How to Avoid Spring Plankton Bloom in Japan: Site-by-Site Data Guide (Mar–May)
2026-03-11
March 11, 2026 · 12 dive sites analyzed from real observation data
What Is Spring Plankton Bloom and Why Does It Happen?
Spring plankton bloom ('haru-nigori') refers to the explosive proliferation of phytoplankton as water temperatures rise in March–May, dramatically reducing ocean visibility. Since phytoplankton photosynthesize using nutrients and sunlight, the longer days and rising temperatures of spring create peak growth conditions. At many Izu Peninsula sites, winter (January) visibility of 15–18m crashes to 9–10m by April–May. We compare 12 sites to distinguish those affected versus those immune to spring murk.
Site Classification by Spring Murk Impact
Maintains winter clarity through spring. Blessed with Okinawan oceanic water.
- Yonaguni (Jan 22.4m→May 24.2m)
- Kerama (Jan 18.1m→May 19.4m)
- Ishigaki (Jan 20.9m→May 20.1m)
Some decline but modest. Continued oceanic water influence.
- Ito (Jan 18.8m→May 15.9m)
Clear spring turbidity. Some recovery by May.
- Kushimoto (Jan 13.8m→May 10.8m)
- Kashiwajima (Jan 16.8m→May 11.5m)
- Tajiri (Jan 10.7m→May 8.7m)
- Mikomoto (Jan 13.6m→May 9.7m)
- Hirasawa (Jan 11.5m→May 8.2m)
6m+ difference from winter. Classic Izu Peninsula coastal sites.
- IOP (Jan 18.6m→May 10.6m)
- Futo (Jan 15.6m→May 9.2m)
- Kumomi (Jan 14.2m→May 8.8m)
Month-by-Month Visibility: January–June
| Site | Jan | Mar | Apr | May | Jun | Drop |
|---|---|---|---|---|---|---|
| Yonaguni | 22.4m | 23.5m | 23.9m | 24.2m | 24.4m | ±0 |
| Kerama | 18.1m | 19m | 18.6m | 19.4m | 19.7m | ±0 |
| Ishigaki | 20.9m | 21.9m | 21.2m | 20.1m | 20.1m | -0.8m |
| Ito | 18.8m | 16.4m | 14.8m | 15.9m | 15.7m | -2.9m |
| Kushimoto | 13.8m | 10m | 10m | 10.8m | 12.2m | -3.8m |
| Kashiwajima | 16.8m | 12m | 10.4m | 11.5m | 12.1m | -6.4m |
| Tajiri | 10.7m | 8.2m | 8.2m | 8.7m | 9.7m | -2.5m |
| IOP | 18.6m | 13.7m | 10.1m | 10.6m | 11.3m | -8.5m |
| Futo | 15.6m | 11.6m | 9.5m | 9.2m | 9.8m | -6.4m |
| Kumomi | 14.2m | 10.8m | 10m | 8.8m | 9.2m | -5.4m |
| Mikomoto | 13.6m | 11.3m | 9.9m | 9.7m | 12.3m | -3.9m |
| Hirasawa | 11.5m | 9.4m | 8.4m | 8.2m | 8.2m | -3.3m |
'Drop' = difference between Jan and April (worst spring month). Red columns indicate spring bloom peak.
Why Some Areas Escape Spring Bloom
Yonaguni, Kerama, and Ishigaki are largely immune to spring bloom. The reason is direct oceanic water influence. The Kuroshio current or southern oceanic water continuously supplies these coastal areas, preventing the 'nutrient accumulation' necessary for plankton proliferation. Oceanic water is nutrient-poor (oligotrophic), so phytoplankton doesn't explode even in spring.
Ito (Chiba): Unusually Mild Spring Bloom for a Coastal Site
Ito in Chiba drops from 18.8m in January to 14.8m in April, but recovers to 15.9m in May. Located at Tokyo Bay's entrance where water from Sagami Bay flows in, Ito benefits from mild oceanic influence that limits spring bloom. At 14.8m even in April, it remains one of the best options for those wanting to avoid spring murk near the Kanto region.
IOP's Spring Is Harsh: A Dramatic 8.5m Drop
IOP has Japan's best Izu Peninsula winter visibility (18.6m in January), but crashes to 10.1m by April — an 8.5m drop, the largest of all 12 sites. May remains flat at 10.6m, with meaningful recovery only from June (11.3m) onward. Those planning spring trips to Izu must fully account for this risk.
Where Should You Dive in March–May?
| Month | Recommended | Avoid |
|---|---|---|
| March | Ishigaki (21.9m), Yonaguni (23.5m), Kerama (19.0m), Koganezaki (15.1m) | Echizen (7.4m), Hirasawa (9.4m), Aoumi (9.7m) |
| April | Yonaguni (23.9m), Ishigaki (21.2m), Kerama (18.6m), Ito (14.8m) | IOP (10.1m), Futo (9.5m), Echizen (7.6m) |
| May | Yonaguni (24.2m), Kerama (19.4m), Ishigaki (20.1m), Ito (15.9m) | Futo (9.2m), Hirasawa (8.2m), Kumomi (8.8m), Echizen (7.7m) |
Summary
If you want good visibility in spring (March–May), the answer is clear: Okinawa (Yonaguni, Kerama, Ishigaki) is unaffected by spring bloom and maintains 20m+ clarity. In the Kanto region, Ito (Chiba, ~15m) is the best alternative. Spring diving in Izu Peninsula is hard to recommend for visibility, but if you're after spring-specific marine life (nudibranchs, sea perch schools), the lower clarity may be an acceptable trade-off.
Data source: scraped from public dive logs. Monthly averages from real observations.
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