Study Hints That Kelp Forests May Weather Climate Change

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Climate change threatens to radically alter most of California's landscapes, but a recent study suggests that at least one of the state's emblematic ecosystems may actually have a chance to thrive in a greenhouse-gas-enhanced world. The study, published in October in the journal Algae, shows that warmer ocean water with more dissolved carbon dioxide may give California's largest kelp species an advantage over its chief predator, the purple sea urchin.

Sea urchins are voracious consumers of algae, including the giant kelp Macrocystis pyrifera. Healthy kelp forests may contain a couple purple sea urchins per square meter of seabed, but those forests usually have other animals such as sea stars, lobsters, and even otters that keep the urchins in check. When the kelp forest ecosystem is disrupted sea urchin numbers can multiply as much 50-fold, and the giant kelp quickly succumbs to the onslaught. The result: urchin "barrens" containing no kelp, and none of the myriad fish, algae, and invertebrate species the giant kelp shelters.

In their study, Matthew Brown and Matthew Edwards of San Diego State and Kwang Young Kim of South Korea's Chonnam National University grew both giant kelp and purple sea urchins in tanks designed to simulate ocean conditions in a greenhouse gas-enhanced world, and what they found may offer at least a glimmer of hope that California may still have giant kelp forests in a century or two.

The team exposed the kelp and purple sea urchins (Strongylocentrotus purpuratus) to a variety of conditions in the tanks, including current temperatures and dissolved CO₂ levels, warmer water alone, more dissolved CO₂ alone, and both warmer water and extra CO₂.

The team examined an increase in dissolved CO₂ because the more of that greenhouse gas we pump into the atmosphere, the more the oceans will eventually absorb, raising the amount of CO₂ dissolved in the water and consequently making the oceans more acidic.

In the tanks with just elevated CO₂, the giant kelp showed no increase in growth rate over current conditions. In warmer water with no extra CO₂, kelp actually grew less quickly. But in warmer water with more dissolved CO₂, the giant kelp grew significantly faster than in normal conditions, taking more CO₂ out of the water and turning it into organic matter that in the open ocean would be usable by other organisms.

How much faster? Kelp cuttings weighing seven grams wet at the beginning of the experiment grew to just under 12 grams in four weeks under "normal" conditions, with water at 12°C (about 54°F) and current oceanic dissolved CO₂ levels, while cuttings in "future conditions" tanks with water at 15°C (59°F) and three times present-day levels of dissolved CO₂ grew to an average of around 15 grams by the end of the four-week period: approximately a 20 percent increase in mass.

Of course, that assumes a three-fold increase in dissolved CO₂. Without that increase the news would seem to be very bleak indeed: Kelp in tanks where the water was raised to 15°C but CO₂ kept at present levels grew only very slightly for a week or two and then started dying off.

Purple sea urchins didn't fare nearly as well as the kelp in the study. In tanks with warmer water and more CO₂, sea urchins grew significantly less quickly and ate less kelp than those held in the tanks with present-day ocean conditions.

Sea urchins held in future conditions also showed significantly less growth in their gonads, or reproductive organs, which may have implications for the species' ability to reproduce along California's future, climate-enhanced coast. (It definitely has implications for sushi lovers, as sea urchin gonad, a.k.a. uni, is a popular dish with a thriving fishery.)

So kelp grown under future ocean conditions grew better, while purple sea urchins in the same conditions did not. Can we breathe a sigh of relief for California kelp forests currently being ravaged by too many sea urchins? Not quite. The authors note that conditions in the tanks don't come close to matching the complex conditions to be found along the actual coast, where kelp forests have far more than one alga and animal species. And while too many sea urchins is a problem, a healthy kelp forest will have a handful of sea urchins here and there, fulfilling a necessary ecological function by eating pieces of dead kelp.

And even if climate change proved to be wholly beneficial to kelp forests in the short term, there are still plenty of other reasons to work to shut down our greenhouse gas pollution.

Still, it's a reminder that ecosystems are complex and their science surprising, and if a global crisis accidentally gives California's kelp forests a tiny bit of potential slack, we'll take it.