Influences of temperature and pressure on the material and acoustic scattering properties of zooplankton

It is well known that small variations (a few percent) in sound speed and density contrasts of the fluidlike weakly scattering zooplankton result in a significant variation in the target strength of zooplankton and ultimately in acoustically derived estimates of zooplankton biomass. Since pressure and temperature often vary considerably as a function of depth, the material properties of zooplankton are expected to change accordingly. To accurately determine zooplankton biomass from the acoustic scattering data, knowledge of the relationship between their material properties and these oceanographic parameters is required. In situ measurements of sound-speed contrast of the zooplankton species, Calanus finmarchicus, were made with the Acoustic Properties Of zooPlankton (APOP) system in Wilkinson Basin (Gulf of Maine) in August 1999. The sound speed of this animal in the mixed layer above the thermocline (0–18 m) was significantly smaller than that right below the thermocline. Below the thermocline, temperature became nearly constant with depth so that only pressure was changing. The measured sound-speed contrast below the thermocline (60–180 m) increased slowly with increasing pressure. Models of the material properties of weakly scattering zooplankton that take into account the temperature and pressure dependence are proposed and numerical examples are presented. [Work supported by NSF, NOAA/NURP, and ONR.]