The effects of external and internal processes of phosphorus on eutrophication in Tonle Sap Lake (TSL) were assessed in this chapter using the phosphorus dynamics model described in Chap. 25. Spatial sensitivity analyses revealed that the total phosphorus (TP) concentration in TSL is more sensitive to the settling rate and the internal and atmospheric loading rate than the TP concentration in the tributaries and its load from villages on TSL and its floodplain. Regarding the external sources, the TP concentration in the local tributaries is more influential than Tonle Sap River and the villages on the TP concentration in TSL. The baseline distribution of TP was defined as the reproduced results from 1999 to 2003 using the phosphorus dynamics model (Chap. 25), and the eutrophic area in this baseline condition was 7.0%, 5.6%, 2.2%, and 3.3% of the lake area in the low-water, inflow, early outflow, and late outflow periods, respectively. The decrease of the TP concentration in the local tributaries by 75% resulted in the decrease of the eutrophic area by 32%, 31%, 40%, and 24% of the baseline eutrophic area in those four periods, respectively. The TP concentration in Tonle Sap River is highly sensitive to the eutrophication only in the inflow period, whereas the TP load from the villages during 1999–2003 did not promote lake eutrophication significantly. Therefore, an effective countermeasure against eutrophication in TSL is to control the phosphorus concentration in the local tributaries.
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