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1 Article 34 Where the Sea Meets the Sky The ocean’s skin is the richest, most extensive habitat of all by John T. Hardy Charles Darwin was fascinated by an explorer’s account of an American black bear “swimming for hours with widely open mouth, thus catching, like a whale,” thousands of insects, fish, and crustacean larvae that cluster near the water’s surface. The bear that captured Darwin’s imagina- tion was exploiting an ecological niche that has only recently been studied: an im- mense, paper-thin habitat that blankets more than 71 percent of the earth. Known as the sea surface microlayer, it is a remarkable “skin” that separates bod- ies of water from the surrounding atmo- sphere—the familiar dividing line between ocean and sky. Within this complex skin of the seas dwell thousands of species of plants, animals, and microbes, all attracted there by its special ability to nurture life. Since 1989, I have been working with a team of marine biologists, chemists, and toxicologists at the Huxley College of En- vironmental Studies at Western Washing- ton University investigating the biology and chemistry of the surface layer. On clear, relatively calm days, we have sam- pled waters as near as Puget Sound or as far away as the North Sea. Our collecting device is a barrel-sized, teflon-coated ro- tating drum towed alongside our research boat. Organic film from the water’s surface layer adheres to this revolving cylinder and is continuously scraped by a squeegee into a large glass jar. Several quarts of the sur- face layer habitat can be collected by this method in just a few minutes. We also skim the surface with a special plankton net attached to pontoons, to collect sam- ples of surface-dwelling crustaceans, fish eggs, and larvae. Scientists have known for years that the thin aquatic surface layer teems with life. In 1917, a Swedish researcher of freshwa- ter habitats, Ernst Naumann, coined the term neuston to describe certain protozo- ans that use the surface film for support. His coinage was taken from the Greek neustos, which means “floating” but refers to many inhabitants of the upper few inches of oceans and lakes. Since Nau- mann’s time, biologists have discovered scores of plants and animals, ranging from tiny bacteria and algae to large jellyfish and seaweeds, that live, reproduce, or feed within a few inches of the surface. Bacteria adhere to the underside of the surface film, as do some unicellular proto- zoans that attach themselves with a special appendage. Fish eggs are packed with fat globules, which cause them to float in con- tact with the surface. Other organisms, such as snails and some jellyfish and sea- weeds, entrap air bubbles and float on the film. Sargassum seaweed clusters in float- ing mats that nurture many small creatures, including baby sea turtles. Along with the protozoans, a dense blanket of microalgae lives at the surface layer, attracted by both sunlight and the concentration of nutrients found there. Some microalgae actually migrate to the surface at midday and then descend many feet during the night. Capitalizing on this concentration of biota, many seabirds make their living by skimming food from the water’s surface (some are even called skimmers). One of the surface layer’s main attractions for shearwaters, auklets, and petrels is that it provides an important nursery ground for numerous fish species: cod, sole, flounder, hake, menhaden, anchovy, mullet, flying fish, greenling, saury, rockfish, and hali- but. The tremendous risk for so many fish larvae and eggs being so near the surface appears to be balanced by the abundance of food found there and perhaps the lack of predators that live in deeper waters. The northeast Pacific, the U.S. conti- nental shelf, and the North Sea are typical of rich fishery areas where dozens of fish species produce eggs or larvae that con- centrate at the sea surface. In Puget Sound, English sole and sand sole spawn between January and April, releasing billions of eggs that float at the surface until they hatch, generally about a week after fertili- zation. Because of the buoyancy of their large yolk sacs, newly hatched larvae of these flatfish often float upside down near the surface. The ocean’s skin is also a vital habitat for many commercially important shellfish at certain stages of their life cycles. Crab and lobster larvae, for instance, seek the sunlight of the near-surface, where they feed on concentrations of minuscule life

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Page 1: Where the Sea Meets the Sky - University of Massachusetts ...faculty.umb.edu/anamarija.frankic/files... · Where the Sea Meets the Sky The ocean’s skin is the richest, most extensive

Article 34

Where the Sea Meets the Sky

The ocean’s skin is the richest, most extensive habitat of all

by John T. Hardy

Charles Darwin was fascinated by anexplorer’s account of an American blackbear “swimming for hours with widelyopen mouth, thus catching, like a whale,”thousands of insects, fish, and crustaceanlarvae that cluster near the water’s surface.The bear that captured Darwin’s imagina-tion was exploiting an ecological nichethat has only recently been studied: an im-mense, paper-thin habitat that blanketsmore than 71 percent of the earth.

Known as the sea surface microlayer, itis a remarkable “skin” that separates bod-ies of water from the surrounding atmo-sphere—the familiar dividing line betweenocean and sky. Within this complex skin ofthe seas dwell thousands of species ofplants, animals, and microbes, all attractedthere by its special ability to nurture life.

Since 1989, I have been working with ateam of marine biologists, chemists, andtoxicologists at the Huxley College of En-vironmental Studies at Western Washing-ton University investigating the biologyand chemistry of the surface layer. Onclear, relatively calm days, we have sam-pled waters as near as Puget Sound or asfar away as the North Sea. Our collectingdevice is a barrel-sized, teflon-coated ro-tating drum towed alongside our researchboat. Organic film from the water’s surfacelayer adheres to this revolving cylinder andis continuously scraped by a squeegee intoa large glass jar. Several quarts of the sur-face layer habitat can be collected by thismethod in just a few minutes. We also

skim the surface with a special planktonnet attached to pontoons, to collect sam-ples of surface-dwelling crustaceans, fisheggs, and larvae.

Scientists have known for years that thethin aquatic surface layer teems with life.In 1917, a Swedish researcher of freshwa-ter habitats, Ernst Naumann, coined theterm neuston to describe certain protozo-ans that use the surface film for support.His coinage was taken from the Greekneustos, which means “floating” but refersto many inhabitants of the upper fewinches of oceans and lakes. Since Nau-mann’s time, biologists have discoveredscores of plants and animals, ranging fromtiny bacteria and algae to large jellyfishand seaweeds, that live, reproduce, or feedwithin a few inches of the surface.

Bacteria adhere to the underside of thesurface film, as do some unicellular proto-zoans that attach themselves with a specialappendage. Fish eggs are packed with fatglobules, which cause them to float in con-tact with the surface. Other organisms,such as snails and some jellyfish and sea-weeds, entrap air bubbles and float on thefilm. Sargassum seaweed clusters in float-ing mats that nurture many small creatures,including baby sea turtles.

Along with the protozoans, a denseblanket of microalgae lives at the surfacelayer, attracted by both sunlight and theconcentration of nutrients found there.Some microalgae actually migrate to the

surface at midday and then descend manyfeet during the night.

Capitalizing on this concentration ofbiota, many seabirds make their living byskimming food from the water’s surface(some are even called skimmers). One ofthe surface layer’s main attractions forshearwaters, auklets, and petrels is that itprovides an important nursery ground fornumerous fish species: cod, sole, flounder,hake, menhaden, anchovy, mullet, flyingfish, greenling, saury, rockfish, and hali-but. The tremendous risk for so many fishlarvae and eggs being so near the surfaceappears to be balanced by the abundance offood found there and perhaps the lack ofpredators that live in deeper waters.

The northeast Pacific, the U.S. conti-nental shelf, and the North Sea are typicalof rich fishery areas where dozens of fishspecies produce eggs or larvae that con-centrate at the sea surface. In Puget Sound,English sole and sand sole spawn betweenJanuary and April, releasing billions ofeggs that float at the surface until theyhatch, generally about a week after fertili-zation. Because of the buoyancy of theirlarge yolk sacs, newly hatched larvae ofthese flatfish often float upside down nearthe surface.

The ocean’s skin is also a vital habitatfor many commercially important shellfishat certain stages of their life cycles. Craband lobster larvae, for instance, seek thesunlight of the near-surface, where theyfeed on concentrations of minuscule life

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ANNUAL EDITIONS

forms: the microalgae and protozoans. InChesapeake Bay, 99 percent of blue crablarvae migrate to the surface layer andspend several days feeding there. After in-creasing in size, the larvae return to deeperwaters, where they feed upon larger foods.

The extraordinary vitality of the sea’smicrolayer depends on special physicaland chemical properties that are very dif-ferent from those of the waters just below.The topmost three feet of water contains awhole series of sublayers, each with itsown distinctive biological and chemicalfeatures. Within the surface layer (upperfew feet), the first two-thousandths of aninch contains an especially dense concen-tration of minerals, chemicals, protozoans,and microorganisms. The upper few inchescontain a greater density of larger organ-isms: fish eggs, fish larvae, and crusta-ceans. Larger, floating jellyfish andseaweeds may occupy the upper foot. Thesurface layer includes many transients,with plants and animals constantly migrat-ing up and down.

The surfaces of both fresh and marinewaters contain complex mixtures of chem-icals that are often absent or greatly dilutedat lower levels. Yet most of these naturalcompounds are derived from deeper-

dwelling organisms. The billions of tinyplants and animals known as plankton oc-cupy the sunlit photic zone, which may ex-tend downward as far as 400 feet in theopen ocean. The plankton excrete many or-ganic compounds, such as amino acids,proteins, and fatty acids, that serve as nu-trients for bacterial growth. Rising air bub-bles capture these rich materials and carrythem to the surface, where they becomeconcentrated. When plankton die and dis-integrate, some debris sinks to the bottom,but tons of cellular particles, along withoils, fats, and proteins, float to the surface.

Accumulation of these natural organicchemicals modifies the physical and opti-cal properties of the sea surface. Thin or-ganic films, invisible to the naked eye, areubiquitous in lakes, oceans, and rivers.Where currents converge, these filmsmerge and thicken; wave action sometimesmakes them visible as “surface slicks.”Strong surface tension acts on the slicks,resulting in a layer of sandwiched mole-cules, about as thick as a human hair, thatresists turbulent mixing. This unique sur-face layer habitat even extends into the at-mosphere. Just above the surface film,millions of bursting bubbles contribute toan aerosol blanket containing dense con-

centrations of both natural chemicals andman-made pollutants.

Metal ions, common in seawater, bindto the organic molecules and concentratewithin the surface film, creating an envi-ronment that is very different from the sub-surface waters. Some metal ions, such asiron, are necessary and useful to marinelife; others, from human pollution, are poi-sonous. Such toxins as copper, lead, zinc,and cadmium, for instance, have beenfound in the microlayer in concentrationsof 10 to 100 or more times greater than inthe water below. Pesticides have beenfound in concentrations up to millions oftimes greater than in the rest of the water.

This complex aquatic surface is surpris-ingly stable and can hold together despitebuffeting by sixteen-knot winds and four-foot waves. According to Soviet biologistYuri Zaitsev, fish eggs, larvae, and fry cancling tenaciously to the surface layer evenin waves three to six feet high. Generally,winds strong enough to whip up whitecapsand cause surface mixing are not as wide-spread as often imagined, occurring on lessthan 5 percent of the earth’s surface at anygiven time. Even when disturbed andmixed, visible surface slicks can re-form in

JOE LEMONNIERThe ocean's uppermost layer concentrates nutrients and toxins alike, above. Pollutants enter from sewage, drainage systems, or the atmosphere and are ingested by skimming birds, top-feeding fish, and sometimes, by human swimmers.

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Article 34. Where the Sea Meets the Sky

less than an hour after the strong windscalm down.

Because of its unique tendency to col-lect and condense chemicals, this resilientsurface habitat is increasingly threatenedby a variety of human activities, particu-larly the dumping of industrial wastes andwidespread atmospheric pollution. Somenonsoluble pollutants bind to buoyant par-ticles and wind up concentrated within thesurface microlayer. Contaminants that fallfrom a fouled atmosphere collect in thenatural organic films. Like nutmeg powdersprinkled on an eggnog, such particles eas-ily become more concentrated on the sur-face than in the waters below.

We are all familiar with the dramaticdestructiveness of large petroleum spills,although most television images are ofcuddly otters and birds suffering fouled furand feathers. However, the less visible—but much more pervasive—chronic con-tamination of the microlayer may presentan even greater threat to many species. Oil,spreading over the water’s surface at thesame time that fish are releasing their float-ing eggs, can devastate a population’s re-productive success.

Our research team and a few othershave been trying to assess this less obviousdanger to animals and plants that dependon the microsurface habitat for food andreproduction. In the more than 200 micro-layer samples we have collected from riv-ers, estuaries, bays, and oceans (includingPuget Sound, Chesapeake Bay, the NorthSea, and the waters off Southern Californiaand Florida), there is a sadly consistentpicture: the surface microlayer is becom-ing a soup of toxic metals, organic pollut-

ants, bacteria, pesticide residues, and thebyproducts of combustion-derived hydro-carbons from cars, trucks, airplanes, refuseincinerators, and power plants. Coastalsewage waste-water discharges, runofffrom municipal and agricultural drainagesystems, and direct industrial dischargesinto rivers contribute to the contamination.

We assess the effects of pollution byobserving the development of healthy fisheggs placed both in containers attached tomoored buoys and in lab dishes filled withsamples collected from polluted microlay-ers. Using the known data on the healthydevelopment of fish eggs in clean water asa baseline, we have repeatedly observedthat larvae hatched in polluted microlayerseither die, develop slowly, or emerge mal-formed. We have often seen this result insome widely scattered locations through-out the world.

The same physical stability that enablesthe microlayer to support so much life alsofosters the persistence of pollutants in highconcentrations. In the presence of sunlight,some contaminants break down into evenmore harmful chemicals, although othersmay disintegrate and dissolve harmlessly.

Dense populations of microbiota andsmall animals inhabiting the aquatic sur-face layer form the base of an extensivefood chain. While seabirds and other crea-tures feast on the microlayer’s bounty fromabove, many larger organisms from thedeep sea migrate upward to feed at the sur-face. A polluted surface microlayer has thepotential to poison much of the complexmarine food web, including fish, crusta-ceans, whales, and seabirds.

Destruction of the microlayer may evenalter the exchange of materials between theatmosphere and the ocean, thereby affect-ing global climate. According to Robert J.Charlson, an atmospheric chemist at theUniversity of Washington, microscopicplants (phytoplankton) from the ocean’supper layers may be part of a thermostaticsystem that regulates the amount of solarenergy that warms the earth. A complexchain begins when the plankton produce asulfide gas that carries particles of sulfurinto the atmosphere. Cloud droplets formaround them, and their density determineshow much solar heat reaches the earth. Ac-cording to the theories of Charlson andothers, recently backed up by studies inAustralia, these natural emissions are theworld’s main source of the nuclei for cloudcondensation and a major source of sulfur.Where the layer is polluted, as in the NorthSea, more sulfide gases flow into the atmo-sphere, producing denser clouds that coolthe air. This in turn may cause a decreasein phytoplankton and a seesaw warming ofthe region.

Studying the sea’s remarkable “skin”has confirmed our worst suspicions aboutthe global dimensions of the poisoning ofour oceans. This surface habitat is becom-ing as endangered as the dwindling home-lands of the mountain gorilla, but itsdestruction may affect many more lifeforms. It is a vast trough for pollutants toenter into the web of life, a juncture for un-derstanding the complexities of marineecology, a sensitive diagnostic indicator ofenvironmental problems, and quite possi-bly the key to a thermostatic system regu-lating global climates.

From Natural History, May 1991. © 1991 by the American Museum of Natural History. Reprinted by permission.

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