Density- and sound speed contrasts in sub-Arctic zooplankton

Abstract

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).