RELATIONSHIPS BETWEEN ZOOPLANKTON DISPLACEMENT VOLUME, 

 WET WEIGHT, DRY WEIGHT, AND CARBON^ 



Peter H. Wiebe,^ Steven Boyd,- and James L. Cox^ 

 ABSTRACT 



Interconversion of various measures of zooplankton biomass have great utility in studies requiring 

 nondestructive techniques, or for interpretation of past data. In establishing predictive relationships 

 between such measures, the appropriate regression to use is the geometric mean estimate, which 

 provides a regression line in which the regressions of .Y on Y and Y on X are identical. We have 

 employed this type of analysis in determinations on samples from diverse sea areas in different seasons 

 and have determined that statistically significant relationships exist between carbon, wet weight, 

 displacement volume, and dry weight when a constant technique is used. The slope of the regression line 

 for log transformed values for carbon vs. dry weight and wet weight vs. displacement volume was 

 sufficiently close to unity to assume a straight percentage conversion between these values. Carbon was 

 31-33% of dry weight and wet weight was 72-73% of displacement volume, according to our techniques. 

 Comparability of different techniques for a biomass measurement may be poor, especially in the case of 

 displacement volume and wet weight measurements due to variations in the interstitial water content. 

 Moreover, interstitial water content varies inversely with total biomass density, which accounts for the 

 absence of a simple percentage relationship between wet weight or displacement volume and other 

 measures of zooplankton biomass. 



Biomass is a classic and useful measure of the 

 zooplankton standing crop. A number of tech- 

 niques exist to measure it. Four commonly used 

 techniques involve measurement of displacement 

 volume (Yentsch and Hebard 1957; Frolander 

 1957; Sutcliffe 1957; Tranter 1960; Ahlstrom and 

 Thrailkill 1963), wet weight (Nakai and Honjo 

 1962), dry vi^eight (Lovegrove 1966), and carbon 

 (Curl 1962; Piatt et al. 1969). For most studies, 

 especially those determining energy flow through 

 food chains, carbon is the most fundamental of 

 these gross measures. Many zooplankton collec- 

 tions frequently serve several purposes and the 

 destructive techniques required to determine car- 

 bon or dry weight frequently cannot be employed. 

 An alternative is to measure displacement volume 

 or wet weight, and convert the data into either dry 

 weight or carbon. These latter techniques, if done 

 properly, are nondestructive since the organisms 

 can still be identified when re-suspended in liquid. 

 There is an obvious need for conversion factors 

 that reliably define the relationship between the 

 various biomass measures. This need also arises 



'Contribution No. 3433 from the Woods Hole Oceanographic 

 Institution, Woods Hole, MA 02543. This study was supported by 

 NSF GA 29303, ONR N00014-66-CO-241, and NRO 83-004. 



^Woods Hole Oceanographic Institution, Woods Hole, MA 

 02747. 



'Southeastern Massachusetts University, North Dartmouth, 

 MA 02747. 



when data based on different techniques are com- 

 pared. Although conversion factors exist in the 

 literature, they often are based on data from re- 

 stricted sea areas. Further, in some cases, biomass 

 determinations were made by techniques which 

 are no longer recommended (see Lovegrove 1966). 

 The objective of this paper is to more satisfactorily 

 define the relationships between the biomass 

 measures mentioned above. Using both data 

 derived from samples collected from diverse 

 oceanic areas over the past 6 yr and data selected 

 from the literature, we have empirically deter- 

 mined linear regression equations relating pairs 

 of biomass measures. 



THEORETICAL CONSIDERATIONS 



Ideally, any two biomass measures, X and Y 

 should be related by a constant of proportionality, 

 a, such that 



Y=aXP, 



(1) 



Manuscript accepted March 1975. 



FISHERY BULLETIN: VOL. 73, NO. 4, 1975. 



where fi = 1.0. A measurement error or bias which 

 occurs as a constant fraction of the biomass results 

 only in a change in the value of a. When natural 

 variability or an error factor(s) in X or Y is 

 disproportionate or inversely proportional to the 

 amount of biomass, j8 cannot be assumed to equal 



777 



