Shigeru Mizugaki

Rapid shrinkage of Kushiro Mire, the largest mire in Japan, due to increased sedimentation associated with land-use development in the catchment

Abstract The suspended sediment discharge in the Kuchoro River, a tributary of the Kushiro River, showed that wash load was about 90% of the yearly total suspended load carried into the wetland from the entire watershed. Seasonal floods associated with typhoons and snowmelt carried a large volume of wash load, 44% and 37%, respectively, of the yearly total wash load. The deposition of suspended sediment in the channelized section has aggraded the riverbed by 2 m in the past two decades, which has reduced the cross-sectional area of the channel, so that turbid water spills over and carries wash load and suspended sediment deep into the wetlands. Flooding of turbid water, in association with aggradation of the riverbed, was detected by using a Water Turbidity Index. The flooding and turbidity have significantly increased between 1984 and 1994. Similarly, a Normalized Difference Vegetation Index map showed that forest establishment has continued from the wetland margins and in areas adjacent to the river channel. The suspended sediment carried and deposited by floods and sediment-associated nutrients should alter the edaphic environment from wet nutrient-poor soil to dry nutrient-rich soil providing regeneration habitats for tree species. The vegetation in turn adds resistance and friction against flows and contributes to additional sedimentation. However, flooding and the associated high water table causes stress for trees and may lead to premature decay. Environmental variables, such as water level, water content, organic content and grain size, varied significantly along cross-sectional distance, and are likely regulated by deposition of fine sediments transported by floods. Electrical conductivity and total nitrogen in groundwater varied significantly along the longitudinal distance. Basal areas of willow and alder stands correlated with variables related to spread of turbid water, which indicates that eutrophication of groundwater indirectly affects marsh forest expansion.

Baseflow concentrations of nitrogen and phosphorus in forested headwaters in Japan

A comprehensive investigation on all dissolved nitrogen and phosphorus components at both local and regional scales in the headwaters from forested watersheds is valuable to improve our understanding of the factors controlling water quality. Here, we investigated the baseflow concentrations of dissolved nitrogen and phosphorus components, N:P ratio, and their associations with region and vegetation type in forested headwaters in fives regions of Japan. We found that inorganic nitrogen and phosphorus were the dominant components in the 26 temperate forested streams, rather than organic forms. There were significant positive correlations between the concentrations of N and P components. Furthermore, the regional patterns of the concentrations of nitrate, dissolved inorganic P (DIP), and dissolved total N (DTN) and P (DTP) were similar. Our results suggest that the regional patterns of the concentrations of N and P components should be related to the regional atmospheric deposition of both N and P nutrients. We also found that the nitrate and DTN concentrations were higher in man-made evergreen conifer (EC) than those in the natural deciduous broadleaf (DB). In contrast, the DIP and DTP concentrations in EC were lower than those in DB. The uniformly higher N:P ratio in EC- than in DB-forested streams for each region suggest that EC-forested streams could be more affected by P-limited than DB-forested streams when N inputs from atmospheric sources increased.

Is MUSLE apt to small steeply reforested watershed

The reforested headwater watersheds in Japan are very important from the points of view of commercial and environmental aspects. At the present time, much and varied research is running to assess and understand the hydrologic behavior of these watersheds. The present study was conducted to evaluate the applicability of the deterministic model MUSLE in the Mie small steeply reforested watershed. The model was tested and calibrated using accurate continuous suspended sediment data collected during eight storm events in 2004. Results of the original model simulations for storm-wise sediment yield did not match the observed data, while the revised version of the model could imitate the observed values well. The results of the study approved the efficient application of the revised MUSLE in estimating storm-wise sediment yield in the study area with a high level of agreement of beyond 88%, an acceptable estimation error of some 14% and non-significant difference in mean values.

Changes in sedimentation rates and phosphorus accumulation in shallow Japanese lakes during 30 years

Sediment-water exchange of phosphorus strongly influences lake biogeochemical processes and ecosystems, especially in shallow lakes ( e.g., WETZEL 2001 ). In Lake Sebygaard, a shallow Danish lake, the net release of total phosphorus (TP) was calculated by comparisons of the vertical pro files of TP in sediments of different cores taken in different years, and influence of interna) phosphorus loads on lake water could last for more than 30 years, despite reduction of externa) loads (S0NDERGAARD et al. 1999). Phosphorus is deposited with sedimentation of particulate matter (FUKUSHIMA et al. 1987); thus, calculation of sedimentation rates and investigation of changes in vertical profiles of TP contents in sediments during severa) decades are important for determining the phosphorus balance in a lake. In this study, we analyzed the changes in sedimentation rates and TP contents in sediment over the 1ast 30 years in Lakes Kasumigaura and Suwa, shallow eutrophic 1akes in Japan. Based on these analyses, we compared TP accumulation rates estimated by: ( 1) net 1oads; (2) difference in vertical profiles ofTP content; and (3) mass sedimentation rates multiplied by TP contents.