Lake Timsah is considered as the biggest water body at Ismailia City with a surface area of 14 km². It is a saline shallow water basin lies approximately mid-way between the south city of Suez and the north city of Port Said at 30^o35'46.55"N and 32^o19'30.54″E. Because it receives water with high and low salinities, salinity stratification is producing in the Lake Timsah, with values of 14-40‰ for the surface water and over 40‰ for the bottom water. The temperature of the lake water decreased to below 19 °C in the winter and rose to above 29 °C in the summer; the concentration of dissolved oxygen ranged between 6.5 and 12.2 l^-1 and the pH fluctuated between 7.9 in its lower value and 8.2 in its higher value. Water transparency was very low as indicated by Secchi disc readings recorded during this study and varied between 0.3 and 2.7 m. The main chemical nutrient (phosphorus) reached its highest levels of 96 µg l^-1 in winter and their lowest values of 24 µg l^-1 during summer. This nutrient concentration is high especially by comparing with those of unpolluted marine waters, but is typical of the more eutrophic coastal waters worldwide. The composition and abundance of phytoplankton with dominancy of diatoms and increased population density (20,986 cell l^-1) reflect the eutrophic condition of the lake. The intensive growth of phytoplankton was enriched by high concentration of chlorophyll a with annual values ranged between 6.5 and 56 µg l^-1. The objective of the present work was quantitative assessment of the quality of the water of the Lake Timsah using different approaches. During the present study, three different approaches were applied for the quantitative assessment of Lake Timsah water quality: the trophic state index (TST); trophic level index (TLI) and water quality index (WQI). Application of the trophic state and trophic level indices (TSI & TLI) revealed that the Lake Timsah has trophic indices of 60 and 5.2 for TSI and TLI, respectively. Both indices reflected the eutrophic condition of the lake waters and confirmed that the eutrophication is a major threat in the Lake Timsah. On the other hand, the WQI calculated for the Lake Timsah during the present study with an average of 49 demonstrated that the water of the Lake Timsah is bad and unsuitable for main and/or several uses. Moreover, WQI allows accounting for several water resource uses and can serve a more robust than TSI and/or TLI and can be used effectively as a comprehensive tool for water quality quantification. In conclusion, the three subjective indices used for the assessment process for the lake water are more suitable and effective for needs of the sustainable water resources protection and management of the Lake Timsah.

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