Assessing the Impacts of Four Land Use Types on the Water Quality of Wetlands in Japan

This study examined how changes in the composition of land use can affect wetland water quality. Twenty-four wetlands located in Hiroshima prefecture in the western part of Japan were selected for this purpose. The water quality parameters that were explored include: pH, electrical conductivity, turbidity, dissolved oxygen, total dissolved solid, temperature and different forms of nitrogen. These important indicators of the water quality in the study area were measured from December 2005 to December 2006. The composition of land uses was determined for the catchments of the wetlands. They were then categorized into three classes, including non-disturbed, moderately-disturbed and highly-disturbed wetlands, based on the extent of urban area (as the most disruptive land use type within the catchment of the wetlands). The relationship between land use types and water quality parameters for the wetlands was statistically examined. The findings indicated that there were significant positive relationships between the proportion (%) of urban areas within catchments of the wetlands and EC (r = 0.67, p < 0.01), TDS (r = 0.69, p < 0.01), TN (r = 0.92, p < 0.01), DON (r = 0.6, p < 0.01), NH₄ ⁺(r = 0.47, p < 0.05), NO₂ ⁻ (r = 0.50, p < 0.05), while negative relationships were observed between the proportion (%) of forest area in these wetlands and EC (r = −0.62, p < 0.01), TDS (r = −0.68, p < 0.01), TN (r = −0.68, p < 0.01), DON (r = -0.43, p < 0.05), and NH₄ ⁺ (r = −0.55, p < 0.01). Analysis of the variance also revealed significant differences within the wetland groups in terms of the annual mean of electrical conductivity, total dissolved solids, total nitrogen, nitrite, dissolved inorganic nitrogen and dissolved organic nitrogen in the study area. Moreover, the study also indicated that the forest area plays a significant role in withholding nutrient loads from the wetlands, and hence, it can act as a sink for surface/subsurface nutrient inputs flowing into such water bodies from the watersheds.