Silhouette photography of oceanic zooplankton

Abstract

No adequate method has been available to characterise a sample of Zooplankton quickly and accurately. As Mullen noted¹, although relatively rapid, measures of standing crop such as displacement volume, dry weight, carbon content and electronic size spectra do not distinguish clearly between animals and plant or detrital material, much less between herbivores and carnivores. Direct microscopic enumeration has remained the only approach giving information detailed enough for many ecological studies. Unfortunately, this method is time-consuming, it cannot be undertaken routinely at sea, and it is feasible only after chemical agents have been added to ‘fix’ and then ‘preserve’ the sample. With time, plankton samples always deteriorate in preservatives, although the degree of this effect varies with species. We report here that a little-known photographic technique—high-speed silhouette photography^2,3—can be used to analyse live Zooplankton samples at sea.