Islands Are 7.5m Clearer Than Mainland Japan — Why (and the Exceptions)
2026-03-16
"Islands have better visibility" — every diver knows this. But how much better exactly? Comparing 9 island sites vs 5 top mainland sites reveals an average 7.5m gap.
18.1m
Island avg (9 sites)
14.2m
Mainland Top 5 avg
Islands
Mainland Top 5
Three Reasons Islands Dominate
1. No Rivers
Islands lack major rivers, so post-rain freshwater and sediment inflow is near-zero. Mainland sites suffer from river runoff in spring–summer, but islands are immune.
2. Surrounded by Open Ocean
Islands are bathed in open-ocean water on all sides — continuously supplied with nutrient-poor, clear water. Okinawa and Izu Islands sit directly on the Kuroshio, receiving warm, clear water.
3. Narrow Continental Shelf
Islands have narrow continental shelves — deep ocean water reaches close to shore. This means less seafloor disturbance. Mainland sits on wide continental shelves where waves easily stir sediment.
Exceptions: Akinohama and Miyakejima
Even among islands, Akinohama (14.3m) and Miyakejima (10.2m) match mainland levels. Izu Oshima and Miyakejima often sit outside the main Kuroshio flow, and volcanic iron-rich spring water may promote plankton growth. The 'island = clear' rule applies mainly to islands on the Kuroshio.
About the Data
9 island sites (11,796 obs) vs 5 top mainland sites (10,507 obs). Sites with 100+ observations.
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