For a long time, brown algae or kelp face every crisis away from the Northern California coast, including major heat waves in the ocean, which are usually calm here. Unusually strong heating in 2014 led to a sudden collapse, eventually destroying 95 percent of the region’s original seaweed and replaced by a type of desert that had a vast population of sea chinins. What is it Biologist Meredith MacPherson from the University of California at Santa Cruz and her team in “Communication Biology” report.
Northern California’s kelp forests generally thrive in cold and nutrient-rich waters and form a species-rich ecosystem and a nursery for many species of fish. During El Niño events and other warm phases in the ocean, deepwater growth is prevented, and nutrient-poor, warm conditions dominate, in which brown algae worsen and cover up. Usually brown algae recover quickly and completely.
Starfish have also suffered from large-scale deaths since 2013 due to frequent and strong hot water bubbles in recent years, as well as water over-fertilization in the region. Lack of oxygen made animals more susceptible to disease, causing them to be attacked by bacteria and melted into mucus. However, starfish are the main predators of some sea urchins, which in turn harbor brown algae.
Without these opponents, sea urchins were able to multiply and intensify the brown algae being polluted by the heat wave. “The coincidence of the two incidents led to the fact that kelp forests were dramatically reduced,” says Rafael Kudela, who was involved in the study. Hungry sea urchins also ensure that all sprouted algae are immediately re-grazing: instead of species and structure-rich kelp forests, bare areas are now expanding into the sea.
Unlike other coastal areas of North America where brown algae grow, there are no sea otters, which also eat sea urchins and can be populated. “Sea otters have not been seen on the northern coast since 1800. Using satellite images of the last 35 years, we could see that the brown algae got along well without them. But when starfish disappeared as predators of sea urchins, there was no one left in the system to keep them in check, ”says MacPherson.
As long as neither the beaver nor the starfish return, things will be bad for the Kel forests – even if other ecological conditions are right. Every algae is eaten immediately. There have been attempts to manually remove sea urchins by divers to give brown algae a head start. But without success. According to this, only an epidemic that leads to mass extinction of sea urchins. But it’s still uncertain whether the ecosystem will actually recover, McPherson says. After all, for the first time since 2014, the heat wave in the ocean has begun to weaken, so scientists have not given up hope that conditions at the site may return to normal again.
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