Mass density assessment : comparison of three methods using Oikopleura dioica (Appendicularia) as a model system

Abstract

Individual and environmental mass densities have an important effect on the dynamics of suspended particulate material in the water column. We used Oikopleura dioica as a model system to compare the accuracy of mass density estimates (rho(s)) using three methods. The first one was based on a morphometric analysis which allowed the construction of a three-dimensional geometric model of the organism to estimate individual body volumes (IBV), while weight was directly measured to calculate density. The second method was derived from Stokes Law, using the settling velocity of Oikopleura houses. Finally, the actual mass density of organisms was measured using a single-density gradient column calibrated with known-density glass floats. Mean h, estimates ranged from 1.0314 g.cm(-3) to 1.0846 g(.)cm(-3), and did not differ significantly among the three methods. Nevertheless, the greatest accuracy was attained with the single-density gradient column. The methodological approach presented in this paper is useful to assess the mass density of small plankton organisms and contribute to the generation of data on the rate at which energy transfer occurs in aquatic ecosystems