Biomass Measured Electronically

Abstract

Until recently, the estimation of zooplankton biomass has been by crude total volume measurements. A biomass vs size histogram can be obtained from the results of differential sieving which separates the plankton into several size categories. This is a laborious procedure, and information on small-scale spatial distribution is still lacking. 

The study described in this paper used an electric sensing zone instrument to make an "in situ" measurement of particles in the range 0.5 to 3.0 mm diameter in the ocean. Now, very few non-biological particles in the indicated range are present in ocean water. Since pulse heights are linearly related to particle volume, 'biomass' and 'pulse height' are virtually synonymous. 

The pulses output by the sensing zone electronics were measured by a LINC-8 computer which had been programmed to reject noise and also keep track of the time between true pulses. Since a typical tow is about one mile long, and the net mouth is only three inches in diameter, we have an essentially one-dimensional transect of the area examined. Thus, the time measurements are equivalent to distance between particles.

In July 1967, a transect was run from Halifax into the Sargasso Sea. For each station, histograms of biomass vs size were derived from the records. Accuracy of the method is discussed.