Reduced nutrient trapping efficiency yet amplified downstream N:P ratio elevation in worldwide cascade reservoirs compared to single reservoirs.

Damming disrupts riverine nutrient transport by changing the magnitude and partitioning of nutrients processed within a river network or exported further downstream. However, whether and how upstream reservoirs affect nutrient retention in downstream reservoirs within closely spaced cascade reservoir systems (CRSs)-which increasingly impound rivers worldwide-remains unknown, leading to large uncertainties in current estimates of the damming effect at regional to global scales. Here, leveraging 3-year water quality data across China, in situ measurements and literature data, we evaluated total phosphorus (TP) and total nitrogen (TN) retention within 19 globally distributed CRSs (44 reservoirs) and 74 single reservoirs. We show that while uppermost reservoirs of the CRSs show comparable retention efficiencies to single reservoirs (47.6 % versus 47.1 % for TP, p > 0.05; 20.4 % versus 34.0 % for TN, p > 0.05), lower reservoirs retain significantly less (0.4 % for TP and -6.9 % for TN; p < 0.001). Despite the reduction, CRSs exhibit an elevation in outflow N:P ratio 4-fold that of single reservoirs (from 71.1 to 119.9 or 68 % versus from 50.9 to 59.0 or 16 %) due to a stronger TN than TP reduction in CRSs. Preferential particulate nutrient removal and progressive enrichment of dissolved, slow-removing nutrients result in more pronounced reduction in TN than TP retention within CRSs. Compared to a hypothesized single-reservoir scenario, we demonstrate that CRSs retain 55 % and 86 % less TP and TN. Disregarding the CRS effects may overestimate nutrient-trapping increase while underestimate downstream N:P elevation and P limitation in increasingly dammed rivers.