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Shedding Light on Light in the Ocean

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    Shedding Light on Light in the Ocean
    New research is illuminating an optically complex environment

    By Sönke Johnsen, Assistant Professor           ing visual field whose brightness increases                        would see looking down and horizontally
    Biology Department, Duke University
                                                    and decreases by a factor of a hundred as                          would be UV. Light passing through water
    and Heidi Sosik, Associate Scientist
    Biology Department, Woods Hole                  each wave passes by, making it impossible                          also becomes polarized, which means its
    Oceanographic Institution                       for eyes to adjust.                                                wave motion vibrates in only one direc-
                                                       If our diver wore ultraviolet (UV)-                             tion, or plane. (This also happens to the

    L    ight in the ocean is like light in no
         other place on Earth. It is a world
    that is visibly different from our famil-
                                                    viewing goggles, almost half the light she                         light reflected as glare from the sea
                                                                                                                       surface or a wet road.) If our diver wore
                                                                                                                       sunglasses that blocked vertically polar-
    iar terrestrial world, and one that marine                                                                         ized light and looked to her side, her view
    animals, plants, and microbes are adapted                                                                          would be dark; but if she looked up or
    to in extraordinary ways. Light behaves                                                                            down, her view would be full of light. The
    very differently when it moves from air                                                                            polarized sunglasses would block light
    into water. It moves through the expansive                                                                         vibrating in the horizontal plane, while
    depths of an ocean that is devoid of solid                                                                         allowing light vibrating in the vertical
    surfaces. These and other factors com-                                                                             plane to pass through.
    bine to create an environment that has no
    equivalent on land.                                                                                                Light frames life
        A scuba diver in the open ocean                                                                                    In many ways, light permeates and
    discovers she is immersed not only in                                                                              creates the environment that ocean or-
    water, but also in an ethereal blue light.                                                                         ganisms experience. The same is true
    Seawater absorbs light much more                                                                                   on land—though we often take light for
    strongly than air does, but visible light is                                                                       granted. We see light as background, when
    made up of a rainbow of different                                                                                  it actually sets the stage and dictates our
    wavelengths, each perceived by us as a                                                                             view of the world. As sure as day follows
    different color. Blue light penetrates                                                                             night and the sun makes plants grow,
    farther into seawater (giving the ocean its                                                                        light frames our existence. So it is, too, in
    distinctive color). At the same time                                                                               the ocean, where animals and plants are
    seawater absorbs red, orange, and yellow                                                                           bathed in different kinds of light com-
    wavelengths, removing these colors. Only                                                                           ing from all directions, in a way we land-
    a few meters below the sea surface, if our                                                                         dwellers never experience. Some animals
    diver looked into a mirror, she would see                                                                          possess visual systems that see ultraviolet
    that her red lips appeared black.                                                                                  and polarized light (which people cannot)
        In calm weather, the diver can look                                                                            and have clever strategies to avoid detec-
                                                                                                   Larry Madin, WHOI




    upward to see the entire hemisphere of                                                                             tion in an environment where one can be
    the sky compressed into a circle over her                                                                          seen from every angle and there is nothing
    head—a phenomenon called Snell’s win-                                                                              to hide behind. Even in the sunless depths,
    dow, caused by the bending of light as it       A scuba diver in the open water is immersed                        light still plays a crucial ecological role,
    enters water. Rough weather and waves           in clear, pure blue light. Water strongly                          with armadas of bioluminescent life mak-
    shatter this window. The waves act like         absorbs red, orange, and yellow light, while                       ing and using their own light.
    lenses to focus light, creating a scintillat-   blue light penetrates into the depths.                                 Humans have had limited ability to ex-


1    Oceanus Magazine • Vol. 43, No.2 • 2004                       Woods Hole Oceanographic Institution • oceanusmag.whoi.edu
    plore the dimension of light in the ocean.             by seawater. Several hundred miles from                          plankton is green and darker for the same
    Today, however, new technology promises                shore, our diver sees extraordinarily clear                      reason that dense tropical forests are: Plant
    to reveal how light operates in the ocean              and pure blue water because water in the                         pigments absorb blue and red light and
    and how it creates a unique environment                open ocean has low concentrations of dis-                        reflect green light, and the cells scatter the
    and assorted phenomena that ocean life is              solved matter and particles, including phy-                      light more than pure water does. While
    adapted to.                                            toplankton. It does not scatter and absorb                       the diver would not be able to detect mi-
                                                           as much light as murkier coastal water                           croscopic phytoplankton cells, she would
    Wavelengths in the waves                               does, and the light that remains is blue.                        see their cumulative effect on the light in
        When light hits a substance, it can do                Nearer the coast, light penetrating                           the ocean.
    one of three things: it can be scattered,              seawater provides the energy to fuel vast
    by hitting molecules of the substance and              photosynthetic hordes of microscopic ma-                         Providing the oxygen we breathe
    bouncing off in different directions; it can           rine plants—phytoplankton—which are                                 Beyond their impact on light, marine
    pass through the substance; or it can be               an essential source of food and oxygen for                       phytoplankton also have enormous impact
    absorbed by the substance—either wholly                the entire planet. Just below the surface,                       on our planet. Photosynthesis by these
    or in only some wavelengths. Much sun-                 our diver will see green light, depending                        abundant cells produces roughly half the
    light reflects off the ocean, but much also            on how much phytoplankton is present.                            oxygen in our air.
    penetrates into it and is strongly absorbed            Water with high concentrations of phyto-                            Phytoplankton have evolved the abil-



                                                                  Light in the Ocean

                                                          Scientists use a satellite that detects light
                                                          reflected from seawater to create a map of
                                                          phytoplankton in the ocean. Warmer colors
                                                          indicate more chlorophyll in the water.
                                                          SeaWiFS image (left) courtesy of Heidi Sosik.



                                                                      ���������


                                                                                                                                             Sunlight reflects off the ocean
                                                                                                                                             surface and penetrates the
                                                                                                                                             water, where it encounters
                                                                                                                                             water molecules of dissolved
                                                                                                                                             substances, suspended par-
                                              Green light reflected
                                                                                                                                             ticles including phytoplankton,
                                              upward from phytoplank-
    Visible and ultraviolet                                                                                                                  and animals.
                                              ton can be detected by
    wavelengths (colors)
                                              Earth-orbiting satellites
    penetrate to different
    depths in clear open
    ocean water.

     �� �������������
                        �����
                         ��
                        ������
                                                Phytoplankton
                                                absorb red and                                                                      ����������
                                                blue light and                                                                                   ����������
                                                reflect green light
                         ���
                                                             �������������
                                                                                                                                                                               Jayne Doucette, WHOI Graphic Services




                                                                      Sunlight loses warm
                                                                      colors as it penetrates                  ����������                 ���������
                                 Ocean animals are                    deeper into the ocean                 �����
                                 adapted to the light
                                                                                                           ���������
                         ���
                                 surrounding them                                                         Light hitting a
                                                                                                          substance can
                                                                                                          scatter in all
                                                                                                          directions

                         ���



2     Oceanus Magazine • Vol. 43, No.2 • 2004                                     Woods Hole Oceanographic Institution • oceanusmag.whoi.edu
    ity to use the blue and green light found
    in the ocean. If water were not relatively     Adaptations to light, from surface to depths
    transparent to this light, aquatic photo-
    synthesis would not be possible, and the                                     The dark backs and light undersides of
    ocean would be largely a dead zone. In ad-                                   these near-surface fish help them match
    dition, if coastal waters become less clear                                  their environment in the open ocean. To
    due to human activities, photosynthesis by                                   a predator looking from above, their dark
    phytoplankton may decrease.                                                  backs seem to blend into the dark depths.
        Phytoplankton form the base of the                                       From the side, their lighter sides blend
    prolific marine food chain, which ulti-                                      with the sunlit water.
    mately also helps feed people and other
    terrestrial life. Throughout Earth’s his-
    tory, phytoplankton have also played an                                      Many open ocean animals use invisibility
    important role in regulating Earth’s cli-                                    to hide in plain view. Adapted to limitless,
    mate. They remove huge amount s of the                                       featureless blue surroundings, this plank-
    greenhouse gas carbon dioxide from the                                       tonic ctenophore, Cestum, lives near the
    atmosphere, turning it into organic matter                                   ocean surface. The complete transparency
    via photosynthesis. Much of this organic                                     if its body makes it almost impossible to
    carbon is consumed by animals in upper                                       see against the open ocean waters.
    ocean waters. Some falls to the seafloor, as
    dead organisms or fecal pellets, where it is
    consumed or converted over time into oil
    and gas deposits.                                                            The hatchet fish is well prepared for the
                                                                                 midwater ocean’s light levels. Bright silver
    The threat of UV radiation                                                   sides reflect whatever light surrounds it.
        Marine photosynthesis is confined to                                     Long, tubular eyes capture and detect
    the tiny fraction of the ocean where sun-                                    low light levels. Living at depths from 200
    light penetrates—at most, the upper 200                                      to 1000 meters, it has ventral (underside)
    meters. UV light also penetrates into                                        light organs that can produce biolumi-
    this region, which may have increasingly                                     nescence to match light coming from
    profound consequences. UV radiation                                          above, making it less visible from below.
    can cause damage to organisms on both                                        With enormous upward-looking eyes that
    land and sea. Recently, scientists have                                      fill half its otherwise transparent body,
    discovered that ultraviolet radiation can                                    the deep-living shrimplike amphipod
    harm organisms deeper down than previ-                                       Cystisoma is well-suited to its dim world.
    ously thought.                                                               It needs such large eyes to detect the little
        Decreasing ozone levels in the atmo-                                     light available in its midwater environ-
    sphere, including the ozone hole over                                        ment (800 meters), and red eyes, at that
    Antarctica, may exacerbate the problem,                                      depth, look black—and invisible.
    because ozone blocks UV radiation from
    reaching Earth. Higher levels of UV can
    kill phytoplankton, slow their growth, or                                    Atolla is a jellyfish common from mid-
    disrupt the delicate balance of species that                                 water, about 500 meters deep—where
    interact in ocean ecosystems.                                                there is still a small amount of sunlight,
                                                                                                                                 Images courtesy of Larry Madin/WHOI




        Marine organisms have evolved ways                                       to bathyal depths of 4500 meters—far
    to protect themselves from UV, including                                     below the limit of sunlight’s penetration.
    UV-absorbing pigments, the ability to re-                                    Where there is light, its red color looks
    pair DNA damaged by UV, and developing                                       black, making it hard to see. It also pro-
    behavior to avoid UV by staying in deeper                                    duces brilliant bioluminescence, possibly
    water. However, the recent ozone changes                                     to frighten predators.
    may be occurring too fast for


3    Oceanus Magazine • Vol. 43, No.2 • 2004           Woods Hole Oceanographic Institution • oceanusmag.whoi.edu
    organisms to adapt. Given the fundamen-
    tal role of phytoplankton in Earth’s biol-
    ogy, chemistry, and climate, these changes
    may affect us all.




                                                                                                                                                            Courtesy of Tom Cronin
    Into the darker depths
        As our diver continues to descend only
    a few more meters, she begins to go from
    day to night. She can see blue light to her
    sides, and white light above, but below         The advantage of UV vision shows in reef views in visible (left) and ultraviolet (right) light. In UV
    her the view is dark. As she moves down-        light, the fish are in much higher contrast to the background.
    ward, the UV, green, and violet wave-
    lengths disappear, and the light becomes
    an intense, almost laser-like, pure blue.
    At 200 meters deep, the diver would cross
    from the surface realm (called the epipe-
    lagic zone), where there is enough sunlight
    for photosynthesis, to the twilight realm
    (called the mesopelagic zone), where
    enough sunlight penetrates for vision, but
    not for photosynthesis.




                                                                                                                                                            Courtesy of Nadav Shashar
        By now, our descending diver would
    notice nearly continuous blue flashes
    around her—bioluminescent light pro-
    duced by animals in the midwater zone,
    in response to the disturbance in the
    water that she caused. Below 850 meters,        Bottom: Prey species like the copepod (Labidocera), are nearly transparent in visible light (left),
    though, the diver would no longer be able       but are brightly visible when photographed in polarized light (right), or to a predator that can
    to see anything, even looking up. Human         see polarization of light.
    eyes aren’t sensitive enough to detect the
    minute amounts of sunlight that haven’t         sorbing UV light. This makes the animals             enormous crustacean eyes that fill the
    been absorbed by the water. At 1,000 me-        appear dark and silhouetted against an               animal’s entire head capture as much light
    ters, even the most visually sensitive deep-    otherwise bright background of UV light.             as possible.
    sea animals can longer see the sun. The         With UV vision, animals can see animals
    region below this is known as the aphotic       that are transparent in visible light.               Prey—using light to camouflage
    (no-light) zone, but this is only true for          Some ocean animals, such as shrimp                   But seeing is only part of the equation.
    sunlight, as bioluminescence is common.         and squid, can even see the polarization             Ocean organisms’ visual adaptations are
                                                    of underwater light—due to a special geo-            matched by clever strategies to avoid
    Predators—using light to hunt                   metric arrangement of their retinas. With            being seen in an open ocean where it is
        Not surprisingly, aquatic animals pos-      this ability, they can actually navigate by          difficult to hide. Some color themselves to
    sess visual systems that are specially adapt-   the skylight polarization pattern, or detect         match the background water. Others have
    ed to the nature and properties of light        otherwise transparent, or silvery-scaled             mirrored sides, because a mirror in the
    underwater. Animals living near well-lit        prey by seeing its effect on the polarization        ocean only reflects more of the ocean, and
    surface regions have eyes similar to terres-    of light.                                            so is invisible.
    trial species. They have color vision, since        In deeper, mesopelagic regions with                   Still others camouflage themselves
    light near the surface still has color.         less light, the animals often have bizarre           with light, hiding their silhouettes with
        Many also have UV vision, which             adaptations to increase their visual sensi-          light-producing organs on their down-
    advantageously extends their range of vi-       tivity. They see in extremely low light lev-         ward-facing surfaces that mimic the sur-
    sion. Many animals contain compounds in         els, though at the expense of acuity, with a         rounding illumination. Many are simply
    their tissues that protect them against UV      reduced ability to detect rapid movements.           transparent, matching their background in
    radiation by scattering, reflecting, or ab-     Long, tubular, telescope eyes in fish, or            all situations.


4    Oceanus Magazine • Vol. 43, No.2 • 2004                        Woods Hole Oceanographic Institution • oceanusmag.whoi.edu
       Finally, some use light and dark for           to insights on satellite-based ocean color              questions about the operation of light in
    disguise. They hide in the depths during          measurements, global phytoplankton pro-                 the ocean, and its role in the lives of ma-
    the day, rising to feed at night, or they stay    ductivity, and how marine animals use                   rine phytoplankton and animals. We are
    near the surface, hiding in the glittering        light and camouflage.                                   gaining new understanding of this hereto-
    background of the lensing waves.                      The door is now open to answering                   fore shadowy—but not lightless—realm.

    Instruments to measure light
        To study light in the sea, scientists use a
    wide range of instruments—submerged in
    the depths and sent into space. Submers-
    ible radiometers, or light meters, measure




                                                                                                                                                                    Left photo and inset by Larry Madin, right photo by Terry Rioux, WHOI.
    ultraviolet and visible light and detect
    extremely low light levels in the deep sea.
    They numerically describe the shape of
    the light field in the ocean and measure
    how light is absorbed and scattered in wa-
    ter over small spatial scales. They can be
    lowered from ships, placed on submersible
    vehicles, or even carried by scuba divers to
    investigate the optical environment of spe-
    cific areas or depths.
        One of the most exciting advances in
    oceanography has been the developing
    ability to measure changing color over
    wide swaths of the ocean, using Earth-or-         DIVING INTO DIFFERENT WORLDS: Open ocean water (left) contains few particles and absorbs
    biting satellites that carry spectral radi-       warm colors, so blue light penetrates far into the clear distance. Near the coast, high nutrient
    ometers that measure light reflected from         levels allow dense growth of phytoplankton (center), making the water appear green and
    the surface layer of the ocean. These color       darker (right). Forests appear dark and green for the same reason—plant pigments absorb red
    changes indicate changes in the global dis-       and blue wavelengths of light, and reflect remaining green light.
    tribution of phytoplankton. For the first
    time, oceanographers can see how phyto-
    plankton populations bloom, collapse, and                            Sönke Johnsen entered biology with backgrounds in math, physics, and art and has
    change over time in any area, and these                              since used all three fields to investigate the visual ecology of oceanic zooplankton.
    satellite views have revolutionized how we                           After a frustrating graduate career, in which he studied the vision and behavior
                                                                         of animals with neither eyes nor brains, he completed postdoctoral fellowships at
    think about the upper ocean.
                                                                         Harbor Branch Oceanographic Institution and WHOI. After a year as an assistant
                                                                         scientist at WHOI, he accepted a position in the Biology Department at Duke Uni-
    A bright future                                                      versity. He is interested in all aspects of vision in oceanic species, with a particular
        Despite continually improving satellite       emphasis on strategies for camouflage. The camouflage research has involved investigations into UV
                                                      vision, whole-body transparency, cryptic coloration, and counterillumination using techniques rang-
    data, optical oceanographers still have too       ing from blue water diving to protein biochemistry to Monte Carlo modeling of photon trajectories. He
    few observations. Satellites view only the        gives numerous talks to the general public and his work has been featured in newspapers, magazines
    upper few meters of the water. With un-           and a John Updike poem*. (*http://www.swarthmore.edu/NatSci/SigmaXi/archive/SX_johnsen_an-
                                                      nouncement.pdf)
    derwater instruments, we can sample only
    relatively few specific places and times in
                                                                           Heidi Sosik received bachelor and master of science degrees in Civil and Environ-
    the ocean. What’s lacking is a way to visu-                            mental Engineering from MIT in 1988 and a doctorate in Oceanography from
    alize the entire ocean—surface to depths                               Scripps Institution of Oceanography in 1993. She joined the Woods Hole Oceano-
    and across the globe.                                                  graphic Institution as a Postdoctoral Scholar in 1993 and is now an Associate
                                                                           Scientist in the Biology Department. Her research interests focus on phytoplankton
        Computer models offer a way to ap-                                 ecology and factors that influence light in the marine environment. She received a
    proach this deficit. Combining available                               Presidential Early Career Award for Scientists and Engineers in 1996 and is cur-
    observations with new, accurate computer          rently a joint Fellow of WHOI’s Ocean Life Institute and Coastal Ocean Institute. “Never a dull mo-
                                                      ment” is an apt description of life for Heidi, whose family—one husband, three kids, two dogs, and a
    models that simulate how light behaves            cat—keep her keel even and her sails full through the ups and downs of a career as an oceanographer.
    and propagates through the ocean has led


5    Oceanus Magazine • Vol. 43, No.2 • 2004                           Woods Hole Oceanographic Institution • oceanusmag.whoi.edu

				
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