Avoiding Spring's Worst: Predicting the Lowest Visibility Weeks in March–April

2026-03-16

Every year from March to April, dive sites along Japan's main island of Honshu suffer from 'spring turbidity.' Phytoplankton blooms turn the water green, pushing visibility to its annual lowest. But the timing of this bottom varies by site. Using real observation data, we can predict when the worst weeks hit — and plan around them.

7.1m

Kushimoto Mar (lowest)

10.1m

IOP Apr

24.9m

Yonaguni Mar

15.8m

Koganezaki Mar

Sites That Hit Bottom in Spring: Worst Month & Visibility

Site	Worst Month	Lowest Vis	Annual Avg	Drop
Kushimoto	Mar	7.1m	11.9m	-4.8m
Futo	Apr	9.5m	11.2m	-1.7m
IOP	Apr	10.1m	13.4m	-3.3m
Kumomi	Apr	7.8m	9.7m	-1.9m
Hirasawa	Apr	5.6m	7.3m	-1.7m

Kushimoto bottoms out in March, while Izu Peninsula sites (IOP, Futo, Kumomi, Hirasawa) hit their lowest in April. Kushimoto, at the southern tip of the Kii Peninsula, receives direct Kuroshio influence, which likely shifts the timing of its spring bloom.

Why Visibility Drops in Spring

1. Spring Phytoplankton Bloom

Nutrients (nitrogen, phosphorus) accumulated during winter trigger explosive phytoplankton growth as daylight increases and water warms. This colors the water green and significantly reduces visibility.

Reference: JAMSTEC Spring Bloom Mechanisms (https://www.jamstec.go.jp/e/)

2. Snowmelt Runoff

Snowmelt from mountains flows into the sea via rivers, carrying terrestrial sediment and organic matter. Kushimoto's March bottom may reflect earlier snowmelt from the Kii Mountains.

3. Vertical Water Mixing

In winter, surface and deep water temperatures equalize, promoting vertical mixing. This stirs up bottom sediment, extending turbidity into early spring.

Reference: JMA Ocean Temperature Knowledge Base (https://www.data.jma.go.jp/gmd/kaiyou/)

Clear Water in Spring: Escape Destinations

Site	Mar	Apr
Yonaguni	24.9m	26.1m
Kerama	18.8m	19.5m
Ishigaki	21.2m	22m
Koganezaki	15.8m	13.2m

Conclusion: Go South in Spring, or Try Koganezaki

Okinawa and remote islands are largely unaffected by spring bloom, offering 20m+ visibility even in March. If staying on Honshu, Koganezaki (15.8m in March) is the exceptional choice — the most reliable clear-water site on the Izu Peninsula in spring.

Koganezaki is located in West Izu, in a bay sheltered from open-ocean influence. Since spring turbidity mainly spreads from the open ocean side, the bay's inner position delays the impact, keeping conditions clearer for longer.

Can AI Predict the Spring Visibility Bottom?

Our AI model achieves 82% prediction accuracy at IOP. For spring turbidity prediction, satellite data (chlorophyll concentration and light attenuation) are especially important impact factors. Since plankton blooms can be observed in real-time via satellite imagery, the model can predict visibility drops several days in advance.

However, predicting the exact peak date of spring turbidity to the week remains challenging. Bloom onset can shift by 2–3 weeks between years, limiting long-range forecasts. The best current approach is to combine recent satellite data with weather forecasts and make your final decision one week before the dive.

Spring Planning: Practical Advice

If Diving the Izu Peninsula

Early March is often still tolerable. Early to mid-April carries the highest risk. Consider bay-sheltered sites like Koganezaki or Osezaki.

If Diving Kushimoto / Kii Peninsula

March is the riskiest month (7.1m). Visibility tends to recover by April. Autumn (October, 14.2m) is Kushimoto's clear best season.

If Budget Allows

Yonaguni (Mar 24.9m), Kerama (18.8m), and Ishigaki (21.2m) are completely unaffected by spring bloom. Great escape from Honshu. March off-season rates make Okinawa relatively affordable.

About the Data

Data in this article is aggregated from our database of 46,000+ real dive log observations. Monthly average visibility by site is used. Kushimoto's March figure (7.1m) references nearby Kannoura data as an indicator of southern Kii Peninsula trends. Spring bloom timing can vary by 2–3 weeks between years.

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