surface of the disk, and a clean, thin (No. 

 or No. 1) cover glass is carefully lowered on 

 the preparation. The slide is dried at room 

 temperature or in an oven at about 40° C. 



Identification of Organisms 



The method is well suited for field opera- 

 tions where a vacuum system is available and 

 has been used extensively on shipboard for 

 both water samples and aliquots of samples 

 collected with fine nylon nets (mesh size 

 30 X 30^). 



Various organisms and groups of organisms 

 respond differently to the treatment. Sum- 

 marized below are the results obtained with 

 a variety of planktonic plant groups. 



Bacillariophyceae. — The majority of the 

 pelagic diatom species respond well and suf- 

 fer few if any morphological changes which 

 complicate identification. Frustule deforma- 

 tion has been observed in only a few thin-walled 

 Chaetoceros species. Positive identification in 

 this instance demands other techniques. 



It is not always possible to resolve the fine 

 detail necessary for the identification of very 

 small centrate and pennate diatoms. A mount- 

 ing medium with a different refractive index 

 may reduce these difficulties. If resolution is 

 still impossible, the use of the electron 

 microscope is indicated. 



Dinophyceae . — Experience with thecate 

 members of this class has been limited. It is 

 apparent, however, that certain thecate genera 

 respond well to the treatment and others do 

 not. Species belonging to the genera Exuviaella , 

 Phalachroma > Prorocentrum , Dinophysis ,Orni- 

 thocercus , Histioneis , Goniaulax , Heterodinium , 

 Podalampas , and Ceratium are well preserved, 

 usually with visible plate sutures. In species 

 of the genera Pendinium and Oxytoxum the 

 plate structure is not consistently visible, and 

 occasionally pronounced deformation of the 

 theca occurs. Identification of any thecate 

 dinoflagellate may be difficult once the mount- 

 ing medium has hardened. While the mounting 

 medium is still viscous, it is possible with 

 the aid of needles to exert pressure on the 

 cover glass and orient specimens in different 

 ways, greatly facilitating identification. 



The majority of naked dinoflagellates are 

 difficult to identify after fixation with basic 

 formalin. The iodine fixative mentioned ear- 

 lier yields much better fixation images. Re- 

 gardless of the fixative employed, however, 

 this technique seems to distort and damage 

 many naked dinoflagellate species and there- 

 fore cannot be recommended for samples in 

 which this group is an important component 

 and for which positive identification must be 

 made. In such cases, examination of living 

 material is necessary. 



Chrysophyceae . — The coccolithophorids are 

 well preserved, and no evidence of coccolith 

 deterioration has been evident in 3-year-old 

 preparations. Identification of these organisms 

 is seldom easy, but once a species has become 

 familiar, it can usually be recognized on the 

 filter disk. 



Silicoflagellates . — Representatives of this 

 group have been readily identified on the filter 

 disk. 



Chloro phyceae and Myxophyceae . — On the 

 basis of limited experience, the technique 

 appears satisfactory for these organisms. 



Tintinnidae. — Nonarenacious genera have 

 been readily identified. Too few arenacious 

 genera have been encountered to permit eval- 

 uation of the method. 



Mu flagellates.— Generally members of this 

 large and heterogeneous group of organisms 

 are not well preserved with basic formalin. 

 The iodine fixative (see section on Sample 

 Preparation) is certainly superior. The mem- 

 bers of this group, however, do not seem to 

 respond too well to the multiple filtrations 

 involved. The more delicate species seem 

 to be damaged, presumably by the mechanical 

 stresses. Certain robust species (e.g., C/it/o- 

 monas marina ) have been obtained in good con- 

 dition. Ballantine (1953) says that they can 

 usually be easily identified on the filter disk. 



Enumeration of Organisms 



Although McNabb (1960), using a similar 

 technique, concluded that organisms are ran- 

 domly distributed on the filter, microscopic 

 examination of filter surfaces shows clumping 

 of organisms just inside the edge of the filter 

 and in the central region. Clumping was also 



