolved carbon loss in mangroves caused by SGD, incorporating mangrove area, and the rates of SGD and recirculated submarine groundwater discharge (RSGD), along with the variance in dissolved carbon levels between groundwater and coastal seawater to determine the net carbon loss flow. This study compares results with traditional methods to assess how SGD-driven carbon loss affects the accuracy of mangrove carbon storage estimates. New hydrological insight for the region: The SGD rate in the dry season of Jinhai Bay was 5.80 cm/d. Dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) flux of SGD were 8.39 × 10 6 mol/d and 1.18 × 10 7 mol/d, which accounted for 50% and 69% of the total DIC and DOC sources in the study area, respectively. Additionally, the net dissolved carbon flux loss from Jinhai Bay mangrove ecosystem caused by SGD was estimated at 2.44 × 10 5 mol/d, representing 43% of mangrove primary productivity and 75% of sediment carbon burial flux. This study highlights that ignoring SGD-driven carbon loss overestimates mangrove carbon storage, emphasizing the necessity of integrating SGD into coastal blue carbon budget and mangrove carbon storage assessments. • Organic and inorganic carbon via SGD are the primary carbon sources in Jinhai Bay. • Mangrove dissolved carbon loss driven by SGD can be quantified by the novel formula. • SGD is a key factor for accurately assessing mangrove carbon storage.