Seasonal divergence in dissolved-carbon concentration and export along a cascade-dammed subtropical river continuum: A case study of the Zijiang River, China

Study region The Zijiang River is a subtropical monsoonal tributary of the middle Yangtze River, China, characterized by intensive cascade-reservoir regulation along its mainstem. Study focus We focused on four successive cascade dams and the five longitudinal locations they define (D1–D5), with D1 as the upstream reference and D2–D5 as successive dam-regulated reaches. We examined how hydrological seasonality interacts with repeated cascade regulation to shape DOC and DIC sources, concentrations, and export using seasonal observations integrated with isotope, statistical, and discharge-conditioned analyses. New hydrological insights for the region DOC and DIC displayed a seasonal reversal, with higher DOC in flood season and higher DIC in non-flood season. Yet export timing diverged: in 2023/24, flood-season water carried 63.7–77.7% of annual DOC export, whereas non-flood season carried 57.9–69.6% of DIC export. Plankton-associated DOC dominated D1–D3, then shifted towards mixed plankton–soil contributions in D4–D5. DIC retained a carbonate-weathering background, while soil-CO₂ contribution increased from D3 downstream. Cascade damming was associated with both carbon pools through distinct environmental pathways. These results show that carbon budgets in cascade rivers must resolve season, dam sequence, source composition and in-river processing rather than rely on concentration snapshots.