WIEBE ET AL.: RELATION OF VOLUME, WET AND DRY WEIGHTS, AND CARBON 



DISCUSSION 



Piatt et al. (1969), in a comparison of the 

 seasonal changes in dry weight, carbon, and caloric 

 values of zooplankton collected from St. Mar- 

 garet's Bay, Nova Scotia, found a fivefold varia- 

 tion in caloric content per unit dry weight. As a 

 result they concluded ". . . that there is no single 

 conversion factor that will serve to convert 

 biomass of zooplankton, expressed as dry weight, 

 to its energy equivalent." A similar conclusion was 

 inferred for the conversion of dry weight to car- 

 bon. They found, however, that the carbon content 

 of zooplankton could be used to predict the energy 

 equivalent. These results appear to contradict our 

 finding that a statistically significant relationship 

 does exist between pairs of the different measures 

 of biomass including dry weight and carbon. The 

 explanation for this discrepancy lies in the fact 

 that the data of Piatt et al. represents a small 

 segment of the extensive range of biomass per 

 cubic meter which occurs in marine waters. This 

 fact, coupled with high variation of the dry weight 

 to carbon ratios, appeared to them to provide a 

 nonsignificant relationship. We have used their 

 data (as tabulated by Piatt and Irwin 1968, table 

 4) to examine the fit of their data to our regression 

 line. After transformation to logarithms (base 10), 

 a linear GM regression line was calculated for 

 their 45 pairs of dry weight and carbon values. 

 While the slope of this line was significantly 

 different from zero (P<0.001), it was nonsig- 

 nificantly different (P>0.05) from ours (Table 

 2). However, the intercept was substantially 

 different. This is a reflection of the fact that their 

 carbon values average 14% of dry weight, whereas 

 in our data the average is 32%. The wet-combus- 

 tion method (described by Strickland and Parsons 

 1965) which they used to determine carbon ap- 

 parently provides lower estimates (an average 

 here of 58% lower) than the high temperature 

 combustion technique we used. Sharp (1973) found 

 that persulfate oxidation yields an average 22% 

 lower values than high temperature combustion 

 when these methods are used to measure total or- 

 ganic carbon in seawater. 



In terms of variability, the observations of Piatt 

 et al. (1969) have a variance from the regression 

 line significantly (P<0.01) larger than ours by a 

 factor of 1.6. 



It is clear from the comparisons of biomass 

 measures we have carried out, and from other un- 



published work performed at this laboratory, that 

 the techniques used by various investigators in 

 determining a particular biomass measure (such 

 as displacement volume) provide substantially 

 different answers which are not readily compara- 

 ble. This is particularly true of displacement 

 volume and wet weight and to a lessor degree, 

 carbon. A similar conclusion was reached by Nakai 

 and Honjo (1962). Only the procedure for measur- 

 ing dry weight described by Lovegrove (1966) 

 seems to have been widely adopted and values 

 presented by various investigators using this 

 technique seem to be intercomparable. With 

 displacement volume and wet weight, the problem 

 stems largely from the differing amounts of in- 

 terstitial water adhering to the zooplankton at the 

 time of measurement. We have found, as did Nakai 

 and Honjo (1962), that for a given technique, the 

 amount of interstitial water varies inversely with 

 the amount of biomass being measured. The 

 amount, however, varies from technique to tech- 

 nique. Efforts to significantly reduce the amount 

 of interstitial water present appear to create ad- 

 ditional error. Rather than simply concentrating 

 on the reduction of interstitial water, it is more 

 important to establish a reproduceable procedure 

 that generates values which can be directly related 

 to a more absolute standard such as carhop as we 

 have tried to do. The data on which Equations 1 to 

 6 and 11 to 13 in Table 2 are based were developed 

 using methods which appeared to us to involve the 

 least amount of technique-derived error and which 

 required little complex instrumentation. 



The zooplankton biomass values used in this 

 study encompass a significant part of the range of 

 values an investigator is likely to encounter 

 working in either coastal waters or the open ocean. 

 Thus, the equations we have presented should be 

 useful in a wide variety of situations providing the 

 same techniques to measure biomass are 

 employed. It is important to bear in mind, 

 however, that situations do occur in which these 

 equations may not apply. One example is where 

 marine populations are dominated by salps, 

 doliolids, jellyfish, or chaetognaths. The very high 

 percentage of intracellular water in these or- 

 ganisms may cause the relationships between 

 displacement volume or wet weight and dry 

 weight or carbon to deviate strongly from our 

 predicted relationships. In such cases, which in our 

 experience occur infrequently, we recommend 

 that dry weight or carbon be measured directly. 



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