Sound is an extremely useful tool in biological and fisheries oceanography. The scattering of sound by organisms at many different trophic levels can be used for a variety of purposes. Schools of fish and patches of plankton can be located and tracked acoustically. It may eventually be possible to distinguish living from nonliving scatterers and to identify the biological scatterers by species. It may soon be possible to estimate biomass acoustically as a function of trophic level in the ocean. Sound scattering has been used commercially since the 1930s to locate fish schools, but only recently have multifrequency systems been available for quantitative study of animal plankton. The Multi-Frequency Acoustic Profiling System is capable of profiling zooplankton in the size range of 0.2 to 10 millimeters. General application of acoustical technology will require the development of inexpensive equipment and techniques to analyze and use the large volumes of data generated.
Fluorometers, transmissometers, and spectroradiometers are used to measure phytoplankton populations, the turbidity of the water column, and the amount and wavelength of light that penetrate beneath the ocean surface at a given site. Correlating site measurements with measurements from satellite ocean color sensors provides the means to extrapolate phytoplankton measurements to a global scale. Mooring optical instruments together with current meters and temperature and salinity sensors provides a technique for collecting long (months) and highly resolved (minutes to hours) time-series measurements, permitting biological oceanographers to study what physical factors control phytoplankton populations. Moorings contribute data on variation over time and depth, whereas satellite sensors provide information on variation over the global ocean surface. Flow cytometry is another optically based technology that is extremely useful for characterizing the size and pigment composition of phytoplankton and bacteria and for sorting populations based on these and other criteria.
New techniques for imaging organisms in situ, now available, show promise for widespread application in the 1990s. These