Kelp, Trophic Cascades, and Climate

by nyljaouadi1
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By Lambert Strether of Corrente.

Because I’m a fan of povidone-iodine as a Covid prophylactic (though disclaiming any ability or desire to give medical advice), I thought I would investigate kelp, since I thought that iodine was derived from kelp. Alas, it once was, but that’s “no longer economically viable.” (A substance derived from kelp, algin, is used as an emulsifying and bonding agent in toothpastes, shampoos, salad dressings, puddings, cakes, dairy products, frozen foods, so if you’re a ranch dressing fan, read on.) So, normally when I wander into the biosphere I get lost and don’t come out where I expect; with kelp, I got lost on my very first steps in!

Kelp forests are very beautiful:

And here is a video (whose soundtrack could do double duty as a sleeping aid):

“Wow, what an amazing plant!” you might be tempted to say, but kelp (genus Macrocystis) is a large brown algae, not a plant, but a heterokont (so-called for the flagellated cells[1] that most produce at some point in their lifecycles). Amazing algae[2], then. From

Since the giant kelp is not a plant, it does not have roots. Instead, it obtains all of the necessary nutrients directly from the water and is attached to the rocky bottom by a structure known as a holdfast. Like plants, however, the giant kelp harvests the sun’s energy through photosynthesis and does not feed on other organisms. This species is one of the fastest growing species in the world, and under perfect conditions, it has been known to grow up to two feet (60 cm) in a single day. Once and individual giant kelp reaches the sea surface, it continues to grow horizontally, floating in large mats that shade the water column and sea floor below. In order to remain upright, each giant kelp blade (leaf) includes a gas-filled pod that floats. Several individuals growing together can create dense forests that are an important ecosystem in temperate, coastal areas where they live.

Kelp forests range along 25% of the Earth’s coastlines. Here is a map:

(You will notice no kelp in the tropics; kelp like colder waters.) NASA’s Earth Observatory describes kelp forests:

Giant kelp forests are among Earth’s most productive habitats, and their great diversity of plant and animal species supports many fisheries around the world. The kelp, or Macrocystis, that make up these underwater forests truly are giant. They are the world’s largest marine plants and regularly grow up to 35 meters (115 feet) tall; the largest giant kelp on record stood 65 meters (215 feet) tall. Divers have compared swimming through mature kelp forests to walking through redwood forests.

Unlike redwoods, giant kelp are ephemeral. They live for seven years at most, and often they disappear before that because of winter storms or over-grazing by other species. As fishermen know, giant kelp forests can appear and disappear from season to season, from year to year.

NOAA amplifies the species diversity of kelp forests:

In kelp forests, the most commonly found invertebrates are bristle worms, scud, prawn, snails, and brittle stars. These animals feed on the holdfasts that keep kelp anchored to the bottom of the ocean and algae that are abundant in kelp forests. Sea urchins will often completely remove kelp plants by eating through their holdfasts [ouch!]. Other invertebrates found in kelp forests are sea stars, anemones, crabs, and jellyfish.

Hold that thought on sea urchins. More species:

A wide range of fish can be found in kelp forests, many of which are important to commercial fishermen. For example, many types of rockfish such as black rockfish, blue rockfish, olive rockfish, and kelp rockfish are found in kelp forests and are important to fishermen.

A wide range of marine mammals inhabit kelp forests for protection and food. Sea lions and seals feed on the fish that live in kelp forests. Grey whales have also been observed in kelp forests, most likely using the forest as a safe haven from the predatory killer whale. The grey whale will eat the abundant invertebrates and crustaceans in kelp forests. One of the most important mammals in a kelp forest is the sea otter, who takes refuge from sharks and storms in these forests. The sea otter eats the red sea urchin that can destroy a kelp forest if left to multiply freely.

Kelp forests are a natural buffet for birds such as crows, warblers, starlings, and black phoebes which feed on flies, maggots, and small crustaceans that are abundant in kelp forests. Gulls, terns, egrets, great blue herons, and cormorants dine on the many fish and invertebrates living in the kelp. Kelp forests also provide birds with a refuge from storms.

Ecologists have conceptualized the relations between such species more formally as a “food web.”

A food web consists of all the food chains in a single ecosystem. Each living thing in an ecosystem is part of multiple food chains. Each food chain is one possible path that energy and nutrients may take as they move through the ecosystem. All of the interconnected and overlapping food chains in an ecosystem make up a food web.

Organisms in food webs are grouped into categories called trophic levels. Roughly speaking, these levels are divided into producers (first trophic level), consumers, and decomposers (last trophic level).>

Here is a diagram of a food web that, as it happens, includes kelp, sea urchins, and sea otters:

Turning to paradigms, “as it happens” is, in fact, not a coincidence. The otter -> sea urchin -> kelp relationship is the basis for a paradigmatic example of what ecologists call a a “trophic cascade.” From the Fish and Wildlife Service:

In his famous essay, “Thinking like a Mountain,” Aldo Leopold recounts an epiphany he experienced while watching the “fierce green fire” fade from the eyes of a dying wolf: “I thought that because fewer wolves meant more deer, that no wolves would mean hunter’s paradise. But after seeing the green fire die, I sensed that neither the wolf nor the mountain agreed with such a view.” The larger view that Leopold came to see, and that he tried to help others see, was that the predators his contemporaries vilified and systematically killed were an integral part of the ecosystem—important not only to the plants that the deer consumed but also, seemingly paradoxically, to the deer themselves. “Since then,” Leopold writes, “I have watched the face of many a newly wolfless mountain, and seen the south-facing slopes wrinkle with a maze of new deer trails. I have seen every edible bush and seedling browsed […] to death. I now suspect that just as a deer herd lives in mortal fear of its wolves, so does a mountain live in mortal fear of its deer.”

Like wolves, sea otters were systematically eliminated from most of their native range…. Whereas sea otters were killed during the fur trade for their lush pelts, not for competing with humans for prey, the effects of their removal were parallel to those following the elimination of wolves. [Sea otters eat] the equivalent of about 25 percent of their body mass each day. Calorie-rich sea urchins, herbivores that consume algae, are one of their preferred prey. Just as deer can proliferate and radically alter the landscape in the absence of wolves, hordes of hungry sea urchins, when released from sea otter predation, can turn kelp forests that support a myriad of organisms into urchin barrens. Although factors such as storms can also influence kelp abundance, where there are sea otters, kelp tends to increase. Given their large-scale community effects, sea otters, like wolves, are considered a keystone species.

Removing the keystone, predator, species (wolf or otter) causes a “trophic cascade,” as the consumer species proliferate (deer, urchins) and consume the producer species, collapsing one of the chains in the food web (as many suburbanites have come to learn. See other examples of the same cascade here, here, and here[3]).

Otters -> Sea urchins -> kelp is, in fact, a paradigmatic case of a trophic cascade (it’s Wikipedia’s go-to example for that entry). Interestingly, the paradigm is now under attack[4]. From February of this year in, “New research on sea urchins challenges long-held assumptions about marine reserves,” which provide a nice natural experiment. Sorry to quote so much of it:

But a new study by [Katrina Malakhoff, a doctoral student in UC Santa Barbara’s Interdepartmental Graduate Program in Marine Science] and her advisor, Robert Miller, suggests that the truth is much more nuanced. The researchers examined urchin populations inside and outside marine reserves, where protection from fishing should have enabled urchin predators to rebound and control their populations. But instead of finding fewer urchins, they found that one species was unaffected by the reserves, while the other flourished…. “We predicted that by protecting these areas we’re increasing the number and density of urchin predators that will then control urchin populations and prevent them from overgrazing the kelp forest and turning it into an urchin barren,” Malakhoff said. She sought to investigate this assumption, as well as the tendency of scientists and resource managers to lump the two species together and treat them as ecologically equivalent.

The reserves seemed to have no affect at all on unfished purple urchin populations. What’s more, instead of decreasing in numbers, red urchins proliferated within the borders of some marine reserves. Their size, number and density increased once they no longer faced fishing pressures. Reserves also had no clear effect on giant kelp density.

“I was pretty surprised,” Malakhoff said. “It contradicted what I expected to be happening in the kelp forest.” If a rise in predation within the reserve influenced urchin populations, then both species should have decreased in number, and there ought to have been fewer small urchins, which provide an easier snack for predators.

“There’s a simplistic picture that’s been promulgated of kelp forests versus urchin barrens, and that predators are preventing this phase shift from happening,” Miller said. The corollary is that, by restoring predator populations, reserves should allow the lush kelp forests to return.

Other studies have documented that the urchins’ predators are thriving under the reserves’ protection. . For instance, larval dispersal and recruitment—as well as the oceanographic regimes that affect them—likely have greater effects on urchin populations, Miller said.

This study invites scientists to reconsider how common trophic cascades are, and whether marine reserves will always induce them. “Katrina’s study adds to ,” Miller said.

The two researchers acknowledge that some of their fellow scientists and resource managers might be reluctant to accept their conclusions. “The relationship between marine reserves and trophic cascades has approached paradigm status,” Miller explained, “and it’s always difficult to push back against a paradigm.”

Of course, I have paradigm shifts very much in mind, having followed the aerosol v. droplet tranmission controversy with great interest over the last year. But let’s remember that saying a theory is a paradigm shift means neither that the theory is correct nor that the paradigm shift will take place.

Turning finally to climate, kelp forests are enormous carbon sinks:

The capacity to draw CO2 from the atmosphere has added “climate mitigation” to kelp’s list of benefits. When we talk about ways oceans can sequester carbon, the conversation typically revolves around mangroves, salt marshes, and seagrass meadows. But “the magnitude of carbon sequestered by algal forests is comparable to that of all those three habitats together,” says Carlos Duarte, a professor of marine science at the King Abdullah University of Science and Technology in Saudi Arabia. “Algal forests should not be left behind. They have been hidden for much too long.”

There’s a lot we still don’t understand [of course] about how kelp store CO2. But researchers are starting to build a better picture of this giant seaweed and how we might improve its capacity to help tackle climate change.

WaPo reinforces:

Giant kelp is among the best organisms on the planet for taking planet-warming gases out of the atmosphere. Buoyed by small, gas-filled bulbs called “bladders,” these huge algae grow toward the ocean surface at a pace of up to two feet per day. Their flexible stems and leafy blades form a dense underwater canopy that can store 20 times as much carbon as an equivalent expanse of terrestrial trees.

Unfortunately, since kelp likes cold water, global warning threatens it:

A steady increase in ocean temperatures — nearly 3 degrees Fahrenheit in recent decades — was all it took to doom the once-luxuriant giant kelp forests of eastern Australia and Tasmania: Thick canopies that once covered much of the region’s coastal sea surface have wilted in intolerably warm and nutrient-poor water. Then, a warm-water sea urchin species moved in. Voracious grazers, the invaders have mowed down much of the remaining vegetation and, over vast areas, have formed what scientists call urchin barrens, bleak marine environments largely devoid of life.

The Tasmanian saga is just one of many examples of how climate change and other environmental shifts are driving worldwide losses of giant kelp, a brown algae whose strands can grow to 100 feet. In western Australia, increases in ocean temperatures, accentuated by an extreme spike in 2011, have killed vast beds of an important native kelp, Ecklonia radiata. In southern Norway, ocean temperatures have exceeded the threshold for sugar kelp — Saccharina latissima — which has died en masse since the late 1990s and largely been replaced by thick mats of turf algae, which stifles kelp recovery. In western Europe, the warming Atlantic Ocean poses a serious threat to coastal beds of Laminaria digitata kelp, and researchers have predicted “extirpation of the species as early as the first half of the 21st century” in parts of France, Denmark, and southern England.

Fortunately, restoration efforts are taking place (and in Maine, too). Yes, it’s a startup, but that’s where we are:

Adam Baske strolls through a warehouse on the coast of Harpswell, Maine. Surrounding him are trays of oysters with water circulating between them in small tubes. In another room stands rows of eight-foot-tall tanks of algae growing at different stages. The algae will be food for the oysters. If you’ve never seen a shellfish hatchery, this one looks pretty typical. But it’s not. This year, they’re planning to harvest something new—atmospheric carbon.

His company, called Running Tide Technologies, plans to grow vast quantities of seaweed in drifting ocean mini-farms—farms that the company plans to sink to the bottom of the ocean.

“So this is basically taking the emissions of our fossil-fuel burning, locking them back up into the structure of the kelp and sending it back to the bottom of the ocean, where, you know, it’s at least locked up for hundreds to thousands of years because of the great pressure and the slow movement of the water in the deep ocean.”

Kelp, like other plants, uses photosynthesis to extract carbon dioxide from the atmosphere. Colette Feehan, a marine ecologist at Montclair State University, who does not work with Running Tide Technologies, says that kelp is a no-brainer when it comes to carbon sequestration.

“The productivity of kelp forests has been found to be comparable to tropical rain forests, meaning that they put on a great deal of biomass, and that biomass is stored carbon.”

It can do this because it grows fast (about a foot per month). It also quickly sinks to the seafloor. Once there, it degrades very slowly.

Trees, on the other hand, store carbon but ultimately release it back to the atmosphere when they die and decompose. Kelp can stay effectively buried, its carbon entombed by the crushing pressure and lack of oxygen, for hundreds, maybe thousands of years.

“As a climate change mitigation strategy, there’s mounting evidence that this is a good approach. These forests aren’t taking up land that would otherwise be used for agriculture or housing. So there’s really no negative side to growing kelp forests.”

And here is a similar project in Tasmania, but with a permaculture (1) twist:

“In collaboration with the Climate Foundation as part of its work to regenerate food security, ecosystem services and mitigate climate change, our study aims to establish whether there’s any chance of restoring these important marine communities by identifying individual giant kelp plants that may be genetically better adapted to warmer sea temperatures,” Professor Johnson said.

Dr. Brian von Herzen, Executive Director of the Climate Foundation, said “Marine Permaculture development programs like these increase our collective capacity to regenerate life in the oceans and ensure healthy ecosystems and climate for generations to come.”

Project researcher and Climate Foundation Postdoctoral Fellow Dr Cayne Layton, said that in 2012 southeastern Australia’s giant kelp forests were listed by the Australian Government as an endangered marine community, the first such listing for a marine community in Australia.

“Giant kelp were the foundation for marine communities along much of Tasmania’s East Coast, creating complex habitats that once supported key species of conservation or commercial value, from weedy sea dragons to rock lobsters and abalone,” Dr Layton said.

“Active restoration of these now degraded and disappearing habitats represents a potential approach for conservation of giant kelp forests while at the same time offering new commercial possibilities.

“The same techniques that underpin restoration may also be able to facilitate the development of giant kelp Marine Permaculture for commercial harvest and integrated multi-trophic aquaculture.

Cool! (Note that selecting for kelp that can live in warmer waters seems to be an angle the Maine project has not considered.

Maybe. Worth a shot. One of the nice things about using kelp farms for carbon sequestration, as opposed to tree plantations (I won’t call them forests) is that real estate issues do not arise; the lunatic BECCS project, for example, needed real estate the size of the Indian subcontinent for its trees, so I don’t think so..

* * *

I don’t quite know how to bring this serendipitous trip through the kelp forests to an conclusion, so I will say, with Frank Herbert’s Fremen, “now it’s complete because it’s ended here.” I hope it was fun and useful!


[1] Life is too short to go through the sex life of kelp, but here is a chart of kelp’s life cycle:

The meiospores at top right look like flagella to me.

[2] Also amazing is that as of 2010 is but one single fossil of kelp. Today, the fossil record is considered “sparse,” “as these organisms do not produce hard parts, such as certain calcified red and green algae, nor do they produce resistant spores. Also, in the absence of pigments, fossil brown algae may be almost impossible to distinguish from these other algae, since there are many morphologically convergent forms among the three groups. These difficulties are compounded by the lack of trained paleobiologists who specialize on algae, or of phycologists who examine fossils.”

[3] Thinking of Nabokov’s famous parable where an ape, given charcoal and paper, drew the bars of his cage, consider this passage from Scientific American’s “Living in a Landscape of Fear: How Predators Impact an Ecosystem“:

The keystone species concept lies at the heart of the HSS debate. When Robert Paine introduced it in 1969, he envisioned its mechanisms as a dominant predator consuming and controlling the abundance of a particular prey species and a prey species competing with other species in its trophic class and excluding them from the community.

Granted, I’m going all PoMo here, but this reminds me of academic politics, although some departments (***cough*** economics ***cough***) are no doubt far worse than others. Speculating freely, one might wonder whether these academics find predator-prey relations so powerful and illuminating as an analytical tool because they mirror the relationships that matter most to them in their professional lives. Department chairs, red in tooth and claw, as it were.

[4] One obvious rejoinder would be to attack the distinction between producers (of nutrients; plants) and consumers (herbivores), where the producers are seen as implicitly passive. But from Michael Pollan’s Botany of Desire, we know that’s not so: Plants, as it were, cultivate our senses. See also the communications networks of trees. Granted, kelp lack roots, but perhaps they have evolved other channels. Plants are in no way passive, and I don’t see why the same should not be true of heterokonts too.

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