Hypolimnetic accumulation and seasonal pulsing of total dissolved solids across cascaded reservoirs

However, the mechanisms by which cascade dams regulate the vertical distribution and fluxes of total dissolved solids (TDS) remain unclear. Here, we investigated eight reservoirs across the Wujiang River, using hydrochemical parameters, TDS components (Ca2+, Mg2+, Na+, K+, SO42–, Cl–, HCO3–, and NO3–), dissolved inorganic carbon isotopes (δ13CDIC), and nitrate isotopes (δ15N-NO3–) to elucidate cascade damming effects on TDS distribution and fluxes.

Our findings reveal that the pronounced biological carbon pump during the wet season reduces TDS concentrations in the epilimnion. Meanwhile, persistent thermal stratification results in hypolimnetic TDS concentrations (below 30 m) that are 22.8 ± 9.8 mg L–1 higher than those in the epilimnion, primarily driven by carbonate dissolution and nitrate transformation processes. During the dry season, vertical TDS gradient decreases to 9.1 ± 15.5 mg L–1, with reservoirs exhibiting weak thermal stratification showing a significantly smaller TDS difference (1.6 ± 3.7 mg L–1) than those with strong stratification (21.6 ± 14.1 mg L–1), underscoring the decisive role of thermal stratification intensity in driving vertical TDS variability.