A forest vanishes
Off the coast of Shikotan Island, a dramatic ecological shift is underway. Dense underwater forests of kombu kelp (Saccharina japonica) are disappearing, leaving behind rocky plains covered in mostly crustose coralline algae. This widespread phenomenon, known in Japan as "isoyake" or "sea desertification," transforms a productive habitat into a barren landscape. The cause of this change is the sea urchin, specifically species like Mesocentrotus nudus.
Historically, cold water temperatures limited urchin populations and their grazing pressure. Now, warming sea temperatures are altering this balance. The mean water temperature in the East/Japan Sea has risen by nearly 1.7°C over the last half-century. This increase boosts sea urchin metabolism and reproductive success, allowing their populations to expand. These energized urchins consume kelp faster than it can regrow, clearing vast areas of the seafloor. Once the kelp is gone, the urchins can survive in a starved state for long periods, preventing any potential recovery of the kelp forest and maintaining the barren state.
A tale of two currents
Shikotan’s marine ecosystem is governed by the confluence of two major ocean currents. The Oyashio Current, originating in the subarctic, flows south, bringing cold, nutrient-rich water that has historically supported the island's immense kelp beds. This current is exceptionally high in nutrients, with nitrate concentrations exceeding 10 µmol/L, which fuels the high primary productivity of the region.
From the south, the warmer, more saline Soya Warm Current—a branch of the Tsushima Current—is extending its influence northward. The meeting point of these cold and warm currents creates a productive transition zone. However, as ocean temperatures rise, the warmer Soya Current is strengthening, altering the thermal conditions around the Kuril Islands. This shift favors the sea urchins over the cold-adapted kelp, disrupting the entire coastal food web. The loss of kelp beds affects the populations of abalone, fish, and other invertebrates that rely on the forests for food and shelter, impacting local fisheries.