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

[AHW27-12] Phosphate Loading From a Freshwater Lens and Its Contribution to Exchangeable Phosphate in Seawater: Implications for Coral Reef Ecosystems

*Jun Yasumoto^1,2, Leong Chris¹, Ko Yasumoto³, Minami Ryusei⁵, Song Ke-Han⁶, Mariko Iijima⁴, Takahiro Hosono⁷, Makoto Taniguchi¹, Ryuichi Shinjo¹ (1.Research Institute for Humanity and Nature, 2.Faculty of Agriculture, University of the Ryukyus, 3.School of Marine Biosciences, Kitasato University, 4.National Institute of Advanced Industrial Science and Technology (AIST), 5.Graduate School of Agriculture, 6.Faculty of Science, University of the Ryukyus, 7.International Research Organization for Advanced Science and Technology, Kumamoto University)

Keywords:Phosphate Loading, Freshwater Lens, Exchangeable Phosphate in Seawater, Coral Reef Ecosystems

Coral reefs provide vital ecosystem services, but rising sea temperatures and coastal development have led to severe degradation. In Sekisei Lagoon, Japan’s largest reef system, repeated bleaching events since 1998 have caused significant coral decline. While terrestrial nutrient input is a suspected driver, the lack of quantitative assessments has hindered effective countermeasures. A survey of 31 sites identified species-specific Exchangeable Phosphate in Seawater (EPS) thresholds impacting coral health. High-EPS sites were concentrated near Kuroshima’s cattle farms and shrimp aquaculture discharge areas, suggesting that terrestrial nutrient sources significantly contribute to phosphorus accumulation in reef sediments.
To further investigate this linkage, we conducted a detailed study on Kuroshima Island, where submarine groundwater discharge (SGD) may transport land-derived phosphorus to the reef environment. By developing a single-layer SWI model using MODFLOW 6 and FloPy, we analyzed freshwater lens thickness, tidal influences on saltwater intrusion, and SGD spatial patterns. Key parameters, including hydraulic conductivity, precipitation, and tidally varying groundwater levels, were optimized to improve simulation accuracy. In addition, phosphate concentrations within the freshwater lens were measured at multiple locations to assess terrestrial phosphorus accumulation and its potential transport via SGD. Field validation included resistivity surveys and continuous well groundwater level monitoring.
The results revealed intense SGD along the island’s northeastern margin, coinciding with high phosphate concentrations in adjacent coral reefs. This suggests that submarine groundwater flow, originating from the freshwater lens, acts as a major vector for nutrient transport, likely contributing to the accumulation of EPS in seafloor sediments. Recent studies have demonstrated a significant correlation between EPS accumulation, coral density decline, and increased coral bleaching, highlighting the potential role of SGD-driven phosphate loading in exacerbating these impacts. Given Kuroshima’s cattle-farming activities, nutrient enrichment from SGD may not only enhance phosphate retention in seafloor sediments but also influence coral reef health through changes in coral-algal dynamics.
These findings underscore the need to establish permissible phosphate loading limits for coral reef conservation. As the next step, our research will focus on estimating the maximum allowable phosphate loading based on EPS thresholds to inform management strategies for mitigating land-derived nutrient impacts in coral reef ecosystems.