that sampling methods used to collect demersal 

 zooplankton can have a significant effect on the 

 numbers and kinds of animals collected. The most 

 widely used method to date is emergence trapping, 

 in which zooplankton are collected as they migrate 

 out of the substrate, e.g.. Porter and Porter (1977). 

 Another method, reentry trapping, captures zoo- 

 plankton as they enter the substrate, e.g., Alldredge 

 and King (1980). A primary purpose of this study 

 was to compare results from simultaneous emer- 

 gence and reentry trapping, and verify Alldredge 

 and King's (1980) finding that reentry traps capture 

 significantly more zooplankton than emergence 

 traps. 



Horizontal currents are likely to have major but 

 variable effects on the transport of demersal organ- 

 isms in the plankton. Organisms migrating high into 

 the water column are likely to be transported lateral- 

 ly to a greater extent than those that remain in the 

 near-bottom layer, as Alldredge and King (1985) 

 have suggested. Thus, the implicit assumption of 

 some other investigators that demersal zooplankton 

 are characteristic residents of a habitat needs 

 qualification. Horizontal transport of migrating 

 demersal zooplankton is probably an important 

 recruitment mechanism and a means by which 

 planktivorous fish associated with reefs and other 

 structures are supported. Demersal zooplankton 

 have been shown to be associated with sand bottoms 

 by Alldredge and King (1977, 1980), so sand bottoms 

 might be an important source of demersal zooplank- 

 ton for reef communities. Therefore, our purpose 

 in this study was also to assess the abundance of 

 demersal zooplankton on sand bottom habitat. The 

 study addressed the questions: how abundant are 

 different taxa of demersal zooplankton associated 

 with sand bottom habitat? Can different trapping 

 techniques distinguish zooplankters with different 

 propensities for horizontal transport by currents? 

 How do the abundance and taxonomic composition 

 of demersal zooplankton on open carbonate sand 

 compare with the finding of others studying similar 

 sand bottoms? 



Materials and Methods 



Study sites were located approximately 1.8 km 

 east of Alligator Reef Buoy, ca. 7.5 km southeast 

 of Upper Matecumbe Key in the Florida Keys (lat. 

 24°41.26'N, long. 80°35.68'W). The sites were char- 

 acterized by a sub tidal, carbonate sand bottom that 

 sloped gently from depths of 2-4 m to depths of over 

 40 m. Macrophages occurred sparsely on the bottom 



(principally Thalassia sp.), with the closest coral 

 structure and associated fish populations several 

 hundred meters downstream. Currents were mini- 

 mal (<0.05 m s^') and flowed to the northeast, 

 parallel to the axis of the Gulf Stream just offshore. 



Sampling at the site was conducted with the aid 

 of scuba, supported by the NOAA-National Under- 

 sea Research Program's RV Seahawk, which op- 

 erated in the Florida Keys region during March 

 1985. 



Demersal zooplankton were sampled simulta- 

 neously by emergence trapping and reentry trap- 

 ping. The emergence traps were a simplified ver- 

 sion of those used by Hobson and Chess (1979), 

 consisting of a single cone of 95 f^m mesh netting 

 1 m high and 21 cm diameter at the mouth (Fig. 1). 

 The mouth of the net was weighted with lead 

 weights sewn into the mouth collar to aid placement 

 on the bottom. AIL polyethylene bottle with a 

 polyethylene funnel in the throat was attached to 

 the upper end of the net; the bottles' natural buoy- 

 ancy extended the traps. Reentry traps were square 

 polyethylene pans, 21 cm per side and 5 cm deep, 

 with removable snap-top lids, approximately one- 

 third filled with defaunated local sand. 



Divers deployed six of each type of trap in adja- 

 cent groups just after dusk and retrieved them just 

 after dawn on two successive nights in March 1985. 

 Reentry traps were closed and trap bottles from 

 emergence traps capped by divers before return to 

 the surface. The moon was nearly new and rose late 

 each night, thus providing little illumination. The 

 first night's collections were made for 12.75 hours 

 at a depth of 7 m and the second night's for 11.5 

 hours at a depth of 20 m. 



Animals in the collection bottles of the emergence 

 traps were concentrated on a 95 ^m mesh sieve, 

 washed into sample bottles, and fixed in a buffered 

 formalin-Rose Bengal solution (ca. 5% formalin final 

 strength). Animals that had entered the reentry 

 traps were washed from the sand with fresh water, 

 causing them to release their grasp on sand grains 

 (R. Higgins^). The contents of each trap were 

 washed at least 5 times, and until no further animals 

 could be seen in the wash water. All washings were 

 sieved through a 95 /im mesh sieve. The retained 

 material was then poured into sample bottles and 

 fixed as above. 



The stained, fixed animals in each sample were 

 examined, identified, and counted with a stereo- 

 microscope. Identifications were made to genus and 



'R. Higgins, Smithsonian Institution, Washington, DC 20560. 



839 



