Implications of seston settling on phosphorus dynamics in three reservoirs of contrasting trophic state

Abstract

We performed a comparative study in three reservoirs based on field measurements and laboratory experiments determine the impact of seston sedimentation on phosphorus dynamics. While we found a strong relationship between the trophic state and the settling flux of dry material in the study reservoirs, an increase in the trophic state did not lead to an increase in the export ratio (fraction of the primary production lost by sinking). In laboratory experiments, the analysis of biotic and abiotic contributions to phosphate (PO 4 3- ) regeneration/uptake by the sinking particles showed that an increase in the trophic state led to a significant vertical segregation in the observed patterns. Settled matter collected in both upper and lower sediment traps exhibited similar temporal patterns in the oligotrophic reservoir (Quentar), while clear spatial differences occurred in the more eutrophic systems (Cubillas and Beznar) as result of the vertical distribution of PO 4 3- characterizing warm monomictic eutrophic systems during summer thermal stratification. Chemical analysis of the settling matter in the oligotrophic reservoir (Quentar) showed that the sinking particles became richer in phosphorus during sedimentation, especially in the organic P pools. In contrast, P content in settling matter decreased with depth in the hypertrophic reservoir (Beznar), especially the org-P at the thermocline. This indicates that the mineralization of settling organic matter is important for maintaining high concentration of PO 4 3- in the deeper layers. However, no significant vertical differences in P content in settled matter were detected during the whole study period.