Thalli of Prasiola mexicana J.Agardh (ChLorophya) were collected in the river Magdalena D.F. monthly during a year 2000-2001 (December-November) and the associated diatoms were analyzed. Two different collected points were selected. The first (P1) located in an inclined zone of permanent flow where P mexicana was always in contact with the water: the annual average of current velocity was 0.77 m s(-1). The second (P2) was located in a backwater at river edge where P mexicana was occasionally exposed to drying
the annual average of current velocity was 0.17 m s(-1). A total of 31 species of diatoms were recorded (7 new records for the region), 19% of them compatible with temperate and Nordic-alpine zones. The species richness was 24 at P1 and 31 at P2 and was more constant at P1. The most abundant and frequent diatoms in the two communities were Achnanthes lanceolata, Cocconeis placentula, Navicula cryptocephala and Cymbella silesiaca. The Friedman test are applied for pigments and diversity showed that the collected points were significantly different, and the multiple linear regression in both communities suggested that the environmental and microenvironmental variables evaluated did not display a significant relation to the changes in the structure of the communities of diatoms. Were found that the diversity at P1 had a positive relation whit the concentration of chLorophyll b and the free ash dry weight of Prasiola mexicana (p < 0.05), whereas at P2 this relation only occured with the free ash dry weight (p < 0.05). Observations in field and laboratory suggests that the dynamics of the communities of each of the collected points at the two zones was related with the expression of Prasiola mexicana: the smooth thalli had scarce sediments at P1 mainly epiphytic diatoms whereas the rough thalli had abundant sediments at P2 with epiphytic, epipsammic and edaphic species also, such as Achnanthes exigua, Navicula radiosa, Navicula rhynchocephala, Pinnularia borealis and Rhoicosphenia abbreviata.
