Sedimentation in the semi-arid freshwater wetland Las Tablas de Daimiel (Spain)

Abstract

Despite sedimentation being recognized as a very important process in wetlands, very little is known about its dynamics. This study analyzed sedimentation on a short-term scale in the semiarid floodplain wetland Las Tablas de Daimiel National Park (central Spain). In monthly sampling intervals during 1997 and 1998, we measured seasonal and spatial sedimentation patterns of total solids, organic and mineral matter, nitrogen, and phosphorus using sediment traps. The highest mean mass accumulation rates were obtained at the wetland-input site (135 ± 57 g m−2 d−1), but a decrease in sedimentation rates was noted with increasing distance from the inflow (49 ± 39 g m−2 d−1 in the terminal zone of the wetland). Flow and water level explained only 13% of variance in sedimentation. The sedimentation patterns presented clear seasonality, coinciding with the period of vegetation growth. There existed a marked spatial heterogeneity in sedimentation that can be attributed to differences in vegetative cover. In vegetated areas, sedimentation of solids was related with nutrient deposition coming from detritus. Sedimentation of mineral matter dominated over the organic fraction. Autochthonous material comprised a very important fraction of accumulated sediment (between 31–57% of mineral matter and 45–65% of organic matter). Sedimentation of autochthonous total phosphorus made up about 90% of total settling phosphorus. Reed (Phragmites australis) and sawgrass (Cladium mariscus) plant cover significantly explained sedimentation of organic matter in the wetland (R2= 0.78 and 0.57, respectively). The contribution of phytoplankton primary productivity to total organic carbon sedimentation was negligible and observed only at the deeper sampling stations. The autochthonous mineral matter of the settling seston may originate either from primary precipitation of endogenic calcite and/or rapid mineralization of organic matter on the sediment surface. Accretion rates were very high (1.61–3.87 cm yr−1), suggesting that the wetland will silt up within the next century if present rates are maintained. Thus, sedimentation dynamics must be involved in management strategies in order to conserve important wetlands.