Background and Objectives: Nayband Marine National Park (NMNP) as Iran's first and largest marine national park, is located on the coasts of Bushehr and Hormozgan provinces and features diverse terrestrial, coastal, and marine habitats and species including scleractinian corals, seagrass, seabirds, sea turtles and mangroves. Today, like other global ecosystems, this one is impacted by climate change, especially rising sea surface temperatures (SST) and terrestrial pollutant sources. Recent efforts by various researchers have focused on identifying robust tracers that reveal past environmental conditions in aquatic ecosystems, particularly coral reefs. Today, coral skeletal oxygen isotopes (δ 18O) are crucial tracer for reconstructing SST, while carbon isotopes (δ13C) record photosynthetic rates of coral symbiotic algae and terrestrial carbon delivery to coastal waters. The NMNP, despite exposure to climate change and pollution, lacks continuous historical climate records. Hence this study aims to reconstruct SST variability using δ18O, and identify coral skeletal carbon sources using δ13C, in Faviidae corals.
Methods: The upcoming research can be divided into four stages. Step (I); coral samples were collected in March 2023 from three sites: South Pars (Site I), Nayband Bay (Site II), and Bonood Beach (Site III). After transfer to the laboratory, coral samples were washed with distilled water, sun-dried, and sliced into 1-cm slabs parallel to the axis of maximum growth using a slicer. Step (II); coral slabs were X-rayed to reveal annual growth bands, using low-density bands (light bands) and high-density bands (dark bands) retrospectively from 2018 to 2023. Then powder was prepared from each coral annual growth bands. The powder was ground using a ceramic mortar and homogenized with a 63 μm sieve. Finally 50 mg of homogenized powder in to sterilized plastic vials was transported to Arak University's Stable Isotopes Research Laboratory for oxygen (δ18O) and carbon (δ13C) stable isotope measurement. In this laboratory, isotopic ratios (δ18O and δ13C) were measured using an Isotope Ratio Mass Spectrometer (IRMS) device in the Vienna Peedee belemnite (VPDB) and per thousand (%o) scale. To normalize the δ18O isotope values, NBS-19 and IAEA-603 standards were used as Isotopic Reference Materials (IRM), and internal analytical precision for both δ18O and δ13C was 0.02‰. Step (III); to reconstruct the SST, the coral carbonate skeleton's δ18O value was converted from the VPDB scale to the Vienna Standard Mean Ocean Water (VSMOW) scale, following International :union: of Pure and Applied Chemistry (IUPAC) standards. Then, the SST was obtained using the equation (SST (°C) = -4.35 × δ¹⁸O VSMOW + 8.00). Finally to validate the estimated SST, water temperature from the European Space Agency (ESA) satellite and CTD device readings from 2022-2023 at sampling stations were utilized. Finally, ESA satellite and CTD water temperature records from sampling stations were used to validate the estimated SST. Step (VI); the carbon element type (organic or inorganic) in coral skeletal tissue was determined using δ13C (Huant classification method) to assess the impact of human-induced pollution on the coral ecosystem.
Findings: The results showed that; across the study area, δ¹⁸O values ranged from -3.94‰ to -5.39‰, with an average of -4.95‰. Specifically, Site I exhibited values from -4.94‰ to -4.84‰ (average = -4.41‰), Site II from -4.98‰ to -5.39‰ (average =-5.21‰), and Site III from -5.11‰ to -5.35‰ (average = -5.24‰). According to these results, the δ18O values are ordered as follows: Site I > Site II > Site III. Also the average SST ranged from 25.13 - 31.45 OC with an average of 29.55 OC in the study area. Site I had an estimated temperature range of 25.13–29.03 °C, with an average of 27.19 °C. Site II ranged from 29.66 – 31.45 °C, averaging 30.66 °C. Site III had a range of 30.24 – 31.27 °C, with an average of 30.80 °C. These results showed that SST values were increasing from site I to site II and site III. Site I recorded the lowest SST at 25.13°C in 2018, while Site II registered the highest at 31.45°C in 2019. The results showed that Site I had a higher δ¹⁸O value than the other two sites. This is due to differences in coral species type, deeper species establishment, and higher turbidity.
Also the result show that δ13C values ranged from -3.14 to +1.52 ‰ with an average of -1.05 ‰ in the study area. The average δ13C values for sites I, II, and III are -2.4‰, -0.85‰, and 0.097‰, respectively. Coral δ13C is influenced by water quality, temperature, and depth; lower quality and higher temperatures result in lower δ13C. It is worth noting that from 2017 to 2022, a decline in δ13C values indicates increased marine pollution and rising SST over the last decade. Also the Hunt's partitioning method revealed that the carbon in corals' skeletons is inorganic and originates from the ocean.
Conclusion: In general this study demonstrated high agreement between δ18O-derived SST variations in NMNP Faviidae corals and SST recorded by ESA satellite and CTD, demonstrating effectiveness and accuracy of using the δ18O variation for precise estimations of SST. This research indicated that the Favia pallida and Favia speciose could be used to develop proxies of long-term δ18O-derived SST variability in the Persian Gulf and other aquatic ecosystems. Due to the relatively short, 6-year time series, the relevance of these findings for reconstructing SST in NMNP and the Persian Gulf region is limited. Therefore, future researches requires longer coral time series and in situ SST measurements. |
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