Abstract:
Phytoplankton plays a very important role in gas exchange in the ocean. But until now it has not been considered as acoustically significant as zooplankton. The existence of gas cavities in phytoplankton cells was suggested a long time ago but has not been detected so far because of early methods of observation. The review of recent works on acoustical investigations of gas bubbles associated with phytoplankton cells is given in this paper. Acoustical and related properties of phytoplankton suspensions were studied under laboratory conditions and in situ by several methods including acoustic 6.5-MHz backscattering under ordinary conditions with additional compression up to 5 atm. It was observed that, under the sharp compression gas cavities dissolve and recover under the light exposition. The volume of gas cavity associated with a phytoplankton cell was found to be up to 0.3 of the volume of a cell for different alga species. Such a bubble may increase the acoustical cross section of a pure cell by 2--3 orders. Roles of phytoplankton bubbles for sound propagation, for anomalous migration of some sound scattering layers, and for hydrodynamic cavitation in the subsurface layer and in wakes of sea animals are discussed. Experimental examples are given. [Work was supported by RFBR (Grant No. 95-05-15632).]