Epipelic and Epilithic Diatom Assemblages in Montezuma Well National Monument, Arizona1

Epipelic and epilithic diatom assemblages in Montezuma Well National Monument, Arizona, were identified and analyzed by H' (species diversity), species number, J' (evenness), relative abundance, SIMI (similarity index), and DI (dissimilarity index) values. The epipelic assemblage was found to be a tri‐dominant association ofCyclotella pseudostelligera f. parvaCzar. & Blinn,Cocconeis placentulavar.lineata(Ehr.) V.H. andAchnanthes linearis(W. Sm.) Grun. The epilithic assemblage was a tri‐dominant association ofRhoicosphenia curvata(Kütz.) Grun. ex Rabh.,Cy. Pseudostelligera f. parvaandSynedra ulna(Nitz.) Ehr. Both associations persisted throughout the year. Of the 83 taxa observed, 51 were common to both assemblages, 24 were restricted to the epipelon, and 8 were restricted to the epilithon. On a yearly basis, the epipelic assemblage exhibited less diversity (H') than the epilithon, even though the latter exhibited fewer taxa. The use of mean diversity values was also determined to be unacceptable.Cy. Pseudostelligera f. parvaandS. ulnaappeared more abundant under high light intensity and duration,Fragilaria construensvar.venter(Ehr.) Grun. favored low light intensity and duration, whereasA. linearis, R. curvataandCo. placentulavar.lineatawere tolerant of a variety of intensities and durations. On a yearly basis, both assemblages exhibited slight similarity (SIMI, DI). The use of SIMI values for comparing assemblages with few highly abundant taxa was determined to be unacceptable and under these conditions, the use of DI was recommended.