Nguyen et al. (Prog Earth Planet Sci 7:1, 2020. https://doi.org/10.1186/s40645-019-0311-0) suggest that Landsat 8 OLI can be used to map and monitor soil salinity in the coastal zone of the Mekong River Delta. The authors use empirical correlations between the near-infrared (NIR) band, or vegetation indexes containing the NIR band, and soil salinity. We show that within the coastal portion of the Mekong Delta, extensively ponded due to widespread shrimp farming, about 90% of Landsat 8 pixels are fully or partially covered by water. We then find that, due to strong NIR radiation absorption, NIR reflectance from ponded pixels decreases linearly with increasing water percentage cover, while no significant correlation is found between reflectance and soil salinity. Through detailed new analyses, we conclude that NIR reflectance attenuation cannot be ascribed to vegetation stress caused by soil salinity, but rather to the presence of water ponds. We also show that a similar behavior exists in ponded freshwater inland areas, confirming that the NIR absorption exerted by water is independent of salinity.
The contamination of surface water by pathogenic bacteria of human origin is an important public health issue. Wetlands can be contaminated with fecal bacteria by water originating from different sources, such as wastewater treatment plants and agriculture. Escherichia coli is a commensal of the human gut flora and the major indication of fecal contamination in surface water. Little is known about the association between fecal bacteria and submerged macrophytes and how this may influence the water quality. We questioned whether macrophytes enhance or inhibit the bacterial growth in wetlands. For this purpose, we grew four different species of macrophytes (Mentha aquatica, Baldellia ranunculoides, Sparganium emersum and Elodea canadensis, in mono- or multispecies cultures) in aquatic rhizotrons and inoculated the devices with a fluorescent strain of Escherichia coli (producing a green fluorescent protein) to simulate the fecal contamination of wetlands. Bacterial survival was monitored by measuring the fluorescence for 19 days. We found (i) that contaminated sediments did not release E. coli in the water column in lentic conditions and (ii) that monocultures of E. canadensis, M. aquatica and S. emersum reduced the E. coli concentration in the water column. This suggests that aquatic plant species may be used in constructed wetlands to clear surface freshwater from bacteria of fecal origin.
Abstract. Coastal wetlands are fundamental for climate-change mitigation thanks to their ability to store large amounts of organic carbon in the soil. They also represent the first natural emitter of methane (CH4). Salinity is known to inhibit CH4 production, but its effect in brackish ecosystems is still poorly understood. Our study aims to understand how environmental variables may affect greenhouse gas emissions (GHGs) in coastal temperate wetlands. We present the results of over one year of measurements performed in four wetlands located along a salinity gradient on the northeast Adriatic coast near Ravenna, Italy. Soil properties were determined by collecting soil samples, while carbon dioxide (CO2) and methane (CH4) fluxes from soils and standing waters, water levels, surface, and groundwater physical-chemical parameters (temperature, pH, electrical conductivity, and sulphate concentrations of water) were monthly monitored by a portable gas flux-meter and multiparametric probes, respectively. Principal component analysis (PCA) was used to investigate emergent relationships between GHGs fluxes and environmental variables. Our results suggest that, among all variables, temperature and irradiance play a significant role in CH4 emissions from water and soil whereas water column depth and salinity are limiting factors of GHGs emissions.
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