Warming waters, sea urchins are decimating kelp forests

Climate change, in tandem with a teeming sea urchin population, is killing bull kelp forests in the Salish Sea. To stem losses that already have kelp at historic lows in the Central Strait of Georgia, researchers are searching for the most heat-resistant kelp populations, and working to perfect a method of reintroducing the plants.

Increasingly, spiking summer water temperatures near the ocean’s surface are stunting bull kelp’s dandelion-like reproductive capacity. The adult plants release far fewer spores when water temperatures sit above the mid-teens. When temperatures stall above 18ºC, the plants disintegrate and die.

“The [high] temperature either ends the life of the kelp plant, or it shortens its reproductive season, those are the two options,” said Bill Heath, biologist and program director for the Comox Valley’s Project Watershed.

This article reprinted as a joint venture with Watershed Sentinel Magazine

As if things weren’t tough enough for the plants, sea urchins are also munching through kelp forests, unchecked by starfish, their natural predator. Urchin numbers exploded after a sweeping die-off of starfish in 2013, from California to Alaska, caused by a viral wasting disease (which is also suspected to be magnified by rising water temperatures). The one-two punch of warm water and hungry urchins razed kelp forests along the California coast. Across the Pacific, similar circumstances have decimated kelp forests in Tasmania.

Kelp forests are pivotal to the marine ecosystem, providing habitat, food, and shelter for a diverse spectrum of marine life. When kelp forests die, declines in salmon, rockfish, and invertebrates follow.

Some kelp forests are proving more resilient to warming ocean waters (including one kelp forest right in Vancouver harbour). Kelp beds are also persisting in areas of the North and South Strait of Georgia, where cooler, deeper water mixes with surface water in the water column.

Researchers from Project Watershed and Simon Fraser University are working to re-establish bull kelp in the Salish Sea. The project includes a host of other partners and is funded by the Pacific Salmon Foundation and Fisheries and Oceans Canada.

Looking for heat-resistant varieties

Braeden Schiltroth, a researcher at SFU, is working to find out why some kelp forests are better able to cope with the heat, and to identify the most heat-resistant populations.

Schiltroth’s research focuses on the early stages of the kelp’s life cycle. In kelp reproduction, the adult plant produces spores, which germinate to become a microscopic intermediary generation called gametophytes. The gametophytes produce sperm or eggs, which in turn unite to form the infant sporophyte, that will grow to become the adult plant.

Schiltroth found spore germination dropped by half above 17º, and at 20º there was complete spore death. “We do see these temperatures at some of our hot sites,” said Schiltroth, “and it’s happening relatively quickly.”

While Schiltroth is finding the hardiest kelp population, Heath is studying how to re-establish the kelp.

Spores are first collected from patches called sori on the kelp’s blades (equivalent to leaves). The spores are cultured through to the sporophyte stage, and then allowed to settle onto spools of twine and root themselves. At a test site off of Hornby Island, Heath and his team (who are also his daughter and son-in-law – kelp runs in the family) wrap the twine around larger ropes, which are fastened underwater in a grid formation. The maturing plants eventually anchor themselves to the thicker rope, and grow to become the familiar buoyant adults.

If ocean waters keep warming … Well, things get even more dicey.”

Heath is also working on getting the kelp on the seeding ropes to recruit back to the ocean floor. One problem is that urchins are scarfing the gametophytes as soon as they hit the seabed. To give the kelp a hand, Heath is experimenting with an underwater pen to keep the urchins at bay until the gametophytes can unite, form sporophytes, and root.

Schiltroth said the kelp declines are complex. “It’s what makes the research fairly difficult to do, that there are so many factors. Definitely the urchin grazing, sea star wasting has got those numbers up. Back in 2016 we had this thing called the blob, which was essentially this warm mass of ocean currents that came towards our coast, and that created a lot of warming in our area. So all these things created this perfect storm, and as you can see in California – 90% decreases in kelp in a lot of areas. We’re really starting to see a lot of the same trends, particularly on the inner Salish Sea.”

If ocean waters keep warming, Heath and Schiltroth may only be buying time. “I think we can do something by selecting thermally resistant strains of bull kelp that can withstand the kinds of summers that we’re having.” said Heath. “That’s summers as they are now. What happens if it gets even warmer? Well, things get even more dicey.”

Still, there are some reasons for optimism. Kelp is naturally prolific, and given a weather window of cooler water, Schiltroth found the plants are able to get back on track and complete their reproductive cycle. Heath also has partnering research groups eager to launch new reseeding sites.

The long-term plan is to reintroduce more heat-resistant kelp forests to a string of test sites along the Strait of Georgia. If a half-dozen or more sites could be established, Heath thinks the effect could be significant. Give the kelp a leg up, and, Heath said, “the plants can do the rest quite well.”

Gavin MacRae writes for the Watershed Sentinel