Kelp forests are out of sight and out of mind for most Californians, aside from the periodic encounter with seaweed during a walk on the beach. For marine biologists, kelp forests are out of sight as well, a situation that is now constantly on their minds.

As Staff Writer Mary Callahan reported on Sunday (“Kelp collapse imperils ocean ecosystem”), the stretches of kelp forest that once were abundant along the North Coast are not just disappearing. They are almost gone. An aerial survey has found that kelp beds cover 93 percent less surface area than in years past from San Francisco to Oregon.

The immediate problem is the proliferation of purple urchins which feed on the nutrient-rich kelp. The spread is so significant that one fisherman referred to the underwater environment that now exists just off the coast as “Urchintopia.”

Given how kelp forests and urchin populations rise and fall in different places, it’s not uncommon to see patches of what is described as “urchin barren” along the coast. But this is different.

“You just don’t see the whole forest up and down the entire North Coast go away,” said Mark Carr, a professor of ecology and evolutionary biology at UC Santa Cruz. It’s particularly concerning, biologists say, because kelp forests are considered some of the most productive ecosystems on Earth, and the disappearance of them has profound effects on populations of fish, including rockfish, red abalone and other sea life.

So what’s the root cause of the rise purple urchins? The answers are complex and varied. Much of it is due to warmer water and increases in acidity, which are throwing off the balance of the eco-systems that exist in the cool waters of the North Coast.

But researchers say it’s also a “perfect storm” of environmental shifts dating back some five years when an algal bloom off the coast released toxins that killed off many red abalone and set off a chain reaction of other impacts on sea creatures

Then in 2013 came the devastation of some 20 species of starfish due to what is known as sea star wasting disease. This had a direct impact on other invertebrates, including the rise of the purple urchins. Starfish are primary predators of purple urchins. So are sea otters, whose numbers also are down from yesteryear.

So what are the solutions? Those are as vague as they are elusive. The concern is the virtual disappearance of the bull kelp, which is known for its long, rubber-like stalks that look like bull whips. Such a fate would be disastrous. But biologists point out that kelp is a hearty species that can bounce back quickly if conditions change. The critical step is to continue public funding for researchers who are keeping a close eye on coastal conditions from places such as the UC Davis Bodega Marine Laboratory.

The importance of this kind of work has never been more evident. The search for solutions can’t begin without knowing the extent of the problem and the root of it — and making sure that what’s happening below the ocean surface stays in the public eye.