1. The causes of variations in the composition of microalgal communities on frost-sorted soil polygons on Signy Island, maritime Antarctica, were investigated, based on analyses of physical and chemical conditions and microalgal communities from 65 polygons. 2. Interpolygon variations in all environmental factors measured were small and not much greater than the intrapolygon variations. Microalgal diversity was low. Only seven taxa occurred in sufficient numbers to be included in statistical analyses, two of which, Nostoc spp., were found on only one polygon. The filamentous cyanobacterium Phormidium autumnale occurred on all polygons and usually provided the largest component of the total biovolume. 3. Microalgal communities were qualitatively constant within polygons, but varied markedly between polygons. Principal components analysis indicated that each taxon varied independently of the others, and hence it was not possible to correlate community structure with any specific environmental factor. 4. Correlations between numbers of individual taxa and edaphic factors were low and accounted for a maximum of 18% of the observed interpolygon variation or 12% of the total variation. It is suggested that the observed differences in the microalgal communities were due to the vagaries of the colonization process and reflect the probability of successful colonization of these ecosystems by microalgae.
