Chemical diversity and spatial variability in myriad lakes in Nhecolandia in the Pantanal wetlands of Brazil

Nearly 15,000 shallow, saline or freshwater lakes and ponds coexist in close proximity in the Nhecolandia, a 24,000 km(2) subregion of the Pantanal wetland in Brazil. This study aims to understand the origin of such diversity in surface water, which is a key aspect for the wetland services and biodiversity in the region. Both soil observations and water samples were collected at the regional or local scale and supplemented by previously published data sets. Statistical and geostatistical treatments were carried out and completed by discrete water-table monitoring. The results confirmed the absence of a regional pattern in surface-water salinity, and the independence of electrical conductivity values, even for neighboring lakes. Despite great differences in chemical composition, all surface waters in the region belong to the same chemical family, corresponding to several concentration stages of the Taquari River water that supplies the region. The concentration stage depends on the hydrological functioning of each lake, which is itself controlled by the relative importance of low-permeability soils as a barrier to movement of water into or out of the lakes through the subsurface. In this framework, oligosaline ponds, freshwater lakes, and saline lakes work as recharge, flow-through, and discharge wetlands, respectively. The salinity observed in some lakes results from an ongoing process of accumulation and evaporation under relatively humid climatic conditions and poor drainage.