The Queen of Weeds

Photo: Enric Sala

Submarine giant kelp forests thrive at the ends of the Earth

Essay by Cheryl Lyn Dybas

It’s a land beyond which there is nothing: the literal ends of the Earth. Or so it would seem. The forbidding headlands of Tierra del Fuego are a dare in the face of never-ending rainstorms, slashing waves and winds that howl up to 140 kilometres an hour. Ultimately, the bluffs lose, ceding ground each year to fierce surrounding seas. This archipelago shared by Chile and Argentina at the tip of South America is far from lifeless, however. Here where the cold waters of the Pacific Ocean meet those of the Atlantic and the Southern Oceans, untouched submarine forests thrive.

“The giant kelp forests of southern South America are some of the least disturbed on the planet,” write Alan Friedlander, chief scientist of the National Geographic Society's Pristine Seas project, and colleagues in the journal PLOS ONE. “The remoteness of this region has, until recently, spared it from many of the direct anthropogenic [human-caused] stressors that have affected these ecosystems elsewhere.”

The abundance of giant kelp (Macrocystis pyrifera), sea urchins and sea stars around Tierra del Fuego hasn’t changed in more than 45 years, the scientists report. They’re comparing findings from a 2018 expedition to those of a survey more than four decades ago, in 1973.

Giant kelp according to Darwin:  A century and more before, in the early 1800s, Charles Darwin recognized the importance of giant kelp as food, substrate and shelter for marine life. “The number of animals of all orders, whose existence intimately depends on giant kelp, is wonderful,” Darwin penned in 1839 in The Voyage of the Beagle. “A great volume might be written, describing the inhabitants of one of these beds of seaweed.”

Today, scientists are finding that not only do ocean creatures depend on living seaweed for protection and sustenance, but that giant kelp is an important food source long after its fronds have died and broken off. The sea’s deep-dwelling animals feed on the decaying blades, called drift kelp, as currents carry them into submarine canyons.

Giant kelp is found along the western coasts of North and South America, South Africa, Tasmania, Australia, New Zealand and the islands near Antarctica. The largest marine plant on Earth, it can grow to a length of more than 30 meters in a little more than a year. Tierra del Fuego’s kelp forests stretch as far as the eye can see; their majestic fronds cover several square kilometres.

Dependent on sunlight for growth, giant kelp thrives in ocean waters from 6 to 24 meters deep. Sunlight easily penetrates to these depths, and kelp escapes the destructive force of breaking waves closer to shore. To flourish, giant kelp must have a hard surface for attachment, moderate wave surge, cool, clear ocean water, and high nutrient levels. Dissolved nutrients are especially abundant off coastlines such as Tierra del Fuego and central California, where water flows to the surface from deep regions in a process called upwelling. Nutrients in these deeper waters are often hundreds of times more plentiful than in the surface waters they replace.

During an El Niño, which occurs periodically in the Pacific and brings abnormally warm, nutrient-depleted waters to inshore areas, giant kelp plants tend to die off.  For better or worse, kelp is married to the sea and shaped by its forces. Home to some of Earth's most diverse ecosystems, kelp forests worldwide face threats from climate change and human activities.  A global analysis of changes in kelp abundance over the past half-century identified declines of 38% or more.

“Kelp forests in remote locations such as Tierra del Fuego are understudied, however, limiting the availability of knowledge to inform conservation efforts,” says Friedlander. While these kelp beds remain unchanged in recent decades, in the future they’re likely to suffer from warming ocean waters and increasing human activity.

Photo: Manu San Felix

Kelp forests at the end of the world: Tierra del Fuego was occupied by the Yaghan people long before its “rediscovery” by Dutch navigators in 1616. Isla de los Estados lies at the region’s easternmost edge. “Dangerous seas with strong waves, winds and currents, along with persistent fog, made the island and its surrounding rocky islets a graveyard for early sailors,” state Friedlander and co-authors, and for the lighthouse once built to warn these seafarers.

In 1884, the Argentine government built an ill-fated watchtower on Isla de los Estados that became the basis of Jules Verne’s novel The Lighthouse at the End of the World.  By shortly after the turn of the century, the wooden lighthouse had become a heap of timbers, its structure felled by wind and wave. A replica of the lighthouse was built in 1998, however, and still stands today.

“The entire coast of this island,” wrote Verne, “one long succession of strands and coves and rocky banks, is alive with shells and shellfish, bivalves and others, mussels, periwinkles, oysters, limpets and whelks. Crustaceans crawled in thousands…birds were plentiful. Uncounted numbers of albatrosses white as swans, of snipe and plovers…frequented the island.”

The isle, Verne reported, “was really nothing more than an enormous rock, almost uninhabitable.” But abundant marine life surrounded – and still does -- Isla de los Estados; it owes its existence to the area’s giant kelp forests and the habitat they provide. “The kelp grows on every rock from low-water mark to a great depth, both on the outer coast and within the channels,” wrote Darwin in The Voyage of the Beagle. “Not one rock near the surface was discovered which was not buoyed by this floating weed. The good service it thus affords to vessels navigating near this stormy land is evident; and it certainly has saved many a one from being wrecked.”

Tierra del Fuego’s geographic isolation and hazardous seas aside, National Geographic Pristine Seas project scientists made their way to its kelp forests two years ago. Data collected by scuba divers revealed that populations of kelp, sea urchins, and sea stars remained similar to those catalogued in 1973.

“Every square inch of the bottom was occupied by a living organism: white and yellow sponges, pink encrusting algae, lollipop-like sea squirts,” observed Enric Sala, director of the Pristine Seas project and author of the book The Nature of Nature: Why We Need the Wild (National Geographic Society, 2020).  “Giant kelp bent to the seafloor from the weight of the mussels growing on them,” marvelled Sala. “Blue starfish gorged on the mussels, along with snails and hermit crabs.” 

Life in a kelp forest, in riotous profusion: Surveys of 11 locations showed “no significant differences in the densities of adult and juvenile Macrocystis pyrifera kelp between the two survey periods,” state the scientists. Sea urchin numbers were similar, with the dominant species Loxechinus albus accounting for 66.3% of total sea urchin abundance in 2018 and 61.1% in 1973.

In most kelp forests, sea urchins feed on the holdfasts that anchor the plants to the bottom; urchins often completely remove kelp by eating through these holdfasts.  Off Tierra del Fuego, however, sea urchins were rarely observed grazing on giant kelp. No urchin "barrens" -- signs of destructive sea urchin grazing -- were seen. “This is contrary to many regions of the world that are experiencing transitions from kelp forests to urchin barrens, such as Tasmania, Eastern Australia, Norway and Northern California,” the researchers write.

The main sources of Tierra del Fuego kelp mortality are entanglement with drift algae and heavy settlement of bivalves and other mollusks on fronds. “Drift algae is what feeds the region’s sea urchins,” says Friedlander, “and there is a great deal of wave action, leading to a great deal of drift kelp.”

The research was conducted by scuba divers finning along two 25-meter-long transects placed at the same sampling locations as the 1973 survey. For sessile and mobile invertebrates, the number of individuals was estimated one meter to either side of the transect line. For organisms such as sponges, bryozoans and tunicates, colonies rather than individuals were counted. Divers also tallied the number of giant kelp stipes (stems) and kelp holdfast diameters.

For the first time, the scientists measured the kelp forest’s piscine residents -- such as Magellanic rock cod, South American eelpout and spiny plunderfish. Fish abundance varied from location to location, with changes between sites with different exposures to ocean waves.

Beyond analysing dive data, the researchers evaluated the kelp forests using Landsat satellite images, which first became available for the region in 1998. They didn’t observe long-term trends over the 20 years from 1998-2018, but did note that kelp forest cover appeared to follow approximately four-year cycles that mirrored sea surface temperature changes and El Niño patterns.

Photo: Enric Sala

Queen of the Seaweeds: Seaweeds, or algae, are divided by colour into three groups: reds, greens and browns. Giant kelp, with its amber colour and long length, is the queen of the brown seaweeds.

In place of the roots, stems and leaves of land plants, seaweeds have parallel structures called holdfasts, stipes and blades. An anchor against surging waves, the kelp holdfast secures the plant in place but doesn’t take up water and nutrients as do the roots of land plants; the flattened kelp blade is where the plant converts energy from sunlight into food. The holdfast can withstand wave surges equivalent to 203 kilometre-per-hour winds. As the holdfast grows, new branches sprout from the base, giving the plant a conical shape.

The kelp’s stipe is a tough but pliant stem that washes back and forth in the waves without breaking. It’s a sturdy anchor line and vital conduit between the holdfast below and the blades above. By transporting the products of photosynthesis from top to bottom, this channel allows the plant to grow unusually large. Many kelp plants have hollow floats along their stipes, making the seaweed buoyant and carrying the kelp blades toward the surface for maximum exposure to sunlight.

“Giant kelp is the redwood of the sea, its long strands like pillars,” writes Sala in The Nature of Nature: Why We Need the Wild.  “Giant kelp continues to grow even once it reaches the surface, creating a canopy atop the water’s surface through which sunlight filters as if through the stained glass of a cathedral.”

Specialized blades near the base of the plants produce spores, the start of new generations of giant kelp. Kelp plants can generate billions of spores, which drift on water currents until they come to a suitable place to settle. A microscopic plant called a sporophyte then develops, which eventually grows into a miniature giant kelp.

Within a few weeks, the plant reaches seven to 10 centimetres in height and begins to form new blades. With sunlight, plentiful nutrients, and ocean temperatures between 10 and 18 degrees Celsius, the blades can grow up to one-sixth of a meter per day. They continually develop and mature, with new blades pushing toward the surface even as old ones break away to become drift kelp.

Seasons of a kelp forest: With the changing of the seasons, so, too, marine life in a kelp forest changes. In spring and summer, new giant kelp blades form and stretch toward the surface. These new blades are temporarily free of colonists such as Gaimardia trapesina, a hitchhiking clam that’s frequently found on Tierra del Fuego’s giant kelp.

In late spring, microscopic larvae of marine animals float in to settle onto kelp blades. By mid-summer, bivalves and other species accumulate in enormous numbers. The Tierra del Fuego team found entire kelp plants sunk by the weight of large aggregations of Gaimardia trapesina.

When fall arrives, giant kelp growth slows as nutrients and light in the water diminish. The now-thick canopy slows the flow of water through the forest, so adult fish swim mostly along the edges. As fall turns to winter, storm waves rip through the underwater forest. The seaweed weakens. Older blades decay and are torn from stipes and holdfasts. Blades begin to litter the seafloor and become drift kelp. Since kelp decomposes slowly, drift kelp provides nutrients for benthic species long after it comes to rest on the ocean bottom.

The once and future giant kelp: "The kelp forests of the extreme tip of South America remain some of the most pristine on Earth,” says Friedlander. “Re-examination of this remote region is incredibly valuable in this age of climate change, and gives us a better understanding of how these ecosystems function in the absence of direct human impacts."

Efforts are underway to protect Tierra del Fuego’s giant kelp forests. Argentina recently created the Yaganes Marine National Park, including some 69,000 square kilometres of ocean adjacent to the Diego Ramirez-Drake Passage Marine Park. The marine park encompasses 140,000 square kilometres of waters off Chile.

“Although both areas are administratively and politically separated, they protect the same ecosystem,” state the Pristine Seas project scientists. However, the researchers say, neither includes the high biodiversity nearshore areas of the Cape Horn Archipelago and two islands, Isla Grande de Tierra del Fuego and Isla de los Estados.  Fishing and oil and gas interests have opposed such moves, according to Friedlander.

Now, members of the Tierra del Fuego Parliament have introduced a bill to create a reserve around Isla de los Estados and the Mitre Peninsula on the easternmost part of Isla Grande de Tierra del Fuego, says Alex Muñoz, director of the Pristine Seas project for Latin America.

Tierra del Fuego is among the last global refuges for giant kelp forests. “Designating these nearshore waters as protected areas would help conserve this unique ecosystem,” says Friedlander. Darwin likely would have agreed. “I can only compare these great aquatic forests of the Southern Hemisphere with the terrestrial ones in the intertropical regions,” he wrote in The Voyage of the Beagle. “Yet, if in any country such a forest was destroyed, I do not believe nearly so many species of animals would perish as would here, from the destruction of the kelp.”

Amid the fronds of this plant, Darwin chronicled, “numerous species of fish live, which nowhere else could find food or shelter; with their destruction the many cormorants and other fishing birds, the otters, seals and porpoises, would soon perish also.” Humans – as dependent on the marine food web as any fishing bird -- would also decline in numbers, conjectured Darwin, “and perhaps cease to exist.”

Without protection, the Queen of the Seaweeds may no longer reign over a vast and long undisturbed undersea domain, perhaps the last of its kind, hidden at the ends of the Earth.

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