شیرین فرخانی

Dissolved oxygen is one of the most important indicators of water quality in marine environments and a critical element for the survival of various aquatic species (Breitburg et al., 2018). Lack of oxygen in marine environments can have irreparable environmental consequences, such as changes in biogeochemical cycles, damage to ecosystem functioning, disruptions in the trophic network, and reductions in biodiversity. With the emergence of climate change effects and global warming in recent decades, the Oman Sea, similar to many marine environments, has experienced deoxygenation and the expansion of hypoxic (dissolved oxygen less than 2 mg/L) areas (Queste et al., 2018; Lachkar et al., 2021). Based on measurements made in the 1960s, permanent hypoxic and anoxic conditions existed in the Oman Sea at depths ranging from about 150 to 1000 m (Bopp et al., 2013; Long et al., 2016; Stramma et al., 2010; Paulmier and Ruiz-Pino, 2009). However, the hypoxic conditions prevailed at depths greater than 50 m based on measurements taken in the summer of 2018 (Saleh et al., 2021). Therefore, studying the effective factors influencing changes in dissolved oxygen in this sea is crucial to protect and manage aquatic resources. Since the Oman Sea has water exchanges with the Persian Gulf (Garcia-Soto et al., 2021; Reynolds, 1993), this research investigated how dense outflow from the Persian Gulf (penetrating to a depth range of 150–300 m in the Oman Sea through the Strait of Hormuz) affected dissolved oxygen levels in this sea and their spatial variations, using available global measurement data. Furthermore, this study examined the trends of dissolved oxygen changes in the Persian Gulf over recent decades and their effects on decreasing dissolved oxygen levels in the Oman Sea.

To investigate the influence of high salinity and dense outflow from the Persian Gulf on dissolved oxygen levels in the Oman Sea, we utilized available global measurements of dissolved oxygen in both water bodies. We specifically analyzed long-term trends in dissolved oxygen in the Persian Gulf and their impact on the reduction of dissolved oxygen levels in the Oman Sea. Data on dissolved oxygen profiles at 25 points in the Oman Sea in 2010 and all available dissolved oxygen profiles in the Persian Gulf from 1980 to 2010 were downloaded from the World Ocean Database. At each study point in the Oman Sea, vertical profiles of dissolved oxygen at each depth were averaged, and this result was used as the annual mean profile for each point. The profiles obtained from the 25 study points were then analyzed using Ocean Data View software as four cross-sections to evaluate areas influenced by dissolved oxygen from the Persian Gulf outflow. To evaluate long-term trends in dissolved oxygen in the Persian Gulf, measurement data from 1980 to 2010 were studied as an annual average time series.

The results showed that, in general, the surface mixed layer of the Oman Sea has the maximum dissolved oxygen concentration. With increasing depth, this concentration decreases until it reaches 2 mg/L at a depth of about 80 m. In the layer between depths of 80 and 1000 m, hypoxic and anoxic conditions prevail in the sea. Between depths of 150 and 300 m, a relative increase occurs in dissolved oxygen concentration. The results demonstrated that the dense and salty outflow from the Persian Gulf, which has higher oxygen levels than the low-oxygen waters of the Oman Sea, increases dissolved oxygen levels by 1–4 mg/L in the depth range of 150–300 m in this sea. These effects are greatest along the southern coast; moving toward the Iranian coast, this influence diminishes. Additionally, along the southern coast of the Oman Sea, moving from the Strait of Hormuz to the Arabian Sea, these effects decrease. Considering these analysis results, a criterion of at least a 0.5 mg/L local increase in dissolved oxygen concentration at depths ranging from 150 to 350 m indicated that approximately 76,000 km2 of the Oman Sea is affected by the dense currents coming from the Persian Gulf. Examining dissolved oxygen concentration data from the Persian Gulf during 1980–2010 revealed that deoxygenation occurred in its deep waters at a rate of 0.13 mg/L per decade, with concentrations decreasing from about 4.5 mg/L in 1980 to 4.1 mg/L in 2010. Correspondingly, the influence of the Persian Gulf outflow on dissolved oxygen levels in the Oman Sea has decreased from an average of 1.2 mg/L to 0.7 mg/L.

This study assessed the effects of outflow from the Persian Gulf on dissolved oxygen in the Oman Sea. The results indicated a relative increment in dissolved oxygen concentration between 150 and 300 m in the Oman Sea. Considering that this zone is affected by the Persian Gulf outflow (Ghazi et al., 2021; Pous et al., 2015), this increase in oxygen levels can be attributed to these currents. Shenoi et al. (1993), Prasad et al. (2001), Acharya and Panigrahi (2016), and Queste et al. (2018) also demonstrated the penetration of dense Persian Gulf outflow below the thermocline of both the Oman and Arabian Seas, along with its effects on their temperature and salinity. The results showed that outflows from the Persian Gulf increase dissolved oxygen levels over a wide area of the Oman Sea, extending more than 160 km from the southern coast and up to 550 km from the Strait of Hormuz. These effects are greatest along the southern coast; moving toward the Iranian coast, these effects decrease and reach zero near the shores. The results also demonstrated that along the southern coast of the Oman Sea, moving from the Strait of Hormuz to the Arabian Sea, these effects on dissolved oxygen levels of the Oman Sea decrease. Examining dissolved oxygen concentration data from the Persian Gulf during 1980-2010 showed that dissolved oxygen in its deep waters decreased at a rate of 0.13 mg/L per decade; as a result, the ventilation effect of its outflows in the affected area of the Oman Sea has diminished.

In this study, the relationship between dissolved oxygen reduction in the Gulf of Oman and changes in effective factors such as temperature and thermal stratification, exchanges with nearby water bodies, biological activities, chlorophyll-a concentration, and primary production was studied. The data used in this research were extracted from the measurement data available on global sites such as the World Ocean Atlas and SeaWiFS and MODIS satellite images. Comparing the average annual DO profiles in the Gulf of Oman showed a decrease in the upper 1000 m layer in 2010 compared to 1960. The DO reduction in the surface mixed layer was 0.4 mg/l and reached 3.7 mg/l at 100 m depth. At depths more than 100 m, the amount of change decreased and reached 0.1 mg/l at 1000 m. Due to the deoxygenation, the thickness of the surface layer with DO˃2 mg/l decreased from 150 m to 80 m. The results demonstrated that the drop in DO in the surface mixed layer was consistent with the decrease in gas solubility caused by the temperature increment in this layer. Regarding the severe reduction of DO in the layer below the surface mixed layer, it was shown that the strengthening of thermal vertical stratification and intensification of thermocline (0.01°C/m increase in thermal gradient) were the most important factors.

Rivers are amongst the most important sources of water supply all over the world. Increasing urban development, industrial activities and a significant increase in the volume of urban effluents have polluted rivers and in many places water quality of these vital resources have been seriously endangered. In this study, using the QUAL2K numerical model, the quality status of Haraz River has been investigated and the impact of effluents entering the river has been identified. According to the results of qualitative modeling of the river in 2016, at kilometer 48, where the Amol diversion dam is located, the BOD rate suddenly increases which is due to the reduction of flow velocity and discharge and the increase in the discharge of pollutants to the river related to workshops and factories. In spring and summer the concentration of dissolved oxygen is critical in the reaches from Amol diversion dam to the estuary of the river by the Caspian Sea. In terms of acidity, the water of Haraz River is alkaline. The EC parameter also decreases along the river due to the gradual deposition of soluble and suspended solids into the water. The concentration of nitrate in water also increases after the diversion dam, although it is not significant in total.