Abstract
Freshwater wetlands located in dryland environments are characterised by high evapotranspiration rates and frequent periods of desiccation, which strongly influence the water chemistry and solute budgets of these systems. The transpiration of groundwater, especially by trees, is an important mechanism through which dryland wetlands can lose water. This process can lead to groundwater salinization and the precipitation of substantial quantities of minerals within the soil, the accumulation of which can have profound consequences for wetland structure and function. This paper aims to bring together current knowledge on the processes that result in solute accumulation and chemical sedimentation which assist in maintaining freshwater conditions in many seasonal dryland wetlands. Examples from central and southern Africa, Australia and South America are presented to illustrate the geomorphically diverse settings under which chemical sedimentation can occur, and the importance of these processes for the resilience and longevity of dryland wetlands. We show that the localised development of saline groundwater and subsurface precipitation of minerals within soils can play a key role in creating and maintaining the habitat diversity of dryland wetlands. Wetland vegetation localises the accumulation of deleterious constituents, thereby preventing widespread salinization and playa-lake formation, and thus ensuring that the bulk of the surface water remains fresh. Although such processes remain widely understudied, we suggest that chemical sedimentation could be a common phenomenon in many dryland wetlands and have important implications for the future management of these ecosystems.
Highlights
All wetlands can be described in terms of their inputs and outputs which fall into three categories: water, sediment and the atmosphere (Fig. 1)
The existing literature on chemical sedimentation in wetlands is geographically limited and restricted to a few individual case studies, it is clear that chemical sedimentation processes can play a key role in creating and maintaining the biological and habitat diversity of dryland wetlands
The case studies examined in this paper represent some of the most biodiverse wetland ecosystems on Earth and chemical sedimentation is likely a key process contributing to the spatial complexity and biodiversity of these systems
Summary
All wetlands can be described in terms of their inputs and outputs which fall into three categories: water, sediment and the atmosphere (Fig. 1). Water is lost in the form of evaporation, transpiration, surface flow and groundwater flow; sediment in the form of solutes (particulate sediment is generally trapped within wetlands); and atmospheric gases in the form of CH4 and CO2 Wetland ecosystems exhibit numerous strategies to cope with these dry periods which often involve biogeochemical transformations that play an important role in reducing the impacts of salinity increases Water availability in such systems is strongly influenced by both hydrological inputs and by atmospheric demand. Climate classification based on United Nations Environmental Programme (UNEP) aridity index data and represents average yearly precipitation divided by average yearly potential evapotranspiration
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