Physico-Chemical Consequences of Water-Level Decreases in Wetlands

Due to global change, many wetlands experience water level decreases and changes in their hydrological functioning, modifying their physico-chemical characteristics. Riverine wetlands are particularly complex within this framework because they are supplied by different water sources. This study assessed how long-term water level decreases in riverine wetlands relate to changes in their physico-chemical characteristics (temperature, oxygen, pH, conductivity and nutrient contents: nitrate, ammonium and phosphate). We hypothesized that 1) water level decreases have no effect on water nutrient contents because of the high water renewal by groundwater supplies, which may leach nutrients released from the sediment after the dewatering phase, and 2) the physico-chemical characteristics of the water would become increasingly similar to those of hillslope groundwater due to the increase in their water supplies in wetlands. We compared the surface water physico-chemical characteristics of nine riverine wetlands with contrasting water level decreases between 1993 and 2011. No increase in water nutrient contents occurred. However, depending on the wetland considered, the physico-chemical changes suggested either 1) increased connectivity with hillslope groundwater, 2) a lower rate of water renewal in wetlands or 3) increased river water supplies, potentially related to river lateral migration toward wetlands.