Measurements of sound-speed and density contrasts of zooplankton in Antarctic Waters

Sound-speed and density contrasts (h and g, respectively), two important acoustic material properties, of live zooplankton were measured off the western Antarctic Peninsula during a Southern Ocean GLOBEC cruise conducted from 9 April to 21 May 2002. The work included in situ sound-speed contrast and shipboard density-contrast measurements. The temperature and pressure (depth) dependence of the sound-speed contrast of Euphausia superba and E. crystallorophias as well as that of some other zooplankton species were investigated. The size range of E. superba used in the measurements varied from about 20 mm to 57 mm, with mean length of 36.7 mm and standard deviation of 9.8 mm, which covered life stages from juvenile to adult. For E. superba, there was no statistically significant depth dependence, but there was a moderate dependence of sound-speed and density contrasts on the size of the animals. The measured sound-speed contrast varied between 1.018 and 1.044, with mean value 1.0279 and standard deviation 0.0084, while the measured density contrast varied between 1.007 and 1.036, with mean value 1.0241 and standard deviation 0.0082. For E. crystallorophias and Calanus there was a measurable depth dependence in sound-speed contrast. The in situ sound-speed contrasts for E. crystallorophias were 1.025 ± 0.004 to 1.029 ± 0.009. For Calanus, they were variable, with one set giving a value of 0.949 ± 0.001 and the other giving 1.013 ± 0.002. Shipboard measurements of other taxa/species also showed substantial variation in g and h.

In general, values of g ranged from 0.9402 to 1.051 and h ranged from 0.949 to 1.096. The variation of the material properties is related to species, type, size, stage, and in some cases depth of occurrence. The uncertainty of the estimates of zooplankton biomass attributable to these variations in g and h can be quite large (more than 100 fold). Improvements in making biological inferences from acoustic data depend strongly on increased information about the material properties of zooplankton and the biological causes for their variation, as well as a knowledge of the species composition and abundance.