Spring Diving in Kagoshima: Adventures in the Seaweed Forest - 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 Sakurajima, Kagoshima's iconic volcano, have been attracting growing attention in Japan's diving community.

One of the main reasons this region has become so popular is the underwater seaweed forests that flourish in abundance each spring. "Chosuuro," located right next to downtown Kagoshima, is an area formed by the construction of a breakwater. Though it is nearly 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 a remarkable pace, creating an otherworldly 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 vitally important ecological process. Chosuuro is one of the best sites where this cycle can be observed consistently, offering a window into how seasonal rhythms help keep Japan's seas so rich and productive.

When the Ocean Turns Green — Why Seaweed Begins to Grow in Spring

Throughout winter, Japan's coastal waters are clear and brilliantly blue. As the surface water cools, its density increases and it sinks, drawing nutrients up from the seafloor and mixing the water column thoroughly in the process. This vigorous winter mixing sets the stage for everything that follows in spring.

Then, sometime between February and March each year, as the winter chill fades, daylight hours lengthen, and water temperatures begin to rise, seaweed surges upward all at once. Often called the "forest of the sea," seaweed provides hiding places for small fish while supporting the growth of crustaceans and mollusks — which in turn attract larger predatory fish, bringing the entire food chain to life.

In other words, spring — with its abundance of seaweed — is the season when marine life breeds and grows most actively.

This phenomenon is not unique to Kagoshima. Many coastal waters along Japan's main islands follow the same pattern: as seaweed forests spread across the seafloor, gobies and wrasse come to spawn, and cuttlefish begin their courtship dances. Visibility may not be as sharp as in winter, but the density of biological activity is markedly higher.

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

In spring, the sea often takes on a pale green tint, and visibility can drop to under 10 m. In Japan, this phenomenon is known as "Haru-nigori." Its true cause is a massive bloom of phytoplankton — diatoms, dinoflagellates, cyanobacteria, and the like.

As sunlight strengthens, combined with the nutrients accumulated over winter and increasingly favorable water temperatures, phytoplankton explodes in growth. In fisheries science, this is called the "Spring Bloom" — a seasonal phenomenon observed in temperate waters around the world.

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

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

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

A nursery for juvenile fish
Spring is spawning season for many species — jack mackerel, sweetlips, damselfish, squid, and more. The plankton-rich environment allows juvenile fish to grow rapidly. Even with reduced visibility, this is, for young fish, an ocean overflowing with food.
Sustaining fishery resources

A healthy Haru-nigori ensures that fish stocks continue to grow through summer and autumn, making it a crucial natural mechanism underpinning Japan's coastal fisheries.
A synergy with the seaweed forest

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

Diving Kagoshima's Seaweed Forest — Tips for Taiwanese Divers

Step into a surreal, fairytale world conjured entirely by lush, thriving seaweed forests.

Diving Tips

Bubbles exhaled by divers can easily stir up floating algae near the surface, causing visibility to drop quickly.
A wide-angle lens is recommended for capturing the full expanse of this incredible green underwater world.
Mid-range shots of fish schools, large fish, or sea slugs are also very effective.
Because light scatters easily in these conditions, use your strobe at low power output.
Macro photography during this season often yields surprisingly rewarding results.

Haru-nigori Is the Ocean's "New Year": A Season to Embrace Life's Density Over Clarity

Spring brings dramatic change to the crystal-clear waters of winter, but turbidity is not a flaw — it is a sign of the ocean awakening. Japan's Haru-nigori embodies a distinctive seasonal rhythm, one in which the entire chain of life stirs back into visible motion once a year.

This is not a season for chasing perfect visibility. It is a season for feeling the richness and vitality of life in full swing. Why not make the trip to Kagoshima and experience the true abundance of Japan's spring sea for yourself?

My seasonal journey through Japan's seas continues. Next time, I'll return to my home base — Okinawa — to introduce the vibrant underwater world that comes alive as early summer arrives and marine activity begins to intensify.