Dissolved phosphorous through dry–wet–dry transitions in a small-dammed river basin: integrated understanding on transport patterns, export controls, and fate†

Abstract

An integrated understanding of dissolved phosphorous (DP) export mechanism and controls on export over dry and wet periods is crucial for riverine ecological restorations in dammed river basins considering its high bioavailability and retention rates at dams. Riverine DP transport patterns (composition, sources, and transport pathways), export controls, and fate were investigated over the 2020 wet season (5 events) and dry seasons before and after it (2 events: dry_(before) and dry_(after)) in a semi-arid, small-dammed watershed to comprehend the links between terrestrial DP sources and aquatic DP sinks. Close spatiotemporal monitoring of the full range of phosphorous and total suspended solids (TSSs) and subsequent analyses (hysteresis, hierarchical partitioning, and coefficient of variation) provided the basis for the study. Total-DP (TDP) shared 13–39% (25%) of total-P (TP) through storms, dissolved organic-P (DOP) shared 6–21% (12%), and phosphate-P shared (PO₄-P) 7–22% (13%). DP forms displayed strong connections with discharge trends across the wet season, and marked changes in the shares were reported over dry–wet and wet–dry transitions. The DOP fraction of TDP increased from 4% in dry_(before) baseflow to 64% at the end of the wet season. The DOP flux increment in stormflow was 20 folds compared to dry_(before) baseflow, while that of PO₄-P was 2 folds. DOP displayed the least spatial source heterogeneity with minimum anthropogenic pressure on inherent fluxes. DOP originated from overland and near-stream soil sources and was transported via surface runoff and soil water runoff, respectively. Across the wet season, the attrition of overland DOP sources and activation of near-stream soil DOP sources through strengthened hydrological connectivity governed the seasonal DOP trends. Surface and groundwater runoff pathways were important for PO₄-P delivery during stormflow; nonetheless, wastewater treatment plant (WWTP) effluent was the main PO₄-P source under both baseflow and stormflow regimes, followed by near-stream traditional agriculture lands. The interaction patterns of small dam systems with DP inputs through dry–wet periods were explained. The riverine PO₄-P fluxes were profoundly impacted by in-stream biogeochemical and physical processes and small dam systems, while riverine DOP fluxes were relatively less influenced.