Distribution gradient of fucoid algae (Sargassaceae, Phaeophyta) along the coastline of Okinawa Island, southern Japan: Relationship to environmental factors

Coastal ecosystems can provide significantly high ecosystem services via various processes. Tropical and/or subtropical coastal ecosystems are constituted by reef-building corals, macroalgae, and seagrasses. The study was conducted in a subtropical coastal area, Okinawa Island, Japan, where macroalgal beds are distributed. Since few studies have focused on the macroalgal community compared to corals in the study area, the relationship between macroalgal (especially Sargassaceae species) distribution and environmental factors were evaluated. Environmental parameters used in the analysis were obtained by field observation, geographic analysis, satellite observation of water quality, and numerical simulation of wave height. Sampling was conducted mainly between May and August in 2018 and 2019, and 31 sampling stations were established along the coastline of Okinawa Island. Though hermatypic corals occurred in most of the stations, the distribution of Sargassaceae was spatially skewed from the southern and central part of the east coast. Canonical correspondence analysis (CCA) was applied to obtained environmental variables and presence–absence data on macroalgae (Sargassaceae), corals, and seagrasses. CCA results indicated that the geographical setting (openness) and wave exposure in winter are the primary factors determining the macroalgal distribution, whereas nutrient (dissolved inorganic nitrogen, phosphorous, and silicon) concentrations at each sampling occasion were not significant factors. Since coastal ecosystems are threatened by both local and global impacts, elucidation of the major mechanism limiting Sargassaceae distribution is significant for coastal ecosystem management, here we use Okinawa Island as an example.