[A-HW27] Biodiversity, nutrients and other materials in ecosystems from headwaters to coasts

[AHW27-P15] Relationship between Exchangeable Phosphate in Seawater (EPS) in sediment and coral reef abundance in Sekisei Lagoon

*Alice Yamazaki¹, Toma Tanigami¹, Ryota Ide¹, Nanami Nizusawa¹, Shugo Watabe¹, Mariko Iijima², Takahiro Hosono³, Jun Yasumoto⁴, Takashi Nakamura⁵, Ko Yasumoto¹ (1.Kitasato university School of Marine Biosciences, 2.The National Institute of Advanced Industrial Science and Technology, 3.Faculty of Advanced Science and Technology, Kumamoto University, 4.University of Ryukyus, Faculty of Agriculture, 5.University of Ryukyus, Faculty of Science)

Keywords:Coral decline, Calcareous sediments, Terrestrial loads, Exchangeable phosphate in seawater (EPS)

Sekisei Lagoon, located between Ishigaki and Iriomote Islands, is one of the largest coral reef areas in Japan and is designated as a national park. Since 1998, however, large- scale bleaching events have repeatedly occurred due to rising sea water temperatures, raising concerns about the decline of the ecosystem. In recent years, excessive nutrients input and other terrestrial loads have identified as additional threats to this ecosystem. In particular, it has been pointed out that terrestrial phosphates have been adsorbed and accumulated in calcareous sediments, potentially harming corals. Our laboratory defined "Exchangeable Phosphate in Seawater (EPS) " as phosphate adsorbed and accumulated on the surface of calcareous sediments as a new indicator of terrestrial loading on the corals. In this study, as part of the community monitoring under the Sekisei Lagoon Nature Restoration Project, initiated by the Japanese Ministry of the Environment in 2010, EPS values and the number of coral colonies and algal cover were continuously surveyed at 31 designated monitoring sites (S01 to S31) to investigate the relationship between EPS and coral reef abundance.
Sediment samples (n=3) were collected in September 2024 at the 31 designated monitoring sites in Sekisei Lagoon, and dried at 70℃ for 72 hours. After sieving, 6 g of the sediments with a particle size of 0.5 to 1 mm was mixed with 15 ml of seawater and shaken at 30°C for 72 hours. After centrifugation and filtration, phosphate concentrations were measured spectrophotometrically. PO₄-P in µg per gram of sediment was calculated and used as the EPS value. Spearman correlation analysis was performed using EPS values, density of coral colonies, coral bleaching status, and algal cover data from 2021 to 2024. EPS thresholds were statistically calculated using Threshold Indicator Taxa Analysis (TITAN).
EPS values were low in the outer reef areas, but high in the southwest and southeast of Taketomi Island and east of Kuroshima, suggesting influences of shrimp aquaculture effluent and agricultural discharge. Spearman correlation analysis showed that many dominant coral species had significant negative correlations with EPS, while the Sargassaceae family exhibited significant positive correlations each year, suggesting that higher EPS levels may promote algal blooms. A significant correlation between EPS and bleaching was also observed in 2021, but no such correlation with EPS was found in 2022 and 2024, when large-scale bleaching occurred, partly due to a decline in the number of coral colonies and the number of healthy corals. Statistical analysis determined EPS thresholds of 0.45 ± 0.15 µg/g for juvenile corals and 0.57 ± 0.15 µg/g for adult corals, indicating that juvenile corals are more sensitive to EPS. This study indicates that EPS can serve as a terrestrial load indicator for coral reef abundance and may contribute to coral reef conservation efforts.