Seasonal variation in the structure of three Mediterranean algal communities in various light conditions

Three algal communities subjected to different light conditions were analysed using structure descriptors such as α-diversity (Shannon diversity index), similarity (Kulczynski index), diversity/area and similarity/area curves, the number and size of patches and their spatial distribution (Morisita index). The data used to calculate the structure descriptors were obtained from 20 pictures taken at random in each community in June (Spring) and November (Autumn) to assess seasonal variation. The value of α-diversity was quite similar in all communities. The structural minimal area obtained from the spectra of diversity at increasing areas was higher in the photophilic and the hemisciaphilic communities than in the sciaphilic. Moreover, this area was smaller in November than in June in the three communities. Seasonal variation was also detected for the Kulczynski similarity index in the photophilic and hemisciaphilic communities, with similarity values higher in November than in June, which points to the stronger homogeneity of the communities in November. As for the distribution patterns, the number of species with a clumped distribution decreased from the sciaphilic to the photophilic communities. Species distribution showed seasonal variation in the photophilic and, to a lesser extent, in the hemisciaphilic community. These communities had smaller but more numerous patches in November than in June. It is concluded that most of the used descriptors indicate a higher seasonal variation of community structure in the most illuminated communities (i.e. the photophilic and to a lesser extent the hemisciaphilic communities) than in the least illuminated one (i.e. the sciaphilic).