Unveiling the dominance of submarine groundwater discharge on nutrient sources in the Eastern China Marginal Seas.

• High-resolution coupled 226Ra and 228Ra mass balance model enables an SGD estimate. • SGD flux exceeds rivers by 3.4 times in large marginal seas. • Nutrient fluxes via SGD surpasses rivers and atmospheric deposition. • High N:P ratio SGD reveals the necessity in managing coastal groundwater quality. River and atmosphere are traditionally recognized as the primary nutrient sources impacting coastal ecosystems. Despite the increasing attention towards the often-neglected submarine groundwater discharge (SGD), its understanding and significance in highly human-impacted marginal seas remain limited. This study utilizes unprecedented high-resolution data (561 seawater and 282 groundwater radium samples) to provide precise estimates of 226Ra and 228Ra sources and sinks in the Eastern China Marginal Seas. A coupled 226Ra and 228Ra mass balance model enable an integrated SGD flux of (3.7 ± 2.4) × 1012 m3 yr−1, surpassing rivers by 3.4 times. Furthermore, nutrient delivery from SGD exceeds riverine and atmospheric inputs, potentially inducing substantial changes in coastal nutrient cycles. These alterations have profound implications for primary production and biological communities, deviating significantly from the Redfield ratio. Therefore, comprehending the significance of SGD in nutrient budgets is vital for a comprehensive understanding of biogeochemical dynamics and functionality of marginal sea ecosystems. [Display omitted] [ABSTRACT FROM AUTHOR]