Water, salt and phosphorus balance of the 19 years was calculated in Lake Shinji, a shallow brackish lake in Japan. Average annual freshwater inflow of 19 years into Lake Shinji was in the range of the 1.25 ~ 2.35 × 10 9 m 3 , the average was 1.77 × 10 9 m 3 . Reverse flow from the Lake Nakaumi located downstream of the Lake Shinji was in the range of 0.32 ~ 0.84 × 10 9 m 3 , the average was 0.49 × 10 9 m 3 , corresponding to 27.7% of the freshwater inflow. Retention time in consideration of the amount of freshwater inflow and reverse flow from the Lake Nakaumi were from 47.5 to 76.2 days, and 59.4 days on average. The percentage of annual deposition of TP to inflow TP was in the range of -23.6 to 69.3%, and 20.9% on average. In addition, there was a positive relationship (r = 0.71) between annual TP inflow and sedimentation rate, settlement amount was small drought year when inflow is low.Phosphorus concentrations peaked by released from the sediment in August-September around becomes a normal value almost in November-December. We calculated the ratio of phosphorus resettle to the sediment for the duration from the peak of the TP stock to become normal value, using water amount of outflow from Lake Shinji, inflow load and TP concentration of Lake Shinji. TP sedimentation rate was in the range of 8.8 to 65.6%, and 45.1% of phosphorus which released in summer was resettled to the bottom of the lake again in average, and it was considered to be involved in the release of the following year.In the process of phosphorus concentration decreases, SRP was greatly reduced, but the fact that changes in the PP was hardly seen. The SRP reduction occur when DO at the sediment surface increased at the same time. It was considered that reduction of phosphorus in the water column that released from sediment when there was anoxic is attributed to the SRP adsorption to the sediment surface where become aerobic. At this time sediment is deficiency state of phosphorus because it is after releasing SRP, it was believed to adsorb SRP easily.
The organic carbon concentration of surface sediment influences bottom water hypoxia. To investigate whether the organic carbon concentration of surface sediment can be used to reconstruct temporal changes in organic carbon concentration, surface sediments from Lake Shinji, Shimane Prefecture, Japan were studied. Samples were analyzed using ignition loss. For comparison with earlier studies, the techniques used by these studies were also used herein, and a conversion formula developed to calculate equivalent organic carbon concentrations. The converted organic carbon concentrations at the same locations in 1982, 1997 and 2016 were compared, and found to be significantly lower in 1997. Since the organic carbon concentration decrease in the 1990s was not detected by an earlier study, it was thought that the decrease for around ten years may have been obscured during early diagenesis. In addition, the organic carbon concentration was significantly higher in sediments collected from the western part of Lake Shinji where a halocline exists. When monitoring hypoxia in brackish lakes, attention should be paid to areas with a halocline as well as the deeper areas.
close to the theoretical value for both compounds, and almost all organic carbons in each solution were detectable by TOC equipment. On the other hand, the COD oxidation rate in both the saccharide and amino acid solutions decreased according to the increase in the solutions' concentration, and there were also significant differences in the decomposition rate in each compound. The relationship between field data on COD and TOC in Shinji and Nakaumi lakes may be approximated by a quadratic expression, but the approximated curve did not pass the original point.This result indicates that there are some organic compounds that can be decomposed by TOC equipment but not by COD analytical methods, and the decomposition of COD was insufficient in high concentrations in environmental water. Therefore, we consider COD not to be an appropriate indicator of the organic compounds in public waters;instead, TOC should be used.
The release of nitrogen and phosphorus from sediments at high water temperatures was studied by taking weekly measurements at Lake Shinji, a brackish lake in Japan. A severe phosphorus release was observed at high temperatures from August to September. On the other hand, a nitrogen release was observed in July, but not from August to September when the water temperature was higher. Lower nitrite-nitrogen and nitrate-nitrogen concentrations in the water during high water temperature period suggest that the effect of denitrification was minute. The results showed that nitrogen release from sediments in the lake occurs earlier in the year than the phosphorus release.
Calculating the change of stock of each month of the nitrogen in Lake Shinji, a brackish lagoon in Japan, it is assumed that TN is released from the sediment. This relationship has not been observed for the mean water temperature and TN release from the sediment. With respect to TN in Lake Shinji, a significant release from the sediment was not seen in the summer, which was observed for TP, but the effect of the TN concentration in RiverHii was large as compared to the TN concentration of Lake Shinji. As for TN content in Lake Shinji, a correlation between the TN concentration of River Hii of the same month was observed (r = 0.57, p < 0.001).Nitrogen mass balance for 19 years was calculated. The percentage of annual deposition of TN to the TN inflow was in the range of -4.2 to 38.4%, and 19.8% on average.We calculated the average annual value balance of TN and TP for 19 years. For TN, 1480 t flowed in, 1170 t flowed out, and 310 t settled in the Lake Shinji, and for TP, 134 t flowed in, 106 t flowed out, and 28 t had settled in Lake Shinji. Settling TN/TP ratio was 10.7, which was 11.4 for an annual average TN/TP ratio of Lake Shinji.
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