Both Robichaux, et_ aL_ (1981) and Youngbluth (1982) addressed the question 

 of trap design and both found differences in number and composition of zooplankton 

 between sealed and unsealed traps. Both of these studies, however, were 

 conducted over sandy bottoms where a complete seal was possible and zooplankton 

 movement through the underlying substrata unlikely. In this study, both sealed 

 and unsealed traps were placed over reef substrata; the former were sealed as 

 well as possible through the use of sand and rubble placed over the trap skirts. 

 No differences were observed in the numbers of zooplankton captured between the 

 two treatments, possibly suggesting that there is more movement by zooplankton 

 through interstices of the reef than previously thought. Analysis of differences 

 in species composition between the two treatments is currently underway. 



While Alldredge and King (1977) and Porter and Porter (1977) found the 

 most zooplankton over structurally complex coral substrata, this was not found 

 by Ohlhorst (1980) or Birkeland and Smalley (1981). In the present study, 

 there also was no difference in the number of individuals captured over different 

 substrata. One contributing factor may be the differential ability to seal 

 traps over sand and coral substrata. Robichaux, et al . (1981) and Youngbluth 

 (1982) both found that more zooplankton were captured by unsealed traps than by 

 sealed traps when both were placed over sand substrata. Youngbluth (1982) 

 observed that a gap of 1 cm between trap and substratum resulted in capture of 

 significantly greater numbers of zooplankton than a total seal, while there was 

 no difference between samples from traps with gaps of 1 and 10 cm. Therefore, 

 differences in number of zooplankton collected over different types of substrata 

 might be expected when certain treatments are well sealed and others not. If 

 the sand traps of Alldredge and King (1977) and Porter and Porter (1977) 

 were well sealed in contrast to those over coral substrata, it may be difficult 

 for these researchers to compare their substratum treatments. Birkeland and 

 Smalley (1981) compared coral substrata to algal turf pavement and probably 

 were able to sample both treatments similarly, since in either habitat a total 

 seal is unlikely. Ohlhorst (1980) and this study used unsealed traps to sample 

 both coral and sand substrata. These traps captured the zooplankton moving 

 along the reef bottom over both types of substrata, and the data indicated that 

 the numbers of zooplankton available to planktivores were comparable in both 

 habitats. Analysis of the species composition of zooplankton over various reef 

 substrata is in progress. 



Fewer zooplankton were captured per hour in this study than in previous 

 Caribbean studies by one of the authors (Ohlhorst, 1980, 1982, 1985). The most 

 probable explanation lies in the sole use of polyvinyl traps in the previous 

 studies. In this study, two polyvinyl traps were used in addition to mesh 

 traps (details will be discussed elsewhere); and, while no statistically 

 significant differences were found in abundance of plankton between these 

 treatments (due to high variance and small sample size of polyvinyl traps), the 

 polyvinyl traps usually captured considerably more zooplankton. Both this 

 study and the earlier work by Ohlhorst (1980, 1982, 1985) found considerable 

 variability between nights sampled. 



The bimodal emergence pattern (post-sunset, pre-sunrise) suggested by 

 Glynn's (1973) data from plankton tow nets appears to be supported by this 

 study (fig. 1), although more complete information will be available when all 

 the samples have been analyzed. The present study provides the first documentation 



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