Subtidal macroflora of Althorpe and Haystack Islands, South Australia

The benthic macroflora at sites around the Althorpe and Haystack Is was surveyed in October 1993 and January 2004, Near'shore reefs were dominated by the kelp Ecklonia radiata, the fucoid Acrocarpia paniculata and mixed Cystophora species, whereas reefs at Haystack I. were dominated by a multi-species canopy of mixed Sargassum and Cystophora species, with Ecklonia and Seirococcus axillaris also present. On sandy bottoms off Althorpe L and Haystack 1., seagrass communities comprised mixed beds of up to five seagrass species, within four genera (Amphibolis, Halophila, Posidonia, and Heterozostera). Biomass and species composition of the canopy macroalgae were similar at those sites, which were surveyed in both 1993 and 2004, particularly at sites where few species dominated the canopy, For the upper mid-sublittoral samples at two sites around Althorpe I., a similar average number of understorey species was recorded per m2 in both 1993 and 2004. At comparable sites, the understorey species composition differed between the two time periods; however, Caulerpa flexilis was a dominant component during both 1993 and 2004. The islands are species-rich, with 267 species within 151 genera so far recorded to 27 m depth, During the 1993 survey, at least 180 species of macroalgae were recorded in 29 quadrat samples, many of these from Haystack I. The macroalgal species richness at Althorpe and Haystack Is is comparable with that of the Investigator Group and Nuyts Archipelago. These figures highlight the importance of these islands in terms of macroalgal species richmess in SA.