NOTES 



EFFECT OF SEASON AND LOCATION ON THE 



RELATIONSHIP BETWEEN ZOOPLANKTON 



DISPLACEMENT VOLUME AND 



DRY WEIGHT IN THE 



NORTHWEST ATLANTIC 1 



Biomass or "standing stock" is a routinely mea- 

 sured index of abundance for studies of the inter- 

 actions between trophic levels in the oceanic food 

 web. Zooplankton biomass is usually reported as 

 quantity of zooplankton per unit volume of 

 water. Measures of quantity currently in use in- 

 clude displacement volume (Frolander 1957; 

 Sutcliffe 1957; Yentsch and Hebard 1957; Tran- 

 ter 1960; Ahlstrom and Thrailkill 1963), wet 

 weight (Nakai and Honjo 1962), dry weight 

 (Lovegrove 1966), and carbon (Curl 1962; Piatt et 

 al. 1969). These measures can be applied to a spe- 

 cies at a specific developmental stage, to the en- 

 tire population, or to all members of the commu- 

 nity combined. Carbon and dry weight have been 

 considered preferable because variability 

 caused by interstitial and intracellular water is 

 eliminated by either technique (Ahlstrom and 

 Thrailkill 1963). They are not, however, practi- 

 cal measures in some investigations because spe- 

 cialized equipment is required and the tech- 

 niques' destructive nature prevents further 

 analysis of the sample. Measurement of displace- 

 ment volume and wet weight are nondestructive, 

 rapid, and use simple techniques which provide 

 indexes of abundance, but do measure total mat- 

 ter, including water. 



As an alternative, conversion factors or tables 

 of "equivalents" have been used to transform dis- 

 placement volume or wet weight into carbon or 

 dry weight (Bsharah 1957; Menzel and Ryther 

 1961; Piatt et al. 1969; Be et al. 1971; Le Borgne 

 1975; Wiebe et al. 1975). However, plankton sam- 

 ples represent aggregations of organisms at a 

 particular time and place which change accord- 

 ing to season, geographical location, and local 

 environmental conditions. For these reasons, 

 and because many conversion factors were calcu- 

 lated with data produced by outdated techniques, 

 the accuracy of interconversions between bio- 

 mass measures has been questioned (Lovegrove 



1966; Piatt et al. 1969; Beers 1974). Recently, 

 Wiebe et al. (1975) provided conversion factors 

 based on data collected from different oceanic 

 areas over several years in order to account for 

 seasonal and geographical variation in samples. 

 This study explores whether a conversion 

 equation based on data from numerous samples 

 collected in contiguous areas during different 

 seasons can account for sample variability and 

 more accurately convert between biomass mea- 

 sures than equations derived from smaller and 

 smaller subsets of data. Unlike previous studies, 

 an intense sampling strategy provided the 

 means to derive equations to convert between 

 displacement volume and dry weight for sam- 

 ples from both broad and restricted geographic 

 areas and for different seasons. Interconversion 

 accuracy was verified with subsequent samples 

 by comparing estimated values with field mea- 

 surements. In addition, the relative variability 

 and the values of both measures were compared 

 in order to determine which index is more useful 

 for these types of studies. 



Materials and Methods 



Plankton samples were collected by the Na- 

 tional Marine Fisheries Service Northeast Fish- 

 eries Center in conjunction with the Marine Re- 

 sources Monitoring Assessment and Prediction 

 (MARMAP) program (Sherman 1980). Sampling 

 was conducted six times a year in 1977 and 1978 

 off the northeast coast of the United States in 

 three adjacent areas: Gulf of Maine (GOM), 

 Georges Bank (GB), and Southern New England 

 (SNE). Sampling locations are shown in Figures 

 1 and 2. Paired 61 cm diameter bongo samplers 

 fitted with 0.505 mm and 0.333 mm mesh nets 

 were towed obliquely through the water column 

 at a speed of 1.5-2.0 kn. Maximum sampling 

 depth was 200 m or 5 m from the bottom in shal- 

 lower areas, and tow duration was 5-15 min. A 

 flowmeter was strung inside the bongo frame to 

 measure the volume of water filtered. Plankton 

 samples from the 0.333 mm mesh nets were used 

 in this analysis. Samples were preserved in 5% 

 buffered Formalin 2 for at least 6 mo before anal- 



'MARMAP Contribution MED/NEFC 81-8. 



FISHERY BULLETIN: VOL. 80, NO. 3, 1982. 



2 Reference to trade names does not imply endorsement by 

 the National Marine Fisheries Service, NOAA. 



631 



