3.10: Chapter Summary

Difficulties of Studying the Ocean Environment.

Because water effectively absorbs all electromagnetic radiation, oceanographers can “see” through the ocean waters only by using sound waves, which travel through water with relatively little absorption.

The oceans average almost 4 km in depth. Observation and sampling instruments must be lowered through the water column—a time-consuming and expensive process. Until recently, precise navigation was extremely difficult away from sight of land, and resampling a specific location was nearly impossible.

Submersibles and instruments lowered through the ocean water column must be able to withstand very great pressure changes and the corrosiveness of seawater. They must also avoid fouling by organisms. Electrical equipment must be isolated from contact with seawater because seawater is an electrical conductor.

Oceanographers and their instruments must operate reliably on research ships that vibrate from engine noise, and that roll and pitch with wave action. Faulty equipment must be repaired at sea with available parts and personnel.

Mapping the Ocean Floor.

Until 1920, the only way to determine the depth of water under a ship was to lower a weight on the end of a line or wire until it touched bottom, and then measure the length of wire let out. This method was tedious and inaccurate, especially in deep waters. It was also prone to errors because it was difficult to determine when the weight reached the seafloor and because the path of the line or wire through the water column was distorted by currents.

In 1920, echo sounders were developed that measured the time taken for a sound ping to travel from ship to seafloor and back. Precision echo sounders were operated continuously on all research ships, and the data were used in the 1950s to produce the first detailed maps of seafloor topography.

Newer echo sounders send sound pulses spread over a wide area to either side of a research vessel, and the multiple echoes are processed by computer. The data are used to create detailed three-dimensional seafloor maps. Satellites can measure sea surface height so accurately that the data can be used to map the seafloor based on slight variations of sea surface height caused by gravity differences over the varying seafloor topography.

Seafloor Sediments.

Samples of seafloor sediment are obtained for several purposes by a variety of techniques. Samples of the upper few tens of centimeters of sediment are used to study recent sediment processes or the biology of the sediment. They are usually obtained with grab samplers or box corers.

Long vertical cores are obtained to study the history of sediment that is accumulated layer by layer. They are obtained by corers or by drilling. Drilling ships are very expensive to operate but can retrieve very long cores. Samples of rocks from the seafloor are obtained by dredges.

Indirect methods of studying sediments include gravity and magnetic-field strength measurements and seismic techniques in which sound pulses are echoed off layers of buried sediment.

Chemical and Physical Oceanography.

Water samples must be collected to determine most parameters of seawater chemistry and to study microscopic organisms. Samples are collected in bottles lowered to the desired depth, where they are closed and then retrieved. Sample contamination from the sampler’s materials and wire and from surface waters is often a problem because of the very low concentrations of some constituents. Some parameters, notably pressure (depth), conductivity (salinity), and temperature, can be determined in situ by electronic sensors.

Currents are measured by tracking drifters, drogues, or floats as they move with the current; by the use of fixed current meters that measure the speed of rotation of a rotor or the tilt of the meter in the current; and by acoustic remote sensing of suspended sediment movement.

Living Organisms in the Sea.

Sampling marine organisms is difficult because pelagic species are widely dispersed, species vary greatly in size, some species avoid samplers, and some delicate species are damaged by samplers. Pelagic species are collected by nets, water-sampling devices, traps, and fishing lines. Benthic species are collected by grab samplers, dredges, and trawl nets. Some species can only be collected or observed using scuba or submersibles.

Scuba, Manned and Unmanned Submersibles.

Marine organisms in shallow waters, less than a few tens of meters deep, can be observed and sampled best by scuba divers. Scuba divers can spend only a few hours a day underwater unless they live in expensive underwater habitats. Marine organisms in deeper waters can be observed and sampled from manned submersibles, but these vessels are very expensive and can remain submerged for only a few hours. Unmanned remotely operated vehicles (ROVs) using cameras and robot arms are more economical and can remain submerged longer. Autonomous underwater vehicles (AUVs) are improving rapidly and are being used much more extensively.

Satellites.

Satellite-mounted sensors can be used to measure parameters and characteristics of near-surface waters, including temperature, concentrations of phytoplankton, concentrations of suspended sediment, and surface currents and waves. Satellite observations can cover large areas almost simultaneously and repeat these observations frequently, which is not possible with research ships or fixed monitoring systems. Seafloor topography can be inferred by very precise satellite radar measurements of sea surface elevation. Satellite navigation systems have dramatically improved oceanographers’ abilities to map and return to specific features of the oceans.

Computers and Modeling.

Computers have contributed substantially to the development of ocean sciences, primarily by organizing and displaying geographic data and by performing mathematical modeling of complex systems.