Download 1 One of the most important aspects of understanding ocean life is

HOW DO WE MEASURE FACTORS THAT SUPPORT LIFE IN THE OCEAN?
O
ne of the most important aspects of understanding ocean life is understanding, in a
quantitative way, the physical environment in which that life exists. Biologists and
oceanographers have developed a number of techniques for measuring the physical properties of water and other factors to track what life can survive in various ocean
environments. These include measurements of temperature, salinity, density, light levels,
dissolved oxygen, ocean topography, and ocean color. Measurement platforms are found
both on Earth and in Earth’s orbit. The former provide ground truth and the latter allow a
big picture view.
This theme (Life - Measurements) describes measurements that can be made at, beneath,
and above the ocean surface of physical factors important to ocean life forms. The focus is on in
situ measurements (e.g., from ships and buoys) of abiotic factors, although there is some discussion of satellite measurements.
Related Themes:
• A discussion of abiotic factors that affect ocean life can be found in Life - Scale and
Structure, along with more information on the overall size and classification of marine life forms.
• Biotic and abiotic influences on marine ecology and the oceanic food chain is discussed in Life - Systems and Interactions.
• Photosynthesis and the photic zone are presented in Life - Energy.
• How salinity, temperature, density, and pressure affect the oceans’ vertical structure
is featured in Oceans - Scale and Structure.
• Ocean upwelling is examined in Oceans - Systems and Interactions.
• How we measure the physical condition of our oceans is presented in Oceans - Measurements.
Related Activities:
• Describing and Measuring the Oceans
• Estimating the Population of Pencils in Your School
• Building a Plankton Net
• Measuring the Density of Water
• Oxygen, Carbon Dioxide, and Plankton
• Plankton Identification
INTRODUCTION
Quantitative measurements are the foundation of modern science. Measuring life-affecting ocean
factors helps scientists develop theories about the nature, evolution and distribution of life on
Earth. Ultimately, these measurements could also be used to determine whether other planets
might harbor environments that could support life.
Ocean scientists employ a variety of techniques for measuring the physical properties of the
ocean environment. Both in situ (at the source) and satellite measurements provide information
that scientists use to understand the factors that support or endanger life in the ocean.
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COLLECTING WATER FOR DIRECT MEASUREMENTS
Water, of course, is the most
abundant physical component of
the ocean. A number of techniques are used for sampling
water and measuring its properties.
Surface water can be collected
by simply throwing an empty
bucket over the side of a ship and
hoisting the filled bucket back on
board. When sampling ocean
water in this manner, researchers
must be careful to keep the
bucket away from the ship’s engine or bilge pump, and any other
nearby contaminants.
Subsurface water samples are
commonly obtained with instru- Movie 1. Collecting subsurface water with a Van Dorn bottle.
ments such as Van Dorn bottles. A
Van Dorn bottle is essentially a brass tube with valves on both ends. When the bottle is lowered
into the water on a cable--called a hydrowire--both valves are open so that water can easily pass
through. At the desired sample depth, a device consisting of a collar-like brass weight--called a
messenger--is released and seals the tube [Movie 1].
The ocean water sampling techniques described above have two distinct disadvantages. First,
they tend to be expensive. Because of the expenses associated with operating a research ship at
sea, the cost of obtaining a 750 milliliter (0.79 quarts) sample of sea water is currently in excess of
$20 U.S. Second, direct sampling can only measure small samples of water locally in our vast
oceans. Despite these disadvantages, local samples are necessary for the interpretation of other
data sets including satellite measurements. This type of fundamental data helps scientists develop a comprehensive understanding of the ocean environment.
SHIPBOARD, BUOY, AND LABORATORY MEASUREMENTS OF SEA WATER
Ocean water chemistry, which is the chemical composition and quantity of impurities in ocean
water, can be accurately determined in a shipboard laboratory. The same is true of physical
properties of water such as density and temperature. Measuring the chemistry and physical
properties of ocean water is something like doing a “blood test” on the ocean. Just as the abiotic
chemistry of our blood can tell us about our living body, abiotic chemistry and ocean physical
properties tell us about life in the ocean. Key properties measured in the ocean realm include
density, salinity, dissolved oxygen, pH, temperature, pressure, light levels, sound, and plankton
content.
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Density
Density is a measure of mass per unit volume. To determine the density of a sample of sea
water, the sample is weighed and its volume is measured. The weight of the sample container
must be subtracted to obtain the weight of the water alone. Dividing the weight by the volume
yields the density. Density can also be measured using a device called a hydrometer. It determines density by observing how high the hydrometer floats in a given liquid. The hydrometer
will float higher in water that has a higher density. Similarly, a person will float easier in salt
water than in fresh water because the salt water is denser and has increased buoyant force, upward force that a fluid exerts on an object less dense than itself.
Salinity
Salinity is the total quantity of dissolved solids in sea water. The most abundant dissolved
materials in the oceans are salts, mostly table salt or sodium chloride (NaCl). Thus, salinity can
be generally thought of as a measure of dissolved salts in sea water. Living organisms are often
very sensitive to the salt content of the water.
Table 1. Principal constituents of seawater. The top six substances make up 99.28% of the total dissolved solids in seawater. These are major constituents. The other five are minor constituents (less than
100 parts per million). Together, they comprise 34.482% of the mass of seawater.
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The salinity of a water sample can be measured using various methods. One involves measuring the density and temperature of the sea water. Because density depends primarily on
temperature and salinity, once density variations due to temperature are known, salinity can be
determined.
Another method is to allow a sample of water to evaporate and then measure the amount of
salt that remains; this is called a gravimetric determination of salinity. The residue of evaporated
sea water is typically a mixture of several different salts, along with other mineral and organic
materials.
The electrical conductivity of sea water is strongly dependent on its salinity. Thus measuring
the conductivity of a sample of sea water will determine its salinity. Presently, this is the method
most often used to determine sea water salinity.
Water samples and dissolved solids in those samples can be also studied in the laboratory
using a variety of techniques. A common laboratory technique is called titration. In titration, a
known quantity of some substance, X, that reacts in a well understood way with another substance, Y, is added to a sample of sea water. If a reaction occurs, Y must be present. The amount
of X consumed in the reaction determines the amount of Y in the sample. For example, an alkaline solution is added to a measured volume of an acid of unknown concentration. The amount
of the alkaline solution required to neutralize the acid allows us to calculate the concentration of
the acid. The average chemical makeup of dissolved solids in ocean water is shown in Table 1.
Dissolved Oxygen
Oxygen dissolved in water is
necessary for fish and other marine animals to breathe, just as
oxygen in the air is necessary for
animals on land as well as marine
mammals to breathe. Much of
the dissolved oxygen in water is
produced by the photosynthesis of
phytoplankton, which are microscopic floating plants.
Dissolved oxygen can be measured using specially designed
meters. These often consist of a
membrane that allows oxygen to
pass through to the measurement
sensor, but restricts other parts of
the seawater. Then, dissolved
oxygen content is inferred by
measuring current flow that varies with the dissolved oxygen
content [Movie 2].
Movie 2. Measuring dissolved oxygen.
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pH
The pH, or relative acidity of
a water sample can be roughly
determined using litmus paper. It
can be more precisely determined using the titration method
described above, or by using a
device known as a pH indicator
[Movie 3].
Temperature
The temperature of a surface
water sample can be measured
using a simple thermometer.
Measuring temperatures at
depth is trickier. A typical Nansen
bottle, similar to a Van Dorn
bottle, has two reversing thermometers attached to its exterior.
Movie 3. Determining pH.
To measure subsurface water
temperature, a Nansen bottle is
lowered to the desired depth,
and left there for approximately
ten minutes to give the thermometers time to adjust to the local
temperature. When the messenger triggers the bottle to flip, the
thermometers also flip. This
causes the part of the mercury
column that measures the temperature to separate. When the
Nansen bottle is retrieved, the
separated segment of mercury
shows the water temperature at
the depth where the sample was
taken.
Figure 1. ATLAS buoy. ATLAS mooring with sensors measuring
surface winds, air temperature, relative humidity, sea surface temperature, and ten subsurface temperatures.
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Anchored or moored buoys
are used to measure an array of
ocean conditions, including water temperature. The ATLAS
mooring [Fig. 1] for example,
measures temperature to a depth
of 500 meters (1640 feet). Sea surface and subsurface temperature
are measured by thermistors. At pre-selected depths on the buoy, the cable has been cut and a
thermistor sensor pod inserted. Data from ATLAS moorings are sent to scientists onshore using
Earth-orbiting satellites.
Pressure
Pressure is a measure of force or weight per unit area. The pressure at a given ocean depth is
a function of the weight of the column of water above that depth. Pressure increases with depth.
Ocean pressure increases by about one atmosphere every ten meters (33 feet). One atmosphere is
the average air pressure at sea level, about 1.03 kilograms per square centimeter (14.7 pounds
per square inch). In the deepest part of the ocean, the Mariana Trench (10,920 meters or 35,826
feet below sea level), the pressure is nearly 1,000 atmospheres, which is equivalent to a weight of
approximately 1,000 kilograms per square centimeter (14,700 pounds per square inch).
Light Levels (Photic Zone)
Typically, below a depth of
around 300 meters (984 feet), sunlight cannot penetrate and the
ocean is virtually pitch black.
However, the level of light penetration varies greatly depending
on clarity of the water. The region where enough light penetrates for plants to successfully
photosynthesize, the photic zone,
is less than 100 meters (330 feet).
Light levels in the ocean can be
measured using light meters, similar to the light meters commonly
used in cameras. To determine
approximate depth of light penetration at a given time, one can
also use a Secchi disk: a white disk
that is lowered into the water un- Movie 4. Determining light penetration with a Secchi disk.
til it is no longer visible from
above the water surface [Movie 4].
Sound
In general, sound waves travel faster in denser media, and thus sound travels faster through
water than through air. The speed of sound in water varies between 1400 and 1550 meters (4,593
and 5,085 feet) per second, depending on the local salinity, temperature, and pressure of the
water. In comparison, sound travels 344 meters (1,129 feet) per second in dry air at sea level.
Ocean sounds can be recorded using underwater microphones. The intensity of a sound is
measured in units called decibels. Some marine life forms, such as whales, produce sound for
communication in the form of “songs” [Movie 5]. Whales are capable of generating sounds as
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loud as 200 decibels, well above
the pain threshold for human
ears. Whale songs can carry for
thousands of kilometers through
the ocean.
Sound can also be used to locate marine organisms and other
underwater objects, and to measure the topography of the sea
floor. Sonar (SOund Navigation
And Ranging) uses the measured
travel time of sound waves to
determine the distance and scale
of underwater objects. Sound
generated by carefully controlled
explosions can be used to construct three-dimensional maps of
ocean structures and currents in Movie 5. Whale “songs.” Some whales have complex vocala technique called acoustic tomog- izations that last many minutes. They can sound like squeals,
raphy.
cries, or even groans. Whale “songs” are distinctive and easy to
recognize once heard.
Plankton
Although we focus here on
measurement of abiotic factors that affect ocean life, it is worth mentioning measurements of
plankton. Plankton are often microscopic in size, poor swimmers or floaters, and make up the
bottom of the ocean food chain. Variations in the abundance, location, and type of plankton affects almost all other marine life. They are also very sensitive to abiotic factors in the ocean,
particularly nutrients brought up from depth as a result of upwelling. As such, they are indicators of the abiotic environment as well.
Phytoplankton--which are plants
--photosynthesize and thus directly affect carbon dioxide and oxygen levels. Estimates indicate
that phytoplankton generate half the oxygen we breathe and absorb half the world’s carbon
dioxide.
Plankton are often collected for study using fine meshed plankton nets. Because they are
microscopic organisms, the physical dimensions and characteristics of phytoplankton (plants)
and zooplankton (animals) are usually measured using a microscope. The large-scale distribution
of plankton throughout the ocean, however, can be measured by ships and satellites as discussed
below.
SATELLITE MEASUREMENTS
Satellites in orbit provide a better than “birds-eye” view of our planet. At a sufficiently high
altitude, a satellite can observe an entire hemisphere of Earth. Because of this tremendous perspective, satellites are extremely useful for making large-scale measurements of the ocean. Remote sensing is the term used to describe the process of making measurements from a distance, as
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opposed to making in situ measurements. Passive sensors, such as cameras, measure radiation or
particles passively intercepted by the spacecraft. Radar is an example of an active sensor.
Data from the TOPEX/Poseidon satellite are used to map ocean currents whose patterns
have a profound effect on ocean life. TOPEX/Poseidon actively generates radar pulses that
bounce off the surface of the ocean. The timing of the returned radar signal is used to produce
maps of sea surface height. From these maps, ocean current strength and direction are calculated. An advantage of using such an active radar system is that TOPEX/Poseidon’s signal penetrates clouds and allows continuous observation of our oceans.
Satellites can accurately measure ocean color,
and from that data, infer information about plankton populations. Such satellite instruments include the Coastal Zone Color Scanner that flew
from 1978 to 1986, and SeaWiFS (Sea-viewing Wide
Field-of-view Sensor), which was launched in
August 1997. Subtle changes in ocean color signify various kinds and quantities of plankton.
Plankton blooms [Fig. 2], which are characterized
by the rapid growth of a plankton population, lead
to high concentrations of chlorophyll, a chemical
compound crucial for photosynthesis. Satellites
can detect chlorophyll concentrations by associated changes in ocean water color. Knowing the
locations of plankton blooms is very valuable to
fishermen and marine biologists, because fish and
other marine animals tend to collect near fresh
blooms. However, these types of passive sensors
cannot see through clouds and thus cannot collect data continuously.
Satellite and in situ measurements complement
one another. Combined with well-studied localized in situ measurements, scientists can use satellite data to create comprehensive, calibrated models of the ocean environment. In situ measurements provide the data needed to properly interpret the measurements made by orbiting spacecraft.
Figure 2. Plankton bloom viewed from space.
Plankton, seen from the space as a light blue color,
find a rich feeding ground in the cold waters lying off the Namibian Desert coast.
Although the cost of building and launching a satellite like TOPEX/Poseidon or SeaWiFS can
be great, the amount of data obtained by such satellites is so vast that the cost of individual data
sets is often relatively small. In addition, they provide global coverage which is crucial to understanding the ocean environment. New computer, sensor, and electronic technologies are further
reducing the cost of building satellites, and satellite ocean data are expected to become less expensive and even more powerful in the future.
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CONCLUSION
The measurement of physical quantities is fundamental to science. On ships, scientists can
measure many physical quantities in specific locations at the surface and at great depths: ocean
temperature, salinity, density, current speed, and plankton concentrations. Earth- orbiting satellites can be used to make global measurements of ocean wind speed, wave height, ocean topography, and sea surface temperature. Satellite ocean color sensors can detect the presence of
plankton blooms over large regions. One of the overall goals of gathering these data sets is to
better understand how ocean dynamics affect ocean life and whether patterns of ocean life are
changing over time.
VOCABULARY
abiotic
active sensor
calibration
density
gravimetric determination
hydrowire
litmus paper
pH
photosynthesis
pressure
salinity
thermistor
upwelling
acidity
alkaline
chlorophyll
dynamics
hemisphere
in situ measurement
messenger
passive sensor
phytoplankton
quantitative measurements
Secchi disk
titration
Van Dorn bottle
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acoustic tomography
atmosphere (unit of pressure)
decibels
food chain
hydrometer
light meter
Nansen bottle
photic zone
plankton
remote sensing
sonar
topography
zooplankton