California’s kelp forests struggle to recover a decade after collapse

More than 10 years after unusually high water temperatures in the Pacific Ocean killed off Northern California’s aquatic kelp forests, the region has still not recovered, reports David Helvarg, executive director of ocean conservation group Blue Frontier, for Mongabay. From 2013 to 2017, a mass of unusually warm water nicknamed “the Blob” hugged California’s coast, leading to kelp forests collapsing. An estimated 95% of bull kelp (Nereocystis luetkeana), the main species of the region’s aquatic kelp forests, died. Helvarg dove in Casper Cove, off Fort Bragg, a town in Northern California, and found that kelp forests there had still not recovered on a significant scale, except for some coves where more intensive recovery efforts are underway. In one dive, he counted 120 purple urchins (Strongylocentrotus purpuratus) within what he guesstimated was just 1 square meter (11 urchins per square foot). Urchins devour weakened kelp forests, and for kelp to thrive, the number of urchins should be kept low, at around 2 per square meter, Helvarg notes. Before the Blob, purple urchins were kept in check by the sunflower sea star (Pycnopodia helianthoide), a starfish that can grow up to 24 arms and weighs up to 5.9 kilograms (13 pounds). The critically endangered starfish is the major predator of urchins in Northern California, but the species lost 99% of its population to a disease outbreak that scientists say was helped by the warming waters. Without sea stars, the purple sea urchins multiplied. One 2021 study estimated that their population grew by…This article was originally published on Mongabay