Sedimentation of phosphorus fractions and temporal variation in the C:P ratio in La Concepción reservoir, southern Spain

Abstract

The settling flux and content of organic carbon (C), organic matter, and different phosphorus (P) fractions were measured fortnightly during a 6-month stratified period (from early April to early November) with sediment traps located at five different depths in La Concepción, a mesotrophic and P-limited reservoir in southern Spain. Characterisation of P fractions in the collected material was performed by a sequential extraction procedure. Sedimentation rates exhibited a distinct seasonal pattern, that allowed us to differentiate three periods: (1) a biogenic sedimentation period (BS), in which the sedimentation rates and total suspended matter concentration were lowest and the result of material of autochthonous origin; (2) a calcium carbonate precipitation period (CCP), in which there were high calcium concentrations in the settling seston; and (3) a rainfall period (R), in which the highest sedimentation rates occurred. Organic C, organic matter, and organic P content were highest in the settling material during the BS period and lowest during the R period. However, during the R period, inorganic-P sedimentation rates were highest and the major P fraction was apatite-P. Total organic, and inorganic (Fe- and Al-bound and apatite) P, increased substantially, whereas total and organic C decreased with depth in the material collected by sediment traps during the BS and CCP periods. Consequently, the total C:P (Ct:Pt) and organic C:P (Corg.:Porg.) ratios decreased significantly with increasing depth of sediment traps.