noted in Ballantine's (1953, p, 135) prepara- 

 tions which were made according to Goldberg, 

 Baker, and Fox (1952), 



Using a typical filter disk, a count was 

 made of the numbers of Gymnodinium sp. on 

 each of 537 nonoverlapping, equal area ocular 

 fields. The observed counts (table 1) were 

 tested for conformity to a Poisson distribu- 

 tion, and the hypothesis of a random distribu- 

 tion was strongly discredited (X53^ = 1070; 

 p « 0.001). These results are in agreement 

 with those of Ballantine (1953, Table III). 



TABLE 1. — Number of Gymnodinium sp. 

 per ocular field on entire Millipore 

 filter surface and calculated index 

 of dispersion 



Sum 537 



If a complete count is to be undertaken, it 

 is advisable to make two preparations of each 

 sample. One disk should be small (ca. 0.5 cm. 

 in diameter), the other larger (i.e., 1-2 cm. 

 in diameter). The small and generally more 

 abundant organisms should be counted on 

 the small disk, and the larger organisms 

 on the larger disk. The number of orga- 

 nisms in a water sample naturally varies 

 with location and time of year and other fac- 

 tors, so no firm rules can be given as to 

 the amount of water to be filtered for each of 

 the preparations. In tropical offshore waters, 

 3-5 ml. and 50-100 ml. are appropriate vol- 

 umes for the small and large disks, respec- 

 tively. In richer waters it becomes necessary 

 to reduce the volumes filtered by a factor of 

 2 to 5. 



Comparison of the Molecular Filter 

 and Utermohl Methods 



Since the inverted microscope method of 

 Utermohl (1931) is virtually the standard 

 method of phytoplankton enumeration, several 

 comparisons have been made between it and 

 the molecular filter method. The results of 

 one such analysis are shown in table 2. No 

 differences between counts obtained except 

 in the case of the naked or mu flagellates 

 (see section on Mu flagellates) have been 

 found; the two methods seem to be com- 

 parable. 



Molecular filtration of phytoplankton has a 

 number of desirable features which compen- 

 sate for some of the difficulties previously 

 observed. These are (1) the preparation is 

 long-lasting and therefore is useful for teach- 

 ing purposes, reference, and exchange; (2) 

 the preparations require little storage space; 

 (3) little expensive specialized equipment is 

 required; and (4) the method can be used on 

 shipboard. 



