3.5: Chemical and Physical Oceanography

Oceanographers aim to understand how the concentrations of various chemicals in seawater, whether dissolved or associated with suspended sediment, change temporally or spatially (over time and space). Most of these chemicals are found in seawater at very low concentrations (Chap. 5) making measurement of their concentration very difficult. As a result, most chemical concentrations cannot yet be measured directly by instruments lowered into the ocean. Instead, scientists usually need to collect water samples from specific locations and depths, then analyze them on a research vessel or in a laboratory on land.

Sampling Bottles

In all but shallow water, where samples can be pumped up through a hose, water samples are collected in specially designed bottles. Usually, sampling bottles descend in an open configuration that allows seawater to flow through them, so they are not crushed by the increasing pressure. Several sampling bottles can be attached at intervals on a hydrographic wire to sample at different depths during one lowering. When the bottles are at the desired depths, a brass or stainless-steel messenger is attached to the wire. (Fig. 3-11). The messenger slides down the wire and hits a trigger mechanism on the shallowest bottle. This closes the bottle and releases another messenger attached under the bottle. Thus, each bottle releases a new messenger to slide down the wire and close the next-deeper bottle.

Figure 3-11. Nansen water-sampling bottle. When the messenger hits the trigger mechanism, the top of the bottle is released from the wire, the bottle falls into an inverted position, and a mechanical linkage closes valves at each end of the bottle. The trigger mechanism also releases a second messenger, which slides down the wire to the next sampling bottle.

For many years, most water sampling was done with Nansen bottles (Fig. 3-11). These have been replaced by newer designs developed primarily to collect larger samples or to avoid sample contamination. Because concentrations of some important trace metals and organic compounds in seawater are extremely low (Chap. 5), oceanographers must often collect large volumes (sometimes tens or hundreds of liters) of seawater per sample. They then use sophisticated chemical techniques to extract and concentrate the target chemicals before even the most advanced and sensitive analytical instruments can measure the concentration.

Avoiding Sample Contamination

Contamination of the sample must be avoided during its journey from the ocean depths to the laboratory. Contamination may come from many sources, including the metals, plastics, and other sample bottle materials, the metal of the hydrographic wire, and the grease that covers the wire to protect it from corrosion. Dust, oil, and vapors in the ship and laboratory atmosphere are other potential contaminants.

The thin surface microlayer (about 0.1 mm thick or less) that covers the entire ocean presents an especially difficult contamination problem. The microlayer always contains higher concentrations of many chemicals than the seawater below, and it can be further contaminated by discharges from the research vessel, such as oily cooling water and paint and corrosion chips from the vessel’s hull. Sampling bottles that remain open as they are lowered through the sea surface retain a film on the inside of the sampling bottle deposited by the surface microlayer, which can significantly contaminate the sample. Several ingenious sampler designs, including the GoFlo bottle (Fig. 3-12), prevent surface microlayer contamination.

Figure 3-12. A GoFlo water-sampling bottle. This ingeniously designed bottle is lowered through the sea surface in a closed configuration to avoid contamination. At a depth of a few feet, a pressure-sensitive trigger opens both ends of the bottle. The bottle is closed again by means of a messenger or other triggering device when the bottle is at the required sampling depth.

The GoFlo bottle is carefully cleaned and sealed closed on the research vessel before being lowered through the surface. A pressure-sensitive mechanism opens the bottle automatically once it is a few meters below the surface. The bottle can then be closed at the required sampling depth. For chemical parameters not affected by surface microlayer contamination, one of many other sampler designs can be used.

Determining the Depth of Sampling

Ocean water forms a series of horizontal layers, each of which may be only a few meters thick (Chap. 10). Water moves great distances within these layers, but it mixes only slowly with the water in the next layer above or below. Chemical oceanographers usually want to sample within each of the layers, and sometimes at closer depth intervals across the interfaces between layers. Unless the precise depths of the layers are known, selecting the appropriate spacing of water-sampling bottles along the wire to sample all the layers is impossible. In addition, even if samplers are placed correctly along the wire, the depth at which each sampler is closed can be affected by curvature of the wire caused by currents or vessel drift (Fig. 3-2).

Density increases with depth in the oceans, so depth may be estimated by measuring density. The two primary parameters that determine the water density are temperature and salinity, so density may be calculated and depth estimated by measurement of these two parameters. Salinity, temperature, and density relationships are discussed in CC6 and Chapter 5. The approximate depth from which a sample is obtained can often be determined if the temperature and salinity of the water at the depth at which a sampler is closed are measured. Salinity can be determined by measuring the electrical conductivity of the sample of seawater collected after it has been returned to the laboratory. However, the temperature of the sample changes as the sample bottle is retrieved so, before the development of sensitive electronic thermometers, the temperature had to be recorded at depth when the sampler is closed. This temperature measurement was achieved by a “reversing thermometer” mounted on the outside of the sampler (Fig. 3-13a). The thermometer on a Nansen bottle reverses with the bottle itself (Fig. 3-11), but other sampling bottles, which do not themselves reverse, have an externally mounted thermometer rack that rotates mechanically through 180° as the bottle is closed. To accurately determine the depth of sampling, two reversing thermometers can be used, one protected and one unprotected from pressure changes with depth (Fig. 3-13b). This method is so accurate it is still used today to calibrate the electronic temperature sensors.

Figure 3-13. (a) Reversing thermometers like these are attached in their set position to a water-sampling bottle in a rack. (b) When the bottle is closed, the rack and thermometers are turned upside down (reversed). The protected thermometer records the true temperature, while the unprotected thermometer records a temperature that is slightly too high because the mercury reservoir is squeezed slightly by the increased pressure at depth. The temperature difference between the thermometers can be used to determine the depth at which the thermometers were reversed. Reversing thermometers are still used to calibrate CTDs, which are instrument packages that measure conductivity, temperature, and depth electronically.

Instrument Probes and Rosette Samplers

Using water sampling bottles and reversing thermometers to determine salinity, temperature, and depth is tedious and only provides data only at the sampled depths. This method was replaced in the 1970s by instrument packages containing electronic sensors attached to a wire that make measurements continuously as they are lowered and raised. These packages are called CTDs, for conductivity (electrical), temperature, and depth (or sometimes “STDs,” for salinity, temperature, and depth), because electrical conductivity is measured to determine salinity (Chap. 5). CTDs have many advantages over sampling bottles, especially their ability to read salinity and temperature continuously as a function of depth. Such sensors, or probes, have enabled oceanographers to observe small-scale variations in the layered structure of ocean water that cannot be discerned from widely spaced bottle samples. The wire used to lower CTDs have electrical conductors running throughout their length (often tens of thousands of meters). These wires supply electrical power to the sensor package and also return the sensor signal to a processing unit in the ship’s laboratory. This wire is more expensive than the simple steel wire, but the advantages justify the extra cost. Therefore, these CTDs enable scientists to see the variations of salinity and temperature with depth. The special CTD wires, which have electrical conductors running throughout their length (often tens of thousands of meters), are much more expensive than the simple steel wire used with sample bottles

Although oceanographers no longer need to use water-sampling bottles to determine salinity, temperature, and depth, samples of water must still be collected and returned to the laboratory for analysis of most dissolved chemicals. The dissolved constituents and the importance of variations in their concentrations are discussed in Chapters 5, 12, and 16. If the sampling bottles are mounted around a CTD (Fig. 3-14), samples can be taken at precise locations within the various water layers. Each bottle can be closed when the CTD readings in the shipboard laboratory show that the bottle is at the appropriate depth. For this purpose, a rosette sampler (Fig. 3-14) is used that consists of a rack for mounting 12 or more sample bottles around the CTD sensor package and an electronically operated trip mechanism. Signals sent down the support wire to the trip mechanism close each sampling bottle individually.

Figure 3-14. A rosette water sampler has sample bottles, some fitted with thermometer racks, arranged in a rosette pattern around an electronic device that enables researchers on the ship above to close individual sample bottles at selected depths through an electrical signal sent down the hydrographic cable. The round frame below the water-sampling bottle rack usually contains a CTD and possibly other sensors, such as turbidity-measuring devices.

There is great interest in developing sensors to add to the CTD that would continuously measure dissolved concentrations of important chemicals. Dissolved oxygen, pH, and turbidity are among the relatively few parameters for which reliable sensors exist. To date, none of the other sensors are as reliable and sensitive as the salinity, temperature, and depth sensors are. However, a wide range of chemical sensors are now beginning to reach the stage of development where they are providing useful data. 

Measuring Currents

Oceanographers are interested in studying the movement of water in currents that are present throughout the ocean depths. The speed and direction of currents can be computed from salinity and temperature distributions in the oceans (Chap. 8), and water movements can also be studied using chemical or radioactive tracers dissolved in the water. These indirect methods, especially when combined with mathematical modeling techniques (CC10), are extremely valuable for studying the movements of water averaged over large distances (tens of kilometers or more) and long periods of time (months or longer).

Because tracers are much less useful in studying the small-scale details of current distributions and their variability with time, a variety of systems are used to measure currents directly. There are three basic types:

• Passive devices flow with a current wherever it goes and periodically or continuously report their position.

• Current meters are anchored and periodically or continuously measure the speed and direction of water flowing past them.

• Remote sensing systems can measure currents at various depths beneath a moving research vessel or at various depths from a fixed mooring.

Drifters, Drogues, and Floats

The simplest method of measuring currents in the ocean is to use drifters. Drifters are designed to float on the surface of the water or are weighted and designed to sink very slowly to the seafloor, where they are easily picked up and moved by even the gentlest bottom current (Fig. 3-15a). Drift cards (Fig. 3-15b) are thrown overboard in large numbers at a fixed location. Like a message in a bottle, they drift on the surface with the ocean currents until they wash up on a beach. Each card is numbered and bears a message asking whoever finds it to return it to the oceanographer with details of where (usually on a beach) and when it was found. The finder is often paid a small reward for returning the card. Drift cards are a very inexpensive means of gaining information about mean current directions, especially in coastal regions, where currents are often very complex and variable. Drift cards are particularly valuable in studies of the probable fate of wastes discharged or oil spilled at specific locations. Surface currents can also be studied with floats, whose movements can be followed remotely by radio signals received from a transmitter mounted on the float. 

Figure 3-15. Surface and seabed drifters can be used to study ocean currents. (a) A seabed drifter is weighted such that its bottom tip drags along the seafloor as the plastic disk section “catches” the current and moves with it, while (b) a surface drift card is usually a simple postage-paid return postcard sealed in plastic that floats flat on the water surface and moves with surface currents.

Current speed and direction both vary with depth in the ocean. Currents flowing below the surface can be studied with two types of passive systems. In shallow coastal waters, parachute drogues are often useful. A drogue consists of a parachute attached to a weight and to a measured length of wire, which is attached to a float, usually with a radar reflector mounted on it. The parachute opens and is pulled down to a known depth by the weight (Fig. 3-16). Once at the assigned depth, the parachute drogue “sails” in the current, dragging the surface float behind it. Such devices can only be used at relatively shallow depths because of the excessive drag that is caused by a long wire.

Figure 3-16. A parachute drogue consists of a parachute deployed at a selected depth (usually less than 100 m), where it sails along with the current. The parachute is attached to a surface float that drags the surface float after it. The researchers can follow the changing position of the float using visual observations, radar from a ship, or remotely using a GPS unit and radio transmitter mounted on the float.

Radio transmitters and global positioning system (GPS) sensors can be mounted on some surface drifters and parachute drogue floats. This enables them to be tracked from a remote location which reduces the need for, and cost of, surface vessels. 

Currents in deep waters can be measured with neutrally buoyant floats. The simplest versions are self-contained instrument packages, mounted in one or more hollow tubes and weighted so that their density is precisely the same as the density of the seawater at the depth at which the float is to operate (CC1). These floats are deployed from a research vessel and sink through the water column until they reach neutral buoyancy at a predetermined depth. The float contains a pinger, an electronic system that produces short sound pulses, or “pings” that can be used to triangulate the float’s location when heard by two or more listening stations aboard research vessels or onshore. This type of float may be followed for months as they move in the complex currents and eddies of the ocean depths (Chap. 8). Modern versions of these floats are called “autonomous floats,” and they are capable of moving back and forth vertically through the water column, recording conductivity, temperature and other data. These floats periodically revisit the surface to send their data back to a ship or shore station by radio. These floats do not have a means of recording their exact position during their time below the surface. GPS sensors are small enough that they can be mounted in the floats, but GPS and other electromagnetic wave-based positioning systems cannot work below the ocean surface because ocean water effectively absorbs this radiation. However, average current speed and direction, averaged over the depths visited by the floats and the time interval between surface visits, can be estimated from the precise locations of the floats measured each time the float surfaces to report its data.

Mechanical Current Meters

Many current meters remain in a fixed location, measuring the rate and direction of the water flowing past them. Three types of mechanical current meters were common before the development of acoustic current meters: (1) those having an impeller whose axis is oriented vertically (Fig. 3-17a); (2) those having a propeller oriented to face the current (Fig. 3-17b); and (3) those that rely on the current to tilt the meter body at an angle from its normal vertical position (Fig. 3-17c). In each case, the meter is oriented to align itself with the current direction by one or more vanes or fins, just like a weather vane. Current direction is measured using a magnetic compass inside the instrument housing. These current meters are usually suspended at intervals below the surface on wire moorings (Fig. 3-17d). The entire mooring, including the float at the top, is often deployed well below the water surface. The string of meters is left to record currents for days, weeks, or even months, and then recovered by releasing the weights from the bottom of the wire with an acoustic signal sent from the recovery ship to an acoustic release. This method of deployment below the surface is often necessary to avoid having the meters cut free or stolen by curious ship crews or by misguided fishers concerned about their nets catching on the mooring.

Figure 3-17. Typical designs for current meters measure current speed by recording the (a) rate of rotation of a rotor that behaves much like a waterwheel (this type of rotor is known as a “Savonius rotor” or an “impeller”), (b) rate of rotation of a propeller, and (c) angle to which the meter is pushed by the current. Vanes or fins are used to orient the meter in the current. Current speed and direction are usually recorded on tape in the meter and are read when the meter is retrieved. (d) Current meters are typically deployed in vertical strings, such as the three different configurations shown here, all moored to the seafloor. They are often left in place for weeks or months and then retrieved by activating a timer or an acoustic signal from the recovery ship to release the bottom weights. Often current meters are moored entirely below the surface to discourage theft and to reduce navigation hazards.

Current meters deployed on moorings normally contain internal devices that continuously record current speed and direction. Current direction is recorded by continuous readings of the position of a magnetic compass mounted inside the meter body. Depending on the meter design (Fig. 3-17a–c), current speed is measured by the speed of rotation of the impeller or rotor or by the angle of the meter’s tilt from the vertical.

Most current meters are sophisticated and expensive electronic instruments. However, currents have been measured with much simpler systems in some shallow coastal waters. The most ingenious and least expensive system consists of a sealed glass bottle partially filled with sealing wax and containing a magnetic needle attached to a piece of cork. The neck of the bottle is tied with string to a LifeSaver candy, and a weight, such as a rock, is attached to another string, which is also tied to the candy. The bottle is heated on board the ship to melt the wax, and the “current meter” is dropped into the water, where the weight pulls it to the bottom. Once on the seafloor, the bottle on its string is held at an angle by the current, the wax solidifies in the cold water, and soon the candy dissolves, releasing the buoyant bottle from the weight to float back to the surface. The wax surface, which was horizontal when the wax solidified, lies at an angle to the bottom of the bottle. That angle is a measure of the current speed. The bottle also records the current direction because the magnetic needle, which was facing north as it floated in the wax, is locked in place as the wax sets.

Acoustic Current Meters

Remote sensing Doppler acoustic current meters measure current speed and direction at multiple depths simultaneously. Such meters can be mounted looking upward from the seafloor or a seabed mooring (Fig. 3-18a,b), or mounted on a ship looking downward (Fig. 3-18c). They send several narrow-beam sound pulses into the water column that are angled away from the meter in different directions. The sound echoes off particles in the water column, and these echoes are recorded from each of the beams. The current speed and direction are calculated from the Doppler shift of the sound frequency in the returning echoes. The Doppler shift is the same phenomenon that makes the pitch of a train whistle change as it passes. When the train is approaching— or, in this context, when current is flowing toward the acoustic meter—the pitch is increased. When the train is moving away—or when the current is flowing away from the acoustic meter—the pitch is decreased, producing a deeper (more bass) tone.

Figure 3-18. Doppler acoustic current meters can be (a) mounted on the seabed, (b) moored in mid water, or (c) mounted in a ship’s hull facing downward. The sound beams sent out by the meter are reflected off particles in the water, and the frequency of the sound in the returning echo changes according to the direction and speed of the particles that move with the current.

Remote sensing Doppler current meters have the advantage of simultaneously measuring currents at all depths within the meter’s maximum operating range of 100 m or more. They are particularly useful in locations where current-meter moorings are not possible, such as in busy shipping lanes and estuaries with very fast currents. Such meters are widely used but they are expensive and collect large amounts of data, including sound intensity, Doppler shift, and instrument orientation and tilt, which must be processed through complex computer analyses to yield data on current speed and direction.

Another technique, acoustic tomography, is capable of making simultaneous observations of water movements or currents within large areas of the ocean. The system is the acoustic equivalent of the computerized axial tomography (CAT) scan used to produce three-dimensional images of internal human body parts. The acoustic tomography system consists of several sound sources and receivers moored at different locations within a study area that can be hundreds of kilometers across (Fig. 3-19). Sound is emitted by each source and received by each receiver, so numerous pathways of sound traveling between sources and receivers are possible. The sound velocity along each of the pathways is affected by water characteristics such as depth, temperature, and salinity. When currents flow within the study area, they change the distribution of temperature and salinity within the study area and therefore the average speed of sound along the source-receiver pathways. Powerful computer data analysis techniques convert the small variations in travel times between each source and receiver into a detailed picture of the water property distributions and movements within the study area.

Figure 3-19. In a typical acoustic tomography array, sound pulses are sent from each transmitter and received by each of the receivers.