[A-CG51] Coastal Ecosystems-2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG51-11] Changes in coral connectivity pathways under future climate scenarios along the Japanese coast

*Gabriel Lucas Xavier da Silva¹, Takashi Nakamura¹ (1.Department of Transdisciplinary Science and Engineering, School of Environment and Society, Institute of Science Tokyo)

Keywords:Coral reefs, Coral connectivity, Larval dispersal, Climate change, Kuroshio Current, Ryukyu Islands

Despite being sessile organisms in their adult phase, corals exhibit a high dispersal capacity due to the characteristics of their life cycle. During their reproductive phase, hundreds of larvae are released into the water column, allowing for dispersion over large distances. These coral larvae are transported by ocean currents until they find a suitable habitat to settle or perish, a process known as coral connectivity. As the climate crisis intensifies, coral populations are suffering from bleaching and mortality events due to excessive heating near the tropics. In this context, poleward coral expansion has been reported along the Japanese coast, presenting challenges for coral conservation in a changing climate. This study aims to assess coral connectivity pathways under historical and future climate conditions along the Japanese coast to understand potential variability in coral dispersal patterns and identify key areas that can support long-term coral poleward expansion. 3D larval dispersal simulations were performed using a Lagrangian particle tracking model coupled with hydrodynamic data from the Future Ocean Regional Projection (FORP) JPN02-V4 dataset, incorporating generalist life-history traits of Japanese corals. Simulations were conducted over three years, following the Representative Concentration Pathways (RCPs) climate change scenarios: 2005 (Historical), 2100 (RCP2.6), and 2100 (RCP8.5). Particle release and sink sites were defined based on the coral population distribution along the study area, obtained from open-source databases and literature reports. Release dates were modeled to reflect the seasonal behavior of coral spawning, following a temperature threshold function defined by the accumulation of temperature anomalies above 13°C starting February 1st. Once the accumulated threshold exceeded 1000°C days at a given site, spawning was assumed to occur on the next full moon and continue for 14 days. Each particle was assumed to have a pelagic larval duration of 28 days, a pre-competency period of 5 days, and to be neutrally buoyant with seawater. Preliminary results indicate a weakening of larval dispersal potential along the Kuroshio Current system between 25–32°N under RCP8.5 for the year 2100. This weakening may be linked to future changes in Kuroshio Current patterns, such as decreases in overall transport or increases in eddy activity. Contrary to current observations, the results suggest that coral spawning dates may become more uniform along the Japanese coast, with most coral sites reaching the spawning temperature threshold approximately 60 days after February 1st, compared to the historical range of 75 to 125 days. This shift could lead to a coral spawning anticipation of up to three months at certain locations. The combination of these factors led to a decrease in coral connectivity among the coral populations along the Ryukyu Islands, with connections between key coral hotspots such as Yaeyama, Okinawa, and Amami islands being completely disrupted in future years under RCP8.5. However, northern sites like Kyushu and Honshu Islands experienced an overall increase in coral connectivity under the same scenario. These differences may be linked to the observed changes in the Kuroshio Current system in future years. Although the current results highlight important insights regarding coral connectivity under future climate scenarios, further simulations are required to better understand and validate the observed findings. Ongoing work involves conducting particle tracking simulations over a longer time span (> 10 years), calibrating release dates with spawning reports, and unraveling the main oceanographic or climatic drivers for the observed changes in connectivity pathways. Ultimately, these results may contribute to the planning of management practices and the establishment of conservation units to secure coral conservation under a changing climate in Japan.