Kelp Forests and Coastal Upwelling

Ekman Spiral Transport, Pneumatocyst Buoyancy, and Trophic Cascades

Frozen into the Arctic pack ice aboard the Fram in 1893, Fridtjof Nansen noticed that icebergs drifted 20 to 40 degrees to the right of the wind. Vagn Walfrid Ekman solved the puzzle in 1905. Coriolis deflection twists wind-driven water into a depth spiral whose net transport over the top 100 meters moves 90 degrees right of the wind.

When equatorward winds blow parallel to California, Peru, or Namibia, Ekman transport pushes warm surface water 90 degrees offshore. Frigid deep water rich in nitrate and phosphate wells up from 200 meters to replace it. Covering just 1 percent of the ocean, coastal upwelling zones yield 50 percent of the world's wild fish catch.

Giant kelp (Macrocystis pyrifera) is not a plant, but a heterokont brown alga related to diatoms. Anchored to rock by a holdfast that absorbs zero nutrients, it grows upward at 50 centimeters per day to reach 45 meters. Canopy blades pump sugars down to shaded basal fronds through phloem-like trumpet sieve cells.

Because seaweeds lack rigid wooden trunks, giant kelp suspends its heavy fronds in the sun using gas-filled bladders called pneumatocysts at the base of each blade. Rather than filling these floats strictly with air, the alga secretes up to 10 percent carbon monoxide ($\text{CO}$) inside them, maintaining buoyancy against three atmospheres of pressure.

Water is 800 times denser than air, so a 4-meter-per-second storm wave exerts the drag of a 360-kilometer-per-hour tornado. Mimi Koehl and Mark Denny showed how kelp survives surf that would snap an oak trunk. Flexible fronds fold flat into a streamlined bundle while the rubbery stipe stretches by 20 percent like a shock absorber.

Working across the Aleutian Islands in 1974, James Estes uncovered a three-level trophic cascade linking sea otters, urchins, and kelp. Lacking blubber, a 30-kilogram sea otter (Enhydra lutris) must eat 25 percent of its weight in sea urchins daily. Where hunters removed otters, urchins multiplied and mowed kelp forests down into bare urchin barrens.

Why sea urchins do not starve and let kelp regrow after stripping a reef bare became clear only recently. Once purple urchins (Strongylocentrotus purpuratus) devour every kelp frond, they resorb their own gonads and cut their metabolic rate tenfold. As dormant zombie urchins, they survive for years scraping rock and eating new kelp spores.

Although kelp forests grow only along shallow rocky shores, 80 percent of their annual biomass detaches during winter storms as drifting detritus. Swept down steep coastal submarine canyons below 1,000 meters depth, these tough algal mats sequester 170 million metric tons of carbon into the deep sea every year while feeding abyssal fauna.

Every two to seven years during El Niño, slackening Pacific trade winds let warm water slosh eastward against South America and depress the Peruvian thermocline below 150 meters. Upwelling still runs mechanically, but the pump now draws up warm, nitrate-empty water—collapsing the Peruvian anchoveta (Engraulis ringens), Earth's largest single-species fishery.

Inside California kelp forests, predatory sheephead wrasses (Semicossyphus pulcher) help otters control urchins while running a protogynous sex-change clock. Every sheephead hatches and matures as a pink female. Only after reaching 30 centimeters at age eight—if the reef lacks a dominant male—do her ovaries transform into testes as she grows a black-and-white head crest.

Grazing at the base of kelp forests, red abalone (Haliotis rufescens) build shells out of 95 percent brittle aragonite tiles and 5 percent protein glue. Yet their inner nacre is 3,000 times tougher to crack than pure mineral aragonite. Staggered 0.5-micrometer mineral bricks force any crack from an otter's rock to zig-zag across thousands of sliding layers.

Coastal Laminaria kelps concentrate iodide ions ($\text{I}^-$) from seawater 30,000-fold. Frithjof Küpper proved in 2008 that kelp stores iodide outside its cell membranes as an inorganic antioxidant. When low tide exposes blades to sunlight and ozone ($\text{O}_3$), released iodide scavenges the ozone and vents iodine gas that seeds coastal cloud droplets.