الفهرس 
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Abstract
-Ecological study of the four sites was carried out through two successive seasons at August 2007 and January 2008. The four studied sites are; Abou-Shaar, Marine station (at Hurghada harbor), General Beach and Aza’at site (Safaga harbor).
-The survey of the four sites recorded 5 seagrass species; Halophila stipulacea, Halodule uninervis Halophila ovalis, Thalassodendron ciliatum and Syringodium isoetifolium.
-Abou Shaar and Aza’at sites have 3 species; Halophila stipulacea, Halodule uninervis and Thalassodendron ciliatum. Marine Station site contains 4 species; Halophila stipulacea, Halodule uninervis, Halophyla ovalis and Thalassodendron ciliatum. In addition, four species have been recorded in General Beach site; Halophila stipulacea, Halodule uninervis Halophila ovalis, and Syringodium isoetifolium.
-Halophila stipulacea and Halodule uninervis represents the common species in the four sites, while Syringodium isoetifolium are occasional species recorded only in General Beach site in winter season.
-Halophila stipulacea represents the dominant species in the studied sites according to its distribution, biomass and density.
-Newly recorded Halodule uninervis and Syringodium isoetifolium in Marine Station and General Beach sites, respectively is an indicator on environmental changes.
-Water analysis for the four studied sites didn’t show significant difference in the physico-chemical properties. But the water of Aza’at site contains high concentrations of zinc and aluminum than the other sites.
-Soil analysis of the seventeen stands in the four sites show high variation in their construction. The three stands of Abou Shaar and those of Aza’at sites mainly constructed from gravel and coarse sand. While General Beach stands constructed from fine sand.
-Marine station stands show high variation in their construction. However stand 1and 2 mainly constructed from coarse sand, while stands 5 and 6 mainly constructed from fine sand. On the other hand stands 3, 4, 7 and 8 soils contain approximately equal quantities of coarse and fine sand.
-Organic mater showed significant increase in soils those constructed from fine sand than those constructed from rocks or coarse sand.
-Calcium carbonate recorded its highest content in soils of Safaga sites than in soils of Hurghada sites.
-Shoot biomass and density significantly affected by seasonality; however they recorded their highest values in summer than winter for most species.
-Noticeably, shoot biomass and density of the four species significantly affected by sites characteristics specially water depth and transparency.
-Decrease in water depth represents a physiological stress on the studied four species.
-The study indicated that most of the studied species can tolerate stress which represented in high light intensity (in summer), high and low temperature (in summer and winter, respectively). However, water depth represents the factor which controls the light and temperature.
-Chlorophyll a, chlorophyll b and total chlorophyll of H. stipulacea and H. uninervis increased in winter season than in summer. But they increased in H. ovalis and Th. Ciliatum in summer than winter.
-The photosynthetic pigments increased in leaves of H. stipulacea and H. uninervis growing in Marine Station and General Beach than those growing in other sites, while those of H. ovalis in General Beach and Th. Ciliatum in Abou Shaar site.
-Total carbohydrates and crude protein of the four seagrasses in the four sites show a positive correlation with the photosynthetic pigments as it increased with increasing the photosynthetic pigments and decreased with decreasing them.-The results indicated that the soluble compounds (Total soluble sugars, total soluble amino acids and protein) significantly increased in the four species in summer season than winter and increased in seagrasses growing in low depth than deepest locations.
-Nitrogen and phosphorus didn’t show significant variation in seagrasses tissues in the two seasons or sites.
-Sodium ions represent the dominant ions in seagrasses tissues among the studied seagrasses.-Na+, K+, Mg++ and Ca++ cations significantly increased in the four studied seagrasses in summer than in winter and in seagrasses growing in shallow habitats than the deepest ones.
Part II: Experimental study
- Increasing salinity more than 50 psu increased leaf necrosis incidence, mortality and growth rate of H. uninervis leaves and shoots. The injury effect of high salinities increased with increasing time of exposure.
-Recovery of H. uninervis successfully occurred in aquarium with salinities 50 and 55 psu. But mortality was increased to 89 and 96% when the seagrass exposed to 60 and 65 psu, respectively.
-The present study indicated that salinities; 40 and 45 psu represents normal salinity, 50 and 55 psu have moderate effect, while 60 and 65 have lethal effect on H. uninervis.
-The photosynthetic pigments and total carbohydrates decreased when exposed to salinity stress (50 psu) for week but with increasing the time of exposure they re-increased. But the reduction in the photosynthetic pigments high decreased with increase the time of exposure to 60 and 65 psu.
-Exposure the seagrass to 50 psu gradually increased organic osmoprotectants (total soluble sugars, total soluble amino acids and proline). While these compounds increased in plants exposed to 60 and 65 psu for one week and high decreased after two weeks from exposure.
-Sodium content of H. uninervis shoots growing under salinities specially 60 and 65 psu highly increased with increasing the time of exposure.
-Potassium, calcium, magnesium phosphorus and nitrogen ions significantly increased in H. uninervis tissue growing in salinity 50 psu through two weeks of exposure.