Terrestrial and freshwater algae of Aldabra

Abstract

A preliminary account is presented of the terrestrial and freshwater algae found during the wet season 1968/69. Blue-green algae were by far the most abundant group, often conspicuous to the naked eye. Colonization of bare rock is almost entirely by heterocystous blue-green algae, with other algae, lichens and bryophytes playing a negligible role. On champignon and the drier parts of platin Scytonemataceae predom inated. On parts of platin subjected to a frequent cycle of wetting and drying, there were large masses of colonies of Nostoc commune . Thin soils over platin with only a sparse angiosperm cover were often dominated by a Nostoc—Rica community. However, in some areas of South Island sheets of various Oscillatoriaceae covered a similar substratum . It is suggested that this difference may be due to the activity of the tortoises. In the deeper waters covering the platin of South Island forms of Wollea were the predom inant alga. Chlorophyta were represented by a range of species, but these seldom formed a major part of the algal biomass. The rarity of diatoms and the absence of Chrysophyta were noteworthy. Using the assumption that the heterocyst is an indicator of nitrogen-fixing ability, the evidence suggests that deficiency of combined nitrogen is a major factor during early successional stages on bare coral, but not one by the stage that an angiosperm cover has developed. Direct observation suggests that decay of blue-green algae is an important contributor to the formation of humus over bare rock, but gives little clue to the quantitative importance of these organisms as an agent of erosion.