techniques to distinguish migratory from nonmigra- 

 tory populations. 



Comparisons of demersal zooplankton abundances 

 among studies are also made difficult by variation 

 among the trap types and approaches used. All- 

 dredge and King (1980) compared reentry and emer- 

 gence traps, showing as we have that reentry traps 

 captured very much larger numbers and different 

 proportions of demersal organisms. Aside from 

 studies by Stretch (1985) and ourselves (unpubl. 

 data), we are unaware of any effort to use a direct 

 sampling technique to calibrate a trapping tech- 

 nique. Thus, published abundance estimates for 

 demersal zooplankton abundance are probably low 

 and biased, reflecting the preponderant use of 

 emergence trapping. 



Robichaux et al. (1981) pointed out that animals 

 entering traps by crawling can be a significant 

 artifact. Such contamination would probably be a 

 greater problem for reentry traps than for emer- 

 gence traps. Our reentry traps captured large 

 numbers of nematodes and harpacticoid copepods, 

 which can enter by crawling, but even when they 

 are eliminated altogether from the trap totals, reen- 

 try traps still caught twice as many animals as 

 emergence traps (Table 1). Furthermore, at least 

 some nematodes and harpacticoids do swim freely, 

 even if they do not move very far up into the water 

 column, as Alldredge and King (1985) have shown. 

 Thus, we think that reentry trapping reliably yields 

 higher estimates of demersal zooplankton abun- 

 dance that are more realistic than results from 

 emergence trapping but probably not truly accurate. 



Robichaux et al. (1981) also argued that con- 

 tamination of demersal zooplankton traps by holo- 

 planktonic and crawling organisms causes an over- 

 estimate of the actual importance of demersal 

 zooplankton in benthic food webs. We dispute this 

 view on several grounds. First, the emergence trap- 

 ping technique used by Robichaux et al. (1981), as 

 is the case with others' use of emergence trapping, 

 probably yielded significant underestimates of the 

 actual abundance of demersal zooplankton, as we 

 discuss above. Second, we suspect that all trapping- 

 techniques are likely to miss animals that are not 

 migrating actively or that avoid traps, causing fur- 

 ther population underestimates. Finally, estimates 

 of demersal zooplankton populations resident within 

 a given habitat may fail to reflect the actual avail- 

 ability of these animals as consumers or prey via 

 transport. 



Sand bottom habitats may be important sources 

 of demersal zooplankton for consumers in other 



habitats. Currents can carry demersal organisms 

 passively to other habitats. Animals that migrate 

 high into the water column, such as the groups cap- 

 tured especially well by emergence traps, may be 

 carried relatively great distances compared with 

 those that crawl or stay within the near-bottom 

 boundary layer. Furthermore, off-reef foraging by 

 reef dwellers may allow exploitation of demersal 

 organisms on sandy bottoms in the absence of advec- 

 tion. If so, estimates of demersal zooplankton abun- 

 dance derived from reentry trapping will again be 

 realistic, if not accurate, from the standpoint of com- 

 munity ecology. Therefore, demersal zooplankton 

 are potentially quite important to marine benthic 

 communities, even if the techniques used to sample 

 them are imperfect. 



Acknowledgments 



This research was made possible with support pro- 

 vided by the NOAA-National Undersea Research 

 Program at the University of North Carolina, 

 (UNC), Wilmington, to Lawrence B. Cahoon and G. 

 Simmons, who graciously shared shiptime. Support 

 was also provided by UNC Sea Grant (R/MG 84-07 

 and R/MG 85-01), the National Science Foundation 

 (RII 8311 486), the American Philosophical Society, 

 the Lerner-Grey Fund for Marine Research and the 

 North Carolina Collegiate Academy of Sciences. 



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