HOW DO WE MEASURE FACTORS THAT SUPPORT LIFE IN THE OCEAN? One 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. 1
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 instruments such as Van Dorn bottles. A Movie 1. Collecting subsurface water with a Van Dorn bottle. 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. 2
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. 3
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. 4
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. 5 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 until it is no longer visible from Movie 4. Determining light penetration with a Secchi disk. 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 6
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 a technique called acoustic tomography. Movie 5. Whale songs. Some whales have complex vocalizations that last many minutes. They can sound like squeals, 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 7
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. 8
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 acidity acoustic tomography active sensor alkaline atmosphere (unit of pressure) calibration chlorophyll decibels density dynamics food chain gravimetric determination hemisphere hydrometer hydrowire in situ measurement light meter litmus paper messenger Nansen bottle ph passive sensor photic zone photosynthesis phytoplankton plankton pressure quantitative measurements remote sensing salinity Secchi disk sonar thermistor titration topography upwelling Van Dorn bottle zooplankton 9