Composition and spatial variability of terrestrial algal assemblages occurring at the bases of urban walls in Europe

Abstract

Communities of terrestrial green algae occurring at the bases of old, weathered walls are widespread in temperate urban areas, but have been virtually unstudied. In spring and summer 2002 we examined collections from a number of cities in northern Europe (Galway, Dublin, Manchester, Durham, Copenhagen) and southern Europe (Oviedo, León, Bordeaux, Marseilles, Pisa) and from several localities in western Ireland. Seventeen taxa of green and blue-green algae were found. Filamentous green algae were the most common forms and the assemblages could be subdivided into two different types: a Prasiolales assemblage (in which the dominant forms are Rosenvingiella sp. and Prasiola calophylla) and a Klebsormidium assemblage (dominated by species of Klebsormidium). The patterns of distribution of the most abundant species showed marked differences between the cities sampled, but no significant effect of latitude. The vegetation of each of the cities examined was generally assignable either to the Prasiolales assemblage (Galway, Dublin, Durham, Oviedo and León) or the Klebsormidium assemblage (Manchester, Copenhagen, Bordeaux, Marseilles and Pisa). For western Ireland, no significant variation was detected in the assemblages at different distances from the Atlantic. Climatic factors operating at a scale of hundreds of kilometres are considered the most important in determining the relative distribution of these assemblages. The Prasiolales assemblage is primarily associated with the rainy and cool Atlantic regions of Europe, whereas the Klebsormidium assemblage is dominant in continental and Mediterranean areas. The composition of these assemblages shows extremely limited variation on small spatial scales and no significant variation is detectable between different areas of the same city or town for any of the most common species. Physiological attributes of the Prasiolales and Klebsormidium in relation to different climatic regimes are considered to play a fundamental role in the distributional patterns of these algae.