Abstract
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 × 109m3, the average was 1.77 × 109m3. Reverse flow from the Lake Nakaumi located downstream of the Lake Shinji was in the range of 0.32 ～ 0.84 × 109m3, the average was 0.49 × 109m3, 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.
