Where Does 'Spring Turbidity' Actually Occur? Quantitative Analysis of 25 Sites

Every spring, Japanese divers brace for "spring turbidity" (haru-nigori) — a seasonal drop in underwater visibility that can last from March through May. But how bad is it really, and does it affect all dive sites equally? Using over 46,000 actual visibility observations from our database, we quantitatively compared 25 dive sites across Japan to find out. The results were surprisingly varied.

What Is Spring Turbidity?

Spring turbidity refers to the seasonal decline in water clarity during spring months. The primary cause is a massivephytoplankton bloom known as the spring bloom. Nutrients that accumulated during winter are unleashed by increasing daylight and rising water temperatures, triggering explosive phytoplankton growth that turns the water green and murky.

While this phenomenon is well-documented in temperate oceans worldwide, its severity varies enormously depending on geography, ocean currents, depth, and seabed topography. In particular, Pacific-facing sites influenced byKuroshio Current branches,Sea of Japan sites, and subtropical Okinawan sites experience fundamentally different mechanisms and impact levels.

Methodology

We defined "spring" as March through May and compared each site's spring average visibility against its annual average. The change rate is calculated as follows:

Data comes from 46,000+ actual observations collected from dive shop blogs across Japan. Sample sizes vary by site, so results for sites with fewer observations should be treated as indicative.

Full 25-Site Spring Turbidity Comparison

The table below ranks all 25 sites by spring visibility decline. Longer red bars indicate stronger spring turbidity impact, while green bars show sites where visibility actually improves in spring.

Sites with Strong Spring Turbidity (>15% Drop)

Kannoura (-18.1%): The Hardest Hit

Kannoura, located at the border ofTokushima andKochi Prefectures, showed the largest spring turbidity effect in our analysis. Annual average visibility of 9.1m drops to 7.5m in spring — an 18.1% decline. The confluence of nutrient-rich coastal water from theKii Channel and the spring plankton bloom is thought to cause this pronounced visibility reduction.

Mikomoto (-17.2%): The Pelagic Point's Trade-Off

Mikomoto Island, famous for itshammerhead shark aggregations, drops from an annual average of 12.3m to 10.2m in spring (17.2% decline). As an offshore site directly exposed to the Kuroshio, when the current shifts away from the coast, nutrient-rich inshore water moves in and phytoplankton blooms intensify. However, winter (December-January) visibility averages above 13.6m, showing that this site has one of the widest seasonal swings.

Izu Oceanic Park (-16.4%): The Poster Child of Spring Turbidity

Izu Oceanic Park (IOP) is the flagship dive site on the east coast of the Izu Peninsula, with a robust dataset of 3,151 observations. Annual average visibility of 13.8m drops to 11.5m in spring (16.4% decline). Facing Sagami Bay, it is directly exposed to the spring phytoplankton bloom. Our AI prediction model also shows reduced accuracy for IOP during spring (even the best model with AI accuracy 82% has larger errors in spring), suggesting that spring turbidity is inherently difficult to predict.

Echizen (-14.0%): A Different Mechanism on the Sea of Japan

Echizen is located on theSea of Japan coast ofFukui Prefecture. Its annual average of 8.9m drops to 7.6m in spring (14.0% decline). The mechanism here differs from Pacific sites: snowmelt runoff in spring carries sediment and nutrients from rivers into the sea. Since winter diving in the Sea of Japan is nearly impossible due to storms (our database contains only 23 observations from November to February), the effects of snowmelt are still present when the diving season resumes in spring.

Amami Oshima (-14.1%): A Surprising Result

Amami Oshima, despite its subtropical location, shows a relatively large 14.1% spring decline — from 19.2m annual to 16.5m in spring. Amami sits near the bifurcation of the Kuroshio and theTsushima Current, and seasonal shifts in the Kuroshio path may increase exposure to nutrient-rich coastal water. The fact that Amami behaves differently from more southerly sites like Ishigaki and Kerama within the same Okinawan region is noteworthy.

Moderate Spring Turbidity (10-15% Drop)

Seven sites fall into the moderate category: Kushimoto (-13.7%), Futo (-13.1%), Miyakejima (-12.5%), Kashiwajima (-12.4%), Tajiri (-11.0%), Amami Native Sea (-10.8%), and Futone (-10.1%).

A common feature of these sites is that they are open-ocean facing.Kushimoto sits at the tip of theKii Peninsula,Miyakejima is an offshore island, andKashiwajima faces theBungo Channel. While these sites benefit from the Kuroshio during other seasons, they are directly exposed to the large-scale spring plankton bloom.

Tajiri (Iwami, Tottori) is a Sea of Japan site where snowmelt influence is also suspected, though its decline of -11.0% is less severe than Echizen.

Sites with Weak or No Spring Turbidity

Sheltered Bays and Enclosed Environments

Osezaki Bay (0.0%) and Hirasawa (-0.9%) are virtually unaffected by spring turbidity. Both sites are located in sheltered environments withinSuruga Bay, where topography prevents plankton-rich offshore water from directly entering. Even Osezaki Outer (-3.0%), in the same area but facing the open sea, shows only slight impact. This demonstrates that a few hundred meters of topographic difference can dramatically change a site's susceptibility to spring turbidity.

Okinawa and Subtropical Sites

Yonaguni (-2.4%), Kerama (-1.9%), and Ishigaki (+2.2%) are barely affected by spring turbidity.Okinawan waters remain warm year-round (above 20°C even in winter), and do not experience the pronounced seasonal plankton bloom characteristic of temperate seas.Tropical and subtropical waters maintain low nutrient concentrations (oligotrophic conditions) throughout the year, suppressing seasonal mass phytoplankton blooms (JMA: Ocean Nutrients).

Sites That Actually Improve in Spring

Remarkably, Ishigaki (+2.2%) and Shirahama (+2.1%) show improved visibility in spring. For Ishigaki, this is explained by the stable subtropical marine environment described above.Shirahama(Wakayama Prefecture), however, is in the temperate zone, making its spring improvement particularly intriguing. Shirahama is located where the Kuroshio main stream flows relatively close to shore, and it is possible that the Kuroshio's approach in spring brings in clearer offshore water, actually improving visibility (conjecture).

Why Do Sites Differ So Much?

1. Kuroshio Current Influence

The Kuroshio is a warm, clear-water current, but its branch currents can deliver nutrients that fuel coastal plankton blooms. The Kuroshio's path shifts seasonally (JMA: Kuroshio), and when it moves offshore (including the "large meander"), nutrient-rich cold water upwells along the coast, making plankton blooms more severe. Kannoura, Mikomoto, and IOP — all heavily affected sites — are positioned directly in the Kuroshio's zone of influence.

2. Topographic Shielding

The near-zero impact at Osezaki Bay and Hirasawa is due to their inlet and bay topography blocking offshore plankton-rich water. Even on the same Izu Peninsula, the contrast between ocean-facing IOP (-16.4%) and sheltered Osezaki Bay (0.0%) is over 16 percentage points. For divers, choosing sheltered bay sites during the spring turbidity season is a practical countermeasure.

3. Sea of Japan: A Different Mechanism

Sea of Japan sites like Echizen (-14.0%) and Tajiri (-11.0%) experience spring turbidity through a different pathway. Heavy winter snowmelt in spring sends massive river runoff carrying sediment and nutrients into the sea. This causes both physical turbidity from suspended particles and biological turbidity from the resulting plankton bloom. The Tsushima Current (the Sea of Japan's warm current) lacks the Kuroshio's influence, so recovery via warm offshore water is slower.

4. Subtropical Stability

The near-zero spring turbidity at Okinawan sites (Yonaguni, Kerama, Ishigaki) is due to the oceanographic characteristics of subtropical waters. In temperate seas, winter vertical mixing brings deep nutrients to the surface, fueling explosive phytoplankton growth once spring daylight increases. In contrast, subtropical seas maintain a persistentthermocline year-round, limiting nutrient supply from deeper water and preventing large-scale blooms (JAMSTEC: Ocean Basics).

Practical Advice for Divers

Avoiding Spring Turbidity

If visibility is your priority, December to February is the best season for Pacific-side sites. IOP averages 18.6m in January and Mikomoto hits 13.7m in December — their annual peaks. However, water temperatures drop to 14-16°C, making adrysuit essential.

Choosing Low-Impact Sites in Spring

If you must dive in spring, select sheltered bay sites (Osezaki Bay, Hirasawa) orOkinawan sites (Ishigaki, Kerama, Yonaguni) to minimize spring turbidity impact. Ishigaki actually shows improving visibility in spring, making March to May a particularly good time to visit.

Embracing Spring Turbidity

As a counterpoint, spring turbidity means plankton-rich water with abundant food for fish.Macro photography and nudibranch observation do not require high visibility, and the plankton concentration can attract interesting marine life. Osezaki Bay, unaffected by spring turbidity and known as a "macro paradise" (Marine Diving web), is ideal for spring diving.

AI Prediction Model and Spring Turbidity

Our AI visibility prediction model incorporates spring turbidity indicators. It uses satellite-derivedchlorophyll-a concentration (from NOAA ERDDAP) as an input, with plankton abundance indicators andunderwater light penetration indicators serving as key predictors. Additionally, seasonal pattern inputs help the model capture spring turbidity timing.

However, the exact timing of the spring bloom can shift by several weeks year-to-year, making day-level precision challenging. Future improvements in historical data inputs (3-day and 7-day rolling averages of weather data) aim to enhance spring turbidity prediction accuracy.

Conclusion

Spring turbidity is not a uniform nationwide phenomenon — it ranges from -18% to +2% across sites. The variation is driven by a complex interplay of Kuroshio positioning, bay openness, latitude (subtropical vs. temperate), and Sea of Japan snowmelt dynamics.

• Most affected: Kannoura (-18.1%), Mikomoto (-17.2%), IOP (-16.4%)
• Nearly unaffected: Osezaki Bay (0.0%), Hirasawa (-0.9%), Kerama (-1.9%)
• Actually improves in spring: Ishigaki (+2.2%), Shirahama (+2.1%)

When planning your spring diving trip, use this quantitative data to make informed site-selection decisions.