Natasha Benjamin, an East Bay marine biologist and filmmaker, has spent many a weekend surfing and diving along the northern California coast, from West Marin to Mendocino. She remembers times when she got tangled up in bull kelp so thick, it was dangerous.
But then, in what seemed like an instant, the region’s dense undersea forest was gone. Between 2014 and 2016, 95 percent of the bull kelp between San Francisco and Oregon vanished—devastated by…