For a long time, brown algae or kelp forests off the coast of Northern California have withstood every crisis, including the extreme heat waves at sea, which are usually cold here. Unusually strong heating in 2014 caused a sudden collapse, as 95 percent of the area’s original seaweed stock was destroyed and replaced by a kind of desert with large numbers of axolotls. This is what was stated in a report by biologist Meredith Macpherson of the University of California, Santa Cruz and her team at Communications Biology..
Kelp forests off Northern California thrive in cool, nutrient-rich sewage waters and form a species-rich ecosystem and nursery for many species of fish. During El Niño events and other warm phases of the sea, deep water uplift is prevented, and warm conditions are nutrient-poor, with brown algae growing worse and reducing their cover. Brown algae usually recover quickly and completely.
Due to the frequent and strong hot water bubbles as well as the excessive fertilization of water in the area in recent years, there has also been a massive death of starfish from 2013 onwards. The lack of oxygen made the animals more susceptible to disease, so they were attacked by bacteria and dissolved in mucus. However, starfish are the main predators of some sea urchins, which in turn graze aggressively on brown algae.
Without these opponents, axolotls were able to reproduce and intensely attack brown algae that had been polluted by the heat wave. “The coincidence of the two events greatly reduced the kelp forests,” said Raphael Codela, who participated in the study. The hungry axolotls also ensure that all the sprouting algae are immediately grazed again: instead of species and brown-rich kelp forests, the now bare areas expand into the sea.
Unlike other coastal areas of North America where brown algae grow, sea otters are not found here, which also eat axolotls and can contain their inhabitants. Sea otters have not been seen on the northern coast since 1800. Using satellite images from the past 35 years, we can see that brown algae get along well without them. But when the starfish disappeared as a predator to sea urchins, there was no one left in the system to keep them in check, “Macpherson says.
As long as neither the otters nor the starfish are anymore, things will look bad for the kelp forests – even if the other environmental conditions are correct. Every moss is eaten immediately. Attempts have been made to manually remove sea urchins by divers in order to give brown algae a head start. But without success. According to this, only an epidemic that leads to the mass extinction of sea urchins can help. Until then, Macpherson says, it is uncertain whether the ecosystem will truly recover. After all, the heat wave in the sea began to weaken for the first time since 2014, so scientists have not lost hope that conditions at the site can finally return to normal again.