The characteristics physicochemical studies in the northern part of the Oman Sea and Strait of Hormuz within the Iranian waters were conducted during 2007 and 2009.The project was implemented using Ferdous research vessel and with the cooperation of Chah Bahar Research Center. The sampling area included the whole stretch of the Iranian waters along the northern part of the Oman Sea from the northeast of the Iranian marine border of 30 miles Bay of Chabahar to the mouth of the Hormuz Strait in Hormuzgan province. Along this stretch 10 equally distributed transects at a distance of 30 miles away from each other were designated, 4 main sampling stations at a distance of 10 miles from one another were fixed at each transect. Sampling was conducted twice a year ( spring and autumn 2007 and 2009).Physicochemical parameters of water such as temperature, density, salinity, turbidity, dissolved oxygen, pH, EC and chlorophyll-a were recorded at stations using CTD multiparameter probe. Nutrients samples were collected by multi water sampler and were measured by spectrophotometric method. According to achieved results; we can point out to the following outlines below: The beginning of thermocline layer starts in the regional Iranian Territorial Waters of Oman Sea; located at approximate depth of 10m; in spring, 20-40m; in autumn, and at 100m depth in winter; seasons, with an average difference in temperature of 10˚C; between surface and deeper layers. In another word, permanent thermocline exists annually in this body of water; where only the point of start, and the fracture width of thermocline layer; alter with alteration of seasons. Horizontal and vertical distribution of electrical conductivities, obey exactly from thermal degree structure; in such a way that, with increase in temperature; they increase, and with decrease in temperature; they decrease. Level of salinity increase from east to west, but it decrease; from surface to deeper layers. In deeper regions, especially in the midway east of the Oman Sea; the level of salinity in the deeper waters from 150-300m, there is a considerable increase observed in salinity, due to very high salinity waters of The Persian Gulf; that are entering the Sea of Oman, via the Striate of Hormouz from the bottom; where they are having their effect following the whole length of the Sea of Oman; through their path. The level of water density from offshore to onshore, and from surface to deeper levels increase in such a way that; its average in surface layers is 24g/cm^3, and at 400m depth; is 29g/cm^3, and at 600m; reaches to 30g/cm^3. The level of Chlorophyll-a in surface waters, in the northeastern midway; is higher than the northwestern midway of the Sea of Oman, and it decreases from inshore to offshore waters, but its vertical distribution; has caused somehow in a way, that the highest distribution and concentration of chlorophyll-a; to be formed in deeper layers, between about 20-40m depths. Then by the increase in water depth, its concentration decreases drastically; reaching zero in magnitude. Concentration of dissolved oxygen is higher in spring in comparison to that of autumn, and the highest level is achieved in water layers located at 10-40m depth (where the level of chlorophyll-a is higher). In addition to which, that its vertical structure; shows the existence of a permanent oxycline layer at this region, in a way that; by seasonal alterations (similar to that of thermocline layer), only the point of start and that of the thickness of oxycline layer changes. The layer of oxycline is established from seawaters; located at 10m depth in spring, and at20-40m waters in autumn; whereas the same layer is formed at 100m seawaters in winter, and then by water depth increase; it reaches crisis condition. The highest level of water pH; has been achieved in the spring of year 2009 (1388 Iranian Calendar), where some northern and northwestern regions of the Strait of Hormouz; were suffering from the remnants of red tide occurrences (~8.5), where the pH level decreased from water surface, and its vertical alteration trend; obeyed to that of the same for temperature and chlorophyll-a vertical structure, especially for dissolved oxygen parameter, where the maximum decrease was recorded coinciding with formation of oxycline layer. The level of water turbidity increased from offshore to inshore direction, but it decreased from water surface to depth for the Sea of Oman. But nutrient levels increased; at surface water layers from offshore to inshore, and from west to east, for the same body of water. In addition to that, concentration of nutrients in autumn (after monsoon); were more than that of, spring season (before monsoon), where; concentration of their vertical structure increase from water surface toward deeper sections
