Unveiling the potential phosphorus retention effect in small-scale cascade reservoir systems: evidence from the Weiyuan River

The construction of artificial dams profoundly alters nutrient dynamics within reservoir systems, with the phosphorus (P) retention effect of large reservoirs well-established. However, the role of small cascade reservoirs in regulating P transport dynamics remains poorly characterized, and whether their biogeochemical impacts align with those of large-scale reservoirs requires systematic investigation. Traditionally, it is widely believed that reservoir systems retain P, preventing their export downstream and thereby reducing primary productivity downstream of the reservoir. Unexpectedly, our research on the Weiyuan River cascade reservoirs revealed elevated sediment total P (TP) levels of 1208.93 mg/kg, bioavailable P (BAP) at 623.14 mg/kg, and 0.23 mg/L of TP in the overlying water of downstream reservoirs, indicating that P gradually accumulates into a hotspot along the downstream path of the cascade reservoir, especially during the low-water season (LWS). P dynamics within cascade reservoir systems are primarily driven by three interconnected factors: (1) enhanced sediment P remobilization risks in downstream reaches, (2) anthropogenic P loading from external sources, and (3) cascade-induced sedimentological shifts toward elevated organic matter content and finer particle size distributions, which collectively amplify P bioavailability through modified adsorption-desorption equilibria. Notably, the combined effect of elevated P loading (0.17 mg/L) and prolonged hydraulic retention time (HRT: 13.13 days) during low-water seasons triggered pronounced P sequestration in upstream suspended solids (SS) of cascade reservoirs, retaining 30.35 kg (15.73 % of sediment TP). This far exceeds the P transport observed during the high-water season (HWS), where an increase of 34.34 kg (36.69 %) was recorded downstream. The observed sediment retention during LWS exhibits inconsistencies with reservoir-scale P biogeochemical dynamics, potentially driven by the limited P buffering capacity of small cascade reservoirs under hydrological perturbations and shortened sediment residence times. The study results challenged the conventional belief that a singular reservoir is greatly responsible for P retention, which underscores the importance of monitoring P pollution in areas downstream of cascade reservoirs. Our research offers new insights into how river dams affect nutrient cycling and ecosystem functions, aiming to provide theoretical guidance for river management.