Use of spherical and spheroidal models to calculate zooplankton biovolume from particle equivalent spherical diameter as measured by an optical plankton counter

Abstract

Three methods of calculating the biovolume of particles from their shadows as recorded by and optical plankton counter (OPC), based on optical geometry, are presented. In the first method (Vsphere), particles are assumed to be opaque spheres. In the other two methods, particles are represented as opaque spheroids, oriented with their major axes either parallel to the flow thus presenting maximum shadow area (Vmax), or randomly orientated relative to the flow (Vran). The models were tested by comparing with net biovolume, measured from samples of a zooplankton assemblage dominated by Calanus finmarchicus collected during a cruise to the northeast Atlantic during 2001. The randomly orientated spheroidal model (Vran) provided the best fit with the net data: on average the ratio of OPC biovolume to net biovolume was 1.02, compared to ratios of 0.84 when calculating OPC biovolume as Vmax and 1.50 when calculating as Vsphere. The Vran and Vmax methods gave reasonable estimates of net biovolume from OPC measurements without recourse to the use of empirical tuning parameters that are otherwise required. This success was enhanced by the fact that the community chosen for validation purposes was dominated by a single species, C. finmarchicus, which could be approximated by spheroids of known dimension. The calibration methods are less likely to be effective when applied to zooplankton communities incorporating a diverse range of organisms.

Spatial distribution of zooplankton size spectra on the French continental shelf of the Bay of Biscay during spring 2000 and 2001

“…The destruction of fragile gelatinous organisms, such as appendicularians, can also introduce bias in the size structure estimate. The biovolumes, the biomasses and the size structures estimate are also affected by the model to estimate the ESD [ Mustard and Anderson , 2005] and the both length/width and length/carbon ratios. The biomasses and the volumes of zooplankton in the samples were, however, not compared with the OPC estimates because the first validation of ESD and abundance measurements appeared sufficient for our study.…”

Zooplankton biomass estimated from digitalized images in Antarctic waters: A calibration exercise

“…The error in measuring the individual body area here is much less than the measurement of the individual body mass, allowing the use of model I regression [ Legendre and Legendre , 1998]. In contrast, the different position of the organisms in a sample with respect to the optical device will create an error that is difficult to avoid [see Mustard and Anderson , 2005]. This problem is less pronounced in silhouette photography, where organisms adopt a similar position at the bottom of the tray, than in optical plankton counters where organisms pass through the optical beam in different positions.…”

High activity and Lévy searches: jellyfish can search the water column like fish

“…A selection of prey items was measured to the nearest 5 mm using an eyepiece graticule calibrated with a stage micrometer. Equivalent spherical volume or biovolume calculations were carried out based on basic shapes of prey items in each category [34,35]. of IVM was also not simply caused by a large diel change in the depth occupied by the jellyfish.…”