Spring Diving in Japan: Seaweed Forest Adventure in Kagoshima - Japan Underwater Odyssey

This is the fourth installment in my series exploring the waters surrounding the Japanese archipelago. This month, I'd like to introduce Kagoshima, located at the crossroads of Japan's main island and its southwestern island chain. In recent years, the dive sites surrounding Kagoshima's iconic volcano, Sakurajima, have been attracting growing attention within Japan's diving community.

One of the primary reasons this area has become so popular is the spectacular underwater seaweed forest that flourishes here each spring. "Chosuuro," located right beside the city of Kagoshima, is an area formed by the construction of a breakwater. While it is almost entirely enclosed, it remains connected to the open sea through a narrow channel. Seaweed seeds and spores drifting in from the open ocean grow here at an astonishing rate, creating an indescribable underwater spectacle.

For Japan's highly productive coastal waters, the seasonal cycle of seaweed growth — rapid expansion in spring followed by a gradual die-off in early summer — is a critically important ecological process. Chosuuro is one of the best sites for consistently observing and understanding how this seasonal cycle helps keep Japan's coastal waters so abundant.

The Season the Ocean Turns Emerald — Why Seaweed Begins to Grow in Spring

Throughout winter, Japan's coastal waters are clear and a deep blue. As surface water cools and its density increases, it sinks, simultaneously drawing nutrients up from the seafloor to the surface — thoroughly mixing the water column in a vertical direction. This thorough winter mixing lays all the groundwork needed for spring.

Then, between February and March each year, as the winter chill weakens, daylight hours lengthen, and water temperatures begin to rise, seaweed bursts into growth all at once. Often called "forests of the sea," these seaweed beds not only provide hiding spots for small fish but also support the growth of crustaceans and mollusks, which in turn attract larger predatory fish — bringing the entire food chain to life.

In other words, the seaweed-rich spring season is the most important time of year for marine life reproduction and growth.

This phenomenon is not unique to Kagoshima; many coastal waters along Japan's main island follow the same pattern. Once seaweed forests spread across the seafloor, gobies and wrasse spawn among them and cuttlefish begin their courtship displays. Visibility may not match that of winter, but the density of biological activity is noticeably far greater.

What Is Haru-nigori? — Why the Spring Sea Turns Turbid

In spring, the sea often takes on a pale green hue, with visibility sometimes dropping to under 10 m. This phenomenon is known in Japan as "Haru-nigori." Its true cause is the explosive proliferation of phytoplankton — including diatoms, dinoflagellates, and cyanobacteria.

As sunlight intensifies, combined with the nutrients accumulated over winter and increasingly favorable water temperatures, phytoplankton undergo explosive growth. Known in fisheries science as the "Spring Bloom," this is a seasonal phenomenon commonly observed in temperate waters around the world.

For divers, reduced visibility may seem like a drawback; but from an ecological perspective, Haru-nigori is extraordinarily important.

Haru-nigori: The Ocean's "Big Feeding Moment"

• A surge in zooplankton
  Zooplankton such as copepods feed on phytoplankton and reproduce rapidly, while small fish and larval organisms in turn feed on these zooplankton.

• A nursery for juvenile fish
  Spring is the spawning season for many species, including jack mackerel, sweetlips, damselfish, and squid. The plankton-rich environment allows juvenile fish to grow rapidly. Even with reduced visibility, for young fish this remains "a sea full of food."

• Sustaining fishery resources

  Healthy Haru-nigori ensures that fish populations continue to grow through summer and autumn, making it a vital natural mechanism supporting Japan's coastal fishing industry.

• A synergistic effect with seaweed forests

  Seaweed provides shelter; Haru-nigori provides food. Together, they shape one of the most productive coastal ecosystems in the world.

Diving the Kagoshima Seaweed Forest — A Few Tips for Taiwanese Divers

Step into a surreal, fairy-tale world crafted from a lush and thriving seaweed forest.

Diving Tips

• Bubbles exhaled by divers can easily disturb the floating algae near the surface, causing visibility to drop rapidly.

• A wide-angle lens is recommended for capturing the full sweep of this incredible green underwater world.

• Mid-range shots of fish schools, large fish, or sea slugs are also very well-suited to these conditions.

• Because light scatters easily here, keep your strobe output low.

• Macro photography during this season often yields unexpectedly rewarding results.

Haru-nigori Is the Ocean's "New Year": A Season to Embrace Life's Density, Not Clarity

Spring brings dramatic change to winter's clear, blue waters — yet turbidity is not a flaw; it is a symbol of the ocean awakening. Japan's Haru-nigori represents a unique seasonal rhythm, where the chain of life stirs back to visible, tangible activity once a year.

This is not the season to chase perfect visibility. It is a time to feel the richness of life and its vibrant energy. Why not make a trip to Kagoshima and experience for yourself the true abundance of Japan's spring ocean?

And the journey through Japan's seasonal seas continues. Next time, I'll return to my home base — Okinawa — to introduce the colorful underwater world of early summer, as fish activity begins to surge.

Further Reading

• 石野 昇太 IG：shota_gorillahouse

• Where the Kuroshio Strikes: Diving with Hammerhead Sharks at Japan's Westernmost Yonaguni Island - Japan Underwater Odyssey

• The Story of Drift Ice Anchored in the Waters of Japan's World Natural Heritage Site - Japan Underwater Odyssey

• Diving in Ishigaki in Winter and Encounters with Manta Rays - Japan Underwater Odyssey