Density in sub-arctic zooplankton

Abstract

Seasonal variations in specific density were measured for Thysanoessa inermis, Thysanoessa raschii, Meganyctiphanes norvegica, Calanus finmarchicus and Calanus hyperboreus. The density of a 20 mm T. inermis was lowest in December (1.052 g/cm³) and highest in Februar- March (1.065 g/cm³). Foe a 20 mm T. raschii the minimal density was determined in December (1.060 g/cm³), but reached their maximum density in July (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 is 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 1.036 g/cm³ and 1.028 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.