ies of seasonal equations that are restricted to a 

 specific area. A single general conversion equa- 

 tion derived from samples from a widespread 

 area cannot provide estimates of abundance with 

 sufficient accuracy to describe the variation in 

 abundance and community composition neces- 

 sary for detailed studies of trophic structure and 

 community composition. The findings presented 

 here are generally consistent with those of pre- 

 vious investigators (Lovegrove 1966; Piatt et al. 

 1969; Beers 1974), with the exception of Wiebe et 

 al. (1975). It should be recognized that the latter 

 approach (Wiebe et al. 1975) has utility in com- 

 paring disparate data sets from different geo- 

 graphical areas and seasons. It is necessary to 

 recognize the limitations of each approach and 

 select according to the intended use of the data. 



Previous reports have recommended dry 

 weight determinations over displacement vol- 

 umes because both interstitial and intracellular 

 water is eliminated from the sample, removing 

 bias caused by gelatinous organisms (Ahlstrom 

 and Thrailkill 1963; Beers 1974). Further, since 

 only organic and inorganic substances remain in 

 the sample, dry weight should provide informa- 

 tion regarding the potential food value of the 

 plankton standing stock. However, the high cor- 

 relation found between displacement volume 

 and dry weight (r = 0.925, 442 df) implies that 

 both measures provide equivalent assessments of 

 standing stock and potential food value. In two of 

 the three areas investigated, GB and SNE, both 

 measures portray identical ascending and de- 

 scending trends in biomass with maximal and 

 minimal points closely correlated (Fig. 3). In 

 addition, variability, though high for both tech- 

 niques, is higher for dry weight. 



Discrepancies between the two measures ap- 

 pear, however, when one examines GOM data. 

 Standing stock is underestimated by displace- 

 ment volume because samples there have high 

 dry weight to displacement volume ratios (Fig. 

 4). As a consequence, when biomass is compared 

 between the GOM and GB or SNE, each index 

 gives a different interpretation of between-area 

 differences (Fig. 5). For example, in autumn 

 1977, mean dry weight for the GOM was five 

 times higher than for SNE, but mean displace- 

 ment volume was only twice as high. This phe- 

 nomenon is attributed to the life history of C. 

 finmarchicus in the GOM. Dry weight values re- 

 ported by other investigators from different 

 areas are also more readily comparable than dis- 

 placement volumes because Lovegrove's tech- 



160 - 



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Z 140 - 



w — . 



uj E izo - 

 a o 



100 - 

 80 - 

 60 

 40 

 20 



< o 



E 

 o 

 o 



E 



a 



< 



UJ 



2 



® 



DISPLACEMENT VOLUME 



GEORGES BANK 



SOUTHERN NEW ENGLAND 



i i i i i I i — rm — i i i i i t 



DRY WEIGHT 



GEORGES BANK 



GULF OF MAINE 

 / 



SOUTHERN NEW ENGLAND 



I I I I I I 1 I I I I 1 



JFMAMJJASONDJFMAM JJASOND 



1977 



1978 



Figure 5.— Trends in plankton abundance for the three areas 

 in 1977 and 1978 as measured by A) displacement volume, and 

 B) dry weight. Each measure gives a different interpretation 

 of between-area differences in biomass, especially in the Gulf 

 of Maine. 



nique (1966) for measuring them has been widely 

 accepted, while techniques for measuring dis- 

 placement volumes vary, especially in the at- 

 tempt to remove interstitial water (Wiebe et al. 

 1975). Thus, for studies comparing standing 

 stock between different sea areas, and for other 

 advantages previously mentioned, dry weight is 

 the preferred measure. 



Summary 



This report has provided a series of season- and 

 area-specific equations for the interconversion of 

 zooplankton displacement volume and dry 

 weight. In addition, interconversion equations 

 for samples with large amounts of phytoplank- 

 ton and siphonophore fragments have been cal- 

 culated. However, dry weights should be mea- 

 sured directly on samples containing organisms 

 with large amounts of intracellular water be- 

 cause they drastically affect the biomass rela- 



640 



