Warming takes toll on undersea forests of kelp

APPLEDORE ISLAND, Maine — When diving in the Gulf of Maine a few years back, Jennifer Dijkstra expected to be swimming through a flowing kelp forest that had long served as a nursery and food for juvenile fish and lobster.

But Dijkstra, a University of New Hampshire marine biologist, saw only a patchy seafloor before her. The sugar kelp had declined dramatically and been replaced by invasive, shrub-like seaweed that looked like a giant shag rug.

“I remember going to some dive sites and honestly being shocked at how few kelp blades we saw,” she said.

The Gulf of Maine, stretching from Cape Cod to Nova Scotia, is the latest in a growing list of global hot spots losing their kelp, including hundreds of miles in the Mediterranean Sea, off southern Japan and Australia, and parts of the California coast.

Among the world’s most diverse marine ecosystems, kelp forests are found on all continental coastlines except for Antarctica and provide critical food and shelter to myriad fish and other creatures. Kelp also is critical to coastal economies, providing billions of dollars in tourism and fishing.

The likely culprit, according to several scientific studies, is warming oceans from climate change, coupled with the arrival of invasive species. In Maine, the invaders are other seaweeds. In Australia, the Mediterranean and Japan, tropical fish are feasting on the kelp.

“Collectively these changes are part of a recent and increasing global trend of flattening of the world’s kelp forests,” said the University of Western Australia’s Thomas Wern-berg, coauthor of a 2016 study in the Proceedings of the National Academy of Sciences, which found that 38 percent of kelp forest declined over the past 50 years in regions that had data.

Kelp losses on Australia’s Great Southern Reef threaten tourism and fishing industries worth $10 billion. Die-offs contributed to a 60 percent drop in species richness in the Mediterranean and were blamed for the collapse of the abalone fishery in Japan.

The Pacific Coast from Northern California to the Oregon border is one place that suffered dramatic kelp loss, according to Cynthia Catton, a research associate at the Bodega Marine Laboratory at UC Davis. Since 2014, aerial surveys have shown that bull kelp declined by over 90 percent, something Catton blamed on a marine heat wave along with a rapid increase in kelp-eating sea urchins.

Without the kelp to eat, Northern California’s abalone fishery has been harmed.

“It’s pretty devastating to the ecosystem as a whole,” Catton said. “It’s like a redwood forest that has been completely clear-cut. If you lose the trees, you don’t have a forest.”