Impacts of an extreme Changjiang flood on variations in carbon cycle components in the Changjiang Estuary and adjacent East China sea

River flooding is expected to increase in frequency and severity under climate change. However, the impact of extreme river flooding on the coastal carbon cycle has rarely been studied. A severe Changjiang flood occurred in the summer of 2020, which was the largest Changjiang flood in the last 20 years since 2000. This extreme flood resulted in the export of great amounts of nitrate (6.4 × 10⁸ mol d⁻¹), silicate (7.1 × 10⁸ mol d⁻¹), phosphate (5.1 × 10⁶ mol d⁻¹), dissolved organic carbon (DOC; 13.9 × 10⁹ g C d⁻¹), and inorganic carbon (DIC; 145.5 × 10⁹ g C d⁻¹) from Changjiang to the Changjiang Estuary, which were considerably higher (by ∼8–178%) compared to those observed in previous summers since 2006. The increase in nutrient loads, including a considerable increase in PO₄³⁻ concentrations due to the flood, has triggered the development of severe algal blooms in the East China Sea. Consequently, the production of DOC and removal of DIC were observed in the offshore region during the flood. Moreover, the increase in NH₄⁺ concentrations likely indicated enhanced organic material remineralization in flood-influenced coastal waters. The decrease in CO₂ fluxes across the air-sea interface (FCO₂) has been observed in the offshore region, while an increase in FCO₂ was found in the nearshore region during the flood compared to the non-flood condition in July 2018. These findings provide valuable references for assessing the impact of the extreme Changjiang flood on the coastal carbon cycle.