[Variation in the structure of the periphytic community that colonizes artificial substrata in the Medellín river (Colombia)]. / Variación estructural de la comunidad perifítica colonizadora de sustratos artificiales en la zona de ritral del río Medellín, Colombia.

Weekly samples were taken from microscope glass slides attached to acrylic substrata were placed at subsuperficial level in two sampling stations in the higher section of the Medellin river, Colombia, from January through March 1998. Diversity and evenness varied with time (Friedman p<0.05). There were spatial differences in biomass, productivity, diversity and evenness (Friedman p<0.05). Total biomass and productivity increased with the time of colonization at station 1. Station 2, with four times more water flow, had lower diversity and mean evenness values. During the colonization process the were 76 species (22 families). The Chrysophyta, mainly diatoms, were the most abundant (56% of the total), followed by Clorophyta (29%), Cyanophyta (11%) and Cryptophyta (4%).