Determining the mass density of marine copepods and their eggs with a critical focus on some of the previously used methods

We collected Calanus finmarchicus copepodites CIV, CV and CVI females in a deep fjord on the west coast of Norway during April 1996, May 1997 and November 1998. Eggs of C. finmarchicus and Calanus glacialis were collected during May 1989 in the Barents Sea. The sinking speeds of animals and eggs were measured in a homogeneous column with seawater of known density, and Stokes law was applied to estimate their mass density. Also the density contrast between the organisms and seawater was calculated. The mean mass density of C. finmarchicus ranged from 1.0274 to 1.0452 g cm(-3). During spring copepodite stage CV had a significantly lower mean mass density (1.0345 g cm(-3)) compared to CIV(1.0381 g cm(-3)) and CVI females (1.0408 g cm(-3)). Copepods collected during winter had a distinctly lower mass density. The sinking speed of C. glacialis eggs followed a unimodal distribution, with a mean of 25.9 m day(-1), while sinking speeds of C. finmarchicus eggs were bimodal, the two groups of eggs having a mean sinking speed of 23.3 in day(-1) and 35.4 m day(-1) respectively. Correspondingly the mean mass density was 1.0556 g cm(-3) for C. glacialis eggs and for the two groups of C. finmarchicus eggs 1.0639 g cm(-3) and 1.0812 g cm(-3). Results of earlier work, particularly using density gradient methods to determine mass density of zooplankton, are critically reviewed, and it is suggested that this method should not be used to determine the mass density of small organisms the size of C. finmarchicus.