Bay Nature magazineWinter 2015

The Ocean

An Ocean Garden: The Fine Art of Seaweed

January 1, 2015

All text and images excerpted from An Ocean Garden: The Secret Life of Seaweed (Abrams, 2014), by Josie Iselin.

APRIL 21, SAN FRANCISCO, CA. I FELL IN LOVE with seaweed at the kitchen counter.

I had returned with a sack full from the windswept beach at Princeton-by-the-Sea, and as I dropped each specimen into a tub of salt water, its form and color and translucent sensuality awakened. Pale pinks mingled with bright greens and yellow oranges. Rounded fronds, bumpy textures, and slender tendrils unraveled. I focused on one green algal mass. The delicate connections between the razor-thin blades were surprisingly strong, and as I unfolded them, one by one, I could see their tiny serrated edges. I felt like I was discovering a secret that few had seen.

In 1955 Rachel Carson published The Edge of the Sea, a lyrical and intimate prose portrait of the intertidal world—that universe of life that resides just below and between the tides. The rockweeds and kelps are an integral part of her explanations of the rocky intertidal zone; her home base was the seaweed-strewn rocks and beaches of southern Maine, and so how could it be otherwise? Carson saw the integral nature of all organisms living at the seashore. Her ecological view (rare at that time, when most scientists were nose-down in the study of specific species) is as contemporary today as when she wrote about it, and she celebrates seaweed as one of the great ecosystem engineers of our planet. It fixes carbon, generating the base of the food chain, and creates habitat; it is fundamental not only to life in the sea but to all life on earth.

What if Rachel Carson had been able to observe firsthand the intense diversity of the wild California reefs? Marine algae alone number more than 700 species on the Pacific coast. Compared to the monochrome presence of knotweed (Ascophyllum) and bladderwrack (Fucus) on the Maine coast, a California shoreline can seem like a wonderland. The wrack line of its beaches is full of washed-up kelps, and its tide pools are crowded with delicate seaweeds that range in color from reds and magentas to iridescent blues.

My adult life in California includes a close kinship to the beaches I walk regularly, especially Fort Funston in San Francisco, with its wild drifts of massive kelp, and Duxbury Reef in Bolinas, where years ago my seaweed journey began. It was there that I held an innocuous scrap of Cryptopleura violacea up to the sky and, with a gasp of wonder at the intensity of color and the fabulousness of form, decided to bring it back to my studio and place it on my scanner. Among the many treasures the beach has shared with me, seaweed is perhaps the greatest discovery of all.

Ulva lobata, Stone Beach, San Mateo

Some Basics

The common name “seaweed” implies a kinship to plants, but that is misleading. Algae of all sorts were established in the oceans well before the arrival of vascular plants on land, which resulted from the migration of a few ancestral green algae from the top of the surf zone up onto terra firma. Vascular plants inherit their chlorophyll (and thus their green color) from these ancient algal migrants, but the relationship ends there.

The term “algae” covers the domains of microalgae as well as marine algae, multicellular algae, or macroalgae, all three of which are synonyms for seaweed. Microalgae are the invisible single-celled organisms that populate our oceans and waterways and produce more than half the oxygen in our atmosphere. Macroalgae, or seaweed, produce another 20 percent. All of this oxygen is generated as a by-product of photosynthesis, and seaweeds and kelps are astoundingly good at this life-giving process. Both land-based plants and oceanic flora use the energy of sunlight to split water molecules and transform carbon dioxide into organic matter. But seaweeds do not expend precious resources fighting gravity—the buoyancy of the ocean pulls them upward—and, as a result, they are masters of efficiency when it comes to converting light energy into chemical energy, and from there, into metabolic energy, or growth. Giant kelp can fix from 1 to 4.8 kilograms of carbon per square meter of plant per year, growing almost two feet a day. Other species display even higher productivities. The kelp forests of the oceans rival the rain forests of the continents in terms of oxygen production.

Mazzaella volans
Mazzaella volans, Bodega Head

What Color Is Your Seaweed?

Color is perhaps a seaweed’s most striking characteristic: the intensity of its magenta, the subtlety of its golden brown, or the clarity of its kelly green. William Henry Harvey, a colorful Irishman who traveled the world collecting specimens in the 19th century, was the first to use color as the basis for identifying seaweeds. In 1839 Harvey segregated algae into the three taxonomic groups of green, brown, and red that we still use today.

A seaweed’s color is determined by the combination of pigments housed within its cells. Green algae, like plants, have chlorophyll a and b. They are, in fact, typically green. Brown algae, which include kelps and rockweeds, have a third, brown accessory pigment that, when combined in different amounts with the green chlorophyll, creates their array of colors ranging from olive green to golden brown to yellow-orange. The six thousand or so species of red algae have red and blue accessory pigments that overshadow the single chlorophyll a pigment. When these pigments combine, the color can be dazzling: striking scarlet, maroon, pale pink, or deep purple.

While plants on land have the full spectrum of daylight available to them, seaweeds must be resourceful with whatever light filters through the dense ocean waters—some reds and browns live at depths of 100 to 200 feet. Chlorophyll a is the powerhouse activator for photosynthesis and efficiently collects the longer red wavelengths available in the surface waters and on shore, reflecting the greens back to our eyes. It is present in all seaweeds, but brown, red, and blue pigments have evolved alongside it to capture the shorter wavelengths of blue and green light that penetrate the deeper waters where the red and brown seaweeds find their space.

Neoptilota densa, Cambria

Seaweed Architecture

From tiny and intricate to enormous and singular, the diverse shapes found among the tangle of seaweed at the ocean shore are all indicative of strategies to confront the three tasks essential for success in the intertidal zone: holding on, gathering light and nutrients, and defending against being eaten. A scientist would refer to a seaweed’s morphology while a designer would use the term form, but in either case, the complex and varied shapes that exist across all seaweeds derive from some basic building blocks. The overall body of a seaweed is called the “thallus.” What glues the thallus to the benthos, or sea bottom, which is usually a rocky substrate, is a holdfast. Some holdfasts become large and ancient structures in and of themselves. Others are round and flat and remarkably small given the task at hand.

Emerging above the holdfast is the stipe, a “stem” that supports branches and the blades, or the “fronds.” A blade might be as thin as a single cell or as stretchy and tough as contemporary denim, and closer inspection may offer clues to its identity: There might be a distinct midrib, or a network of slender veins. The branching patterns can be as simple as a single stipe reaching to a single blade, a bifurcation (splitting in two), or an enormously complex system that looks like a lifeless tangle when draped over a rock at low tide but reveals a specific growth pattern when floating in water.

Bull Kelp

SEPTEMBER 12, FORT FUNSTON. Today, thousands of Nereocystis (left) were washed up along the sandy expanse of Fort Funston. The giant, mature bull kelps, with hard bulb, sturdy stipe, and streaming blades, collected in huge entanglements tumbling in the surf, their staggering size and quantity only hinting at the profusion of growth beyond the waves. Between the bundles of wrack, I noticed single kelps washed up on the sand: tiny, miniature versions of the giants, their perfectly spherical bulbs as wide as a fingernail, their blades, like golden wings, delicate and luminous in the morning sunshine. While the scale differential is as a seedling to an ancient redwood, the transformation from tiny kelp to giant does not take centuries but merely weeks and months. The remarkable transformation of the inorganic into the organic—of sunlight and seawater into the long stipe, substantial bladder, and winged blades that grow up to 100 feet from the ocean floor toward the surface—happens in a single season, out of sight, in the deep subtidal domain of the cold northern Pacific.

I have a photograph of my nephew at about age 12 trudging along the sandy expanse of Limantour Beach on the Point Reyes National Seashore. He has the stipe of a great bull kelp draped over his shoulder, and he is leaning into the weight of it, the bulb and blades trailing behind him 30 feet or more. I think he must have fancied himself a slave in ancient Egypt, hauling stones to the pyramids. The scale of the enormous kelp brings that kind of thing to mind.

feather boa
Egregia menziesii (feather boa), Half Moon Bay

Feather Boa Kelp

JUNE 8, DUXBURY REEF. I went out to the reef today. It was a minus tide, and the long stretch of exposed rock struck out into the Pacific, pointing south toward the Golden Gate Bridge. I looked for my favorite seaweed, the feather boa kelp, or Egregia menziesii. I went out to the far southern tip of the reef, where the lowest tide teases the rocks with a few hours of exposure and where there are a number of Egregia holdfasts. In February they were battered and worn. Now, from the same holdfasts, the Egregia emerged abundant and fresh, 20 to 40 feet long, olive green fronds splayed along the surf channels, devoted to these rocks for a few hours before floating aloft with the rising tide. What a strange creation this kelp is. Like a bizarre feather boa, its paddle-shaped blades line the edges of a straplike midrib covered with tiny spikes. Intermittent rounded bladders, sporting whimsical, winglike blades, help to keep the massive rope afloat in its subtidal life, to catch the sun’s rays and perform the work of photosynthesis.

How could something this funny-looking be so successful? Without having to devote precious resources to combating gravity, it uses the buoyancy of water to its advantage. And always having the nutrients of the ocean available to it—like other kelps—Egregia is enormously efficient at producing more of itself. While the holdfast might be 15 years old, this 45-foot-long blade had grown in just a few months.

About the Author

Josie Iselin is a San Francisco-based photographer, writer, artist, and book designer, with seven books and many gallery shows to her credit. Find her at

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