Summary
The sound speed was determined for Meganyctiphanes norvegica, for a mixture of Thysanoessa raschii and Thysanoessa inermis and for a mixture of Calanus finmarchicus and Calanus hyperboreus. The sound speed contrasts ranged from 1.014 to 1.044. Seasonal variations in specific density were measured for Thysanoessa inermis, Thysanoessa raschii, Meganyctiphanes norvegica, Calanus finmarchicus and Calanus hyperboreus. The density of 20 mm T. inermis was lowest in November (1.052 g/cm3) and highest in February–March (1.065 g/cm3). For a 20 mm T. raschii the minimal density was determined in December (1.059 g/cm3) and the maximum in February–March (1.074 g/cm3). M. norvegica individuals of 35 mm also had their lowest density in December (1.060 g/cm3), but reached their maximum density in July (1.076 g/cm3).
The density of the euphausiids was found to be size dependent. The density increases as the size decreases. C. finmarchicus and C. hyperboreus had densities less than seawater (1.026 g/cm3) during most of the year. Just before spawning the density increased to 1.028 g/cm3 and 1.036 g/cm3 for C. finmarchicus and C. hyperboreus respectively. The seasonal variations of the density were closely related to the lipid content of the animals.
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Køgeler, J.W., Falk-Petersen, S., Kristensen, Å. et al. Density- and sound speed contrasts in sub-Arctic zooplankton. Polar Biol 7, 231–235 (1987). https://doi.org/10.1007/BF00287419
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DOI: https://doi.org/10.1007/BF00287419