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How
far can you see under water? Travel agents would have you
believe you can see forever in the warm waters of the Caribbean.
Alas, it isn't true. We can't see as far under water as in
the air. Why not?
Light
is attenuated much more by water than by air, limiting the
distance we can see. Attenuation is caused by a combination
of light being scattered and absorbed by water, particles,
and dissolved organic matter.
Although
dissolved organic matter is abundant in the ocean, most of
it is colorless and causes little attenuation of light. The
amount of attenuation by water is the same whether the water
is fresh or salty. Therefore, it is the type and abundance
of particles that control visibility through water in one
location versus another.
Light
attenuation in the ocean is measured with transmissometers
and a-c meters. (See "Tools
of the Trade" for information about instruments discussed
in this article.) The research we are doing with colleagues
and graduate students at Texas A&M University and several
other institutions explains the causes of attenuation and
investigates its effects on visibility in the ocean.
So who
cares about how far you can see under water other than scuba
divers, snorkelers, travel agents, and treasure hunters? Remember
the Gulf War in 1991? The United States Navy had ships ready
to launch an attack from the Persian Gulf against the Iraqis
in Kuwait, but it required that personnel carriers traverse
coastal waters and the beach, and these areas were known to
be littered with explosive mines. The only way to detect some
mines is to see them. Therefore, it is vitally important to
know how far you can see underwater and what sort of atmospheric
and oceanographic processes and conditions can change the
visibility in seawater. For this purpose, the Navy has funded
projects to study coastal mixing and underwater visibility.
To answer
these questions we first need to consider where the particles
in sea-water come from and how fast they settle to the ocean
floor. Are all the particles pollutants? By no means. In coastal
areas, rivers dump tons of sediment and dissolved nutrients
into the sea. As the rivers enter the ocean, coarser materials
quickly settle out near the mouth since there is a rapid decrease
in the turbulent mixing that keeps particles in suspension.
These sediments fill up estuaries and create deltas such as
the huge delta of the Mississippi River.
Finer
materials drift farther from the river mouth until they are
eaten and incorporated into fecal pellets, or react with salt
water and organic matter to form large aggregates (clumps
of particles), which settle faster than tiny, individual particles.
Without aggregation, small particles could take a century
to settle to the bottom of the ocean far from land, whereas
large aggregates or fecal pellets could travel 5000 meters
to the seafloor in a month or two.
See
"As clear as mud: Particles where
you least expect them" to see particles from different
water sources.
The huge
sediment input from the Mississippi and other rivers that
empty into the Gulf of Mexico is one reason that the water
along the Texas coast is much murkier than in the Caribbean.
However, you only have to go a few miles from the coast to
see clearer, greener surface water.
A quick
look through a microscope at a sample of the green water would
reveal very little riverine sediment, but you would see thousands
of tiny marine plants (phytoplankton) and animals (zooplankton).
These organisms thrive in water that has been fertilized by
nutrients brought in from rivers or mixed up from deeper waters.
The green color comes from the chlorophyll in the phytoplankton,
as these phytoplankton are busy converting carbon dioxide,
nutrients, and sunlight to plant material and oxygen through
the process of photosynthesis. The abundance of phytoplankton
can be estimated by the amount they fluoresce when a flash
of light hits them using a fluorometer.
When phytoplankton
die or, more likely, are eaten, they are usually incorporated
into a fecal pellet or an aggregation of sticky particles
and slowly sink. They take with them the nutrients that they
used to make plant parts. As the phytoplankton settle on the
sea-floor, they decompose and turn back into nutrients, carbon
dioxide, and dissolved organic matter. On the continental
shelf, where water ranges from zero to about 100 meters in
depth, storms or winter cooling and mixing can return the
nutrients to surface waters, and phytoplankton and the rest
of the food chain can thrive again.
If we
are concerned about seeing through seawater, we are concerned
not only with the number of particles but also the type of
particles present. Clay and sand absorb and scatter light
differently than phytoplankton. Clay particles are tiny jagged-edged
plates a few thousandths of a millimeter across, while phytoplankton
are somewhat larger spheres, tubes, and other shapes that
are designed to absorb light for the purpose of photosynthesis.
Last fall
our group joined researchers from several other institutions
around the country in the first phase of an intensive study
of a patch of shelf water south of Cape Cod, Massachusetts.
The area was far enough from any rivers that most of the particles
in the upper water column were biological in origin.
The purpose
of our study was to measure all the physical conditions that
cause mixing of the ocean-including wind, waves, currents,
tides, storms, cooling, and heating-and simultaneously measure
the visibility through seawater at different wavelengths.
We also collected and analyzed the particles in the water
that affect visibility.
Visibility
is quantified by using sensitive instruments that measure
the optical properties of seawater-especially scattering,
absorption, and attenuation of light-at several wavelengths
across the visible and even the infrared parts of the spectrum.
Physical and optical conditions were monitored by deploying
highly instrumented moorings to measure meteorological conditions
like air temperature, wind speed, humidity, and sunlight;
and water conditions such as currents, temperature, salinity,
wave height, turbulence, absorption and attenuation at one
to nine wavelengths, chlorophyll abundance, and particle-size
distributions. The moorings were also fitted with traps to
collect samples of settling particles.
During
a three-week period, one ship, the R/V Endeavor,
steamed in small and large box-shaped paths around the site.
She towed an instrument that descended and ascended between
the surface and the seafloor every five minutes to characterize
the spatial variability of temperature, salinity, and optical
properties in the region.
Our ship,
R/V Seward Johnson, generally stayed near the
moorings and made careful measurements through the whole water
column of the same parameters mentioned above.
Our Particle
and Optics Profiling System (POPS) measured the abundance
and size of particles and aggregates in the water from one
micron to several millimeters long. In addition, we collected
water samples to analyze the size and composition of particles
that affect the optical properties of the water. This information
will help to calibrate and interpret the optical signals from
the moored, towed, and profiling instruments. Satellites passing
overhead measured the surface temperature and wave characteristics,
giving us instantaneous "big picture" views of the
region.
The intent
of the project was to study the region first in the late summer
when solar heating had warmed the surface water and it was
hard to mix the water column. The sun had warmed the surface
leaving the cold, dense, nutrient rich water below. Then in
the spring we would study the region after cold winter storms
had cooled the surface and vertically mixed most of the water
column.
All went
well during the first two weeks at sea. The water was well
stratified with a warm surface layer that gradually got colder
with depth, just as a house heated by the sun will be warm
in the attic and cold in the basement.
Chlorophyll
fluorescence profiles through the water column and water samples
showed that phytoplankton were growing happily, especially
in a layer at about 25 meters below the surface. At the seafloor,
sediment was being resuspended about five to ten meters into
the water column by waves and currents. Everything looked
as expected. We even had some subsurface internal waves pass
by that affected most of the water column. These waves cause
no change in sea-surface elevation, but can cause ten- to
forty-meter vertical displacement of layers in the water in
just a few minutes! Undoubtedly these are important mix-masters
on the shelf.
Just
when we began to tire of our daily routine at the same spot,
word came of a hurricane heading our way. We watched the weather
reports with anticipation, and it soon became clear the storm
was headed right for us.
For two
days the swells grew larger. The phytoplankton either slowed
their growth or were getting mixed as the chlorophyll peak
at 20 meters dissipated. Resuspension of bottom sediments
increased. When the swells reached 12 feet and the winds intensified,
the captain headed for the safe harbor of Newport, Rhode Island,
as we tied everything down and cleared the deck.
The next
day, sustained winds of 50 miles per hour at the dock made
us glad we weren't at sea. Hurricane Edouard passed just east
of our site, but the moorings recorded the whole event, showing
massive resuspension of bottom sediments.
After
the hurricane passed we raced back to the site to see how
conditions had changed. The surface-water temperature dropped
from 19°C to 15°C as the hurricane sucked heat from
the water and mixed the colder water upward.
What had
been a water column that gradually grew colder and denser
with depth was now a two-layer system. A cool, upper layer
was fairly homogeneous in temperature and optical properties.
A lower, cold layer was well mixed in temperature but had
strong optical gradients with increasing turbidity toward
the seafloor, the source of the resuspended sediment.
Turbidity
at the bottom decreased rapidly over the next few days as
sediment settled out or cleaner water moved into the area.
A week later Hurricane Hortense passed further to the east,
but we were safely in port by then. Mooring data revealed
that Hortense caused a moderate episode of resuspension. Data
from all instruments indicate that the fine-grained sediment
was most easily resuspended by the hurricanes and that the
large aggregates measuring greater than 0.5 millimeters were
torn up during the intense storms.
If you
were looking for something in the water, you wouldn't do so
during a storm, but it was surprising how rapidly the resuspended
sediment settled out and visibility increased after the hurricane
passed. As our study continues we will learn how visibility
in this region changes with the seasons.
The area
of our study was covered with a fine-grained mud, which presented
a challenge for visibility. Visibility would be much greater
in a region where the bottom is covered with sand rather than
mud for three reasons:
1) Sand
is more difficult to resuspend than mud.
2) Once
resuspended, sand settles much more rapidly than fine-grained
particles.
3) Billions
of fine-grained particles make the water much murkier than
a much smaller number of sand grains, even if the total weight
of both types of particles is the same.
Normally
oceanographers want to avoid hurricanes that landlubbers bill
as going "harmlessly out to sea," but for this project,
the event couldn't have been timed better. As this issue goes
to press, we will be back at our site on a different ship
trying to understand the forces that mix the sea and the particles
that affect underwater visibility. We won't have to worry
about hurricanes in April, but we'll be on the lookout for
nor'easters!
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