Abstract
Over the past two decades, great efforts have been made to restore coastal wetlands through the removal of dikes, but challenges remain because the effects of flooding with saline water on water quality are unknown. We collected soil samples from two adjacent coastal fen peatlands, one drained and diked, the other open to the sea and rewetted, aiming at assessing the mobility and export of various compounds. Microcosm experiments with constant flow-through conditions were conducted to determine the effluent concentrations of dissolved organic carbon (DOC), ammonium ( NH 4 + ), and phosphate ( PO 4 3 − ) during saline–fresh water cycles. Sodium chloride (NaCl) was used to adjust salinity (saline water, NaCl concentration of 0.12 mol L−1; fresh water, NaCl concentration of 0.008 mol L−1) and served as a tracer. A model analysis of the obtained chloride ( Cl − ) and sodium ( Na + ) breakthrough curves indicated that peat soils have a dual porosity structure. Sodium was retarded in peat soils with a retardation factor of 1.4 ± 0.2 due to adsorption. The leaching tests revealed that water salinity has a large impact on DOC, NH 4 + , and PO 4 3 − release. The concentrations of DOC in the effluent decreased with increasing water salinity because the combination of high ionic strength (NaCl concentration of 0.12 mol L−1) and low pH (3.5 to 4.5) caused a solubility reduction. On the contrary, saline water enhanced NH 4 + release through cation exchange processes. The PO 4 3 − concentrations, however, decreased in the effluent with increasing water salinity. Overall, the decommissioning of dikes at coastal wetlands and the flooding of once drained and agriculturally used sites increase the risk that especially nitrogen may be leached at higher rates to the sea.
Highlights
Coastal wetlands—saltmarshes, mangroves, and seagrass beds—only cover approximately 0.3%of the Earth’s land surface [1,2], but hold vast stores of carbon and play a critical role in regulating physical, chemical, and biological processes in tidal and non-tidal systems [3]
While one site was still subjected to drainage, the other site had been rewetted by removing the dike and had been open to the sea since 1993
The fractions of immobile water found in this ranged between 0.5 and 1 (Table 2), which can be taken as an indication that the investigated peat soils constituted a dual-porosity medium containing active and dead-end pores [31,32,33], where the active pores or mobile water region dominated the pore space of the four soil cores
Summary
Coastal wetlands—saltmarshes, mangroves, and seagrass beds—only cover approximately 0.3%of the Earth’s land surface [1,2], but hold vast stores of carbon and play a critical role in regulating physical, chemical, and biological processes in tidal and non-tidal systems [3]. Many marsh wetlands were diked and drained for pastural and agricultural use. Of the world’s coastal tidal wetlands were lost as a result of their direct conversion into agricultural land [2,4]. In addition to the direct land loss, a large number of coastal wetlands has been found to be eutrophic, which is considered one of the main causes for their ecological functional degradation [5,6]. Diking blocks the flow of seawater to wetlands, reducing their salinity [7]. The established drainage lowers the water table, accelerating organic matter decomposition [8,9]. It was reported that after long-term diking and drainage, coastal peatlands revealed low salinity
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