Background and Objectives: Coral reef habitats of the Persian Gulf are ecologically and economically important, supporting biodiversity, food resources, coastal protection, research, and tourism. However, they are increasingly threatened by local and global stressors, including oil pollution, coastal development, nutrient and wastewater inputs, ocean warming, and acidification. Given the key role of carbonate chemistry in coral health, this study investigated seasonal variability of the carbonate system in Iranian waters of the Persian Gulf through one year of monthly monitoring at three reef regions—Khark, Kish, and Qeshm—combined with data from a summer 2021 oceanographic cruise.
Methods: Monthly sampling was carried out from November 2021 to December 2022 at 1 m below surface. Physical and chemical parameters, including temperature, salinity, dissolved oxygen, pH, nutrients, and total alkalinity (AT) were measured. Carbonate system parameters such as dissolved inorganic carbon (DIC), partial pressure of CO2 (pCO2), aragonite saturation state (Ωar), and salinity-normalized indices were calculated. Summer conditions across the Persian Gulf were further evaluated using data from the September 2021 research cruise.
Findings: Mean annual temperatures at Khark, Kish, and Qeshm were 26.6, 28.8, and 28.0 °C, respectively, with corresponding mean salinities of 39.9, 38.3, and 37.2. Mean AT values were 2534, 2456, and 2421 µmol/kg, and mean DIC values were 2147, 2078, and 2062 µmol/kg, respectively. Minimum DIC occurred in summer and maximum values during colder seasons. Annual mean pH (~8.04) and pCO2 (~410–416 µatm) showed no significant spatial differences, and reef waters alternated seasonally between acting as a CO2 source and sink. Waters at all sites remained supersaturated with respect to aragonite throughout the year, with highest saturation states in warmer months. Alkalinity depletion relative to the Indian Ocean source water dilution line indicates substantial biological CaCO3 precipitation within the Gulf, with the greatest reduction observed at Khark and the lowest at Qeshm, consistent with water residence time and proximity to the Strait of Hormuz. The ratio of organic carbon production to CaCO3 formation was estimated at 2.6:1 in summer and approximately 1.1:1 on an annual basis, indicating a relative increase in photosynthetic production compared to calcification in recent decades.
Conclusion: Carbonate chemistry in the reef waters of the Persian Gulf is strongly controlled by temperature, intense evaporation, water residence time, and biogeochemical processes. Although aragonite saturation remains consistently supersaturated, the strong seasonal temperature variability makes the bicarbonate-to-hydrogen ion ratio a more reliable indicator of seasonal effects on coral calcification. The increasing dominance of photosynthesis relative to calcification in recent decades is likely linked to climate change, nutrient enrichment, and declining coral cover. Overall, despite persistent carbonate supersaturation, Persian Gulf reef habitats remain vulnerable to climatic and anthropogenic pressures, highlighting the need for continued carbonate chemistry monitoring. |
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