Sedimentary phosphorus in a cascade of five reservoirs (Lozoya River, Central Spain)

Abstract

The concentration of phosphorus (P), carbon (C), and nitrogen (N) in superficial sediments in a cascade of five reservoirs (Pinilla, Riosequillo, Puentes Viejas, El Villar, and El Atazar) located along the Lozoya river (Central Spain) was determined. The mean reservoir values of sedimentary phosphorus, nitrogen, and carbon increased from the first reservoir (Pinilla) to the central one (Puentes Viejas) where they reached 62 μmolP/g(dw), 5.36 mmolC/g(dw)and 0.56 mmolN/g(dw), respectively. They then followed a decreasing trend until the last reservoir (El Atazar), which presented concentrations close to those of Pinilla. Phosphorus and nitrogen usually tended to increase from the river end to the dam at each reservoir, while carbon did not show a regular trend associated to the river flow. The amount of phosphorus accumulated in sediments of the Lozoya System reservoirs was notably high (>50 μmolP/g (dw)), especially at the three central reservoirs (Riosequillo, Puentes Viejas, and El Villar). Sedimentary phosphorus was positively related to nitrogen, small particles (: 2-10 m), and the ratio of iron to aluminium, Fe/Al, is an indicator of iron precipitation. This last variable explained more than 90% of the sedimentary phosphorus variance in four reservoirs (Pinilla, Riosequillo, El Villar, and El Atazar). In the other reservoir (Puente Viejas), sedimentary phosphorus (Psed) variance was explained by Fe/Al ratio as a main factor, but also by nitrogen as a secondary factor. Hence, association of sedimentary phosphorus to iron oxides with small size appeared as the main factor determining phosphorus accumulation in the sediments of the Lozoya reservoirs. Two main management recommendations arise from these results: (1) reduction in the flow of anoxic water from upstream reservoirs to prevent the transport of dissolved phosphate coming from internal loading, and (2) reduction in high flows that carry fine sediment particles with weakly bound P that may be released when the particle meets a new concentration of soluble reactive phosphorus in the downstream reservoirs. © 2009 Copyright by the North American Lake Management Society.