Download (6Mb) - USP Electronic Research Repository - The ...

Rotuman Distribution
Hapmafau (*HAP281 USP S5: 12).
Habitat and Remarks
Epiphytic on Valonia aegagropila and Dictyota friabilis in sheltered back reef locations.
Phytobiogeographical Analysis of the Rotuman Algal Flora
Comparison of Sites on the Island
These data are graphically presented in figs 209-210. The total flora to date consists of 88
taxa. The Rhodophyta comprise the majority of the flora (46.6%; 41 taxa), with the
Phaeophyta being least represented (12.5%; 11 taxa). Chlorophyta consist of 40.9% of the
flora, or 36 taxa. Owing to the uncertain taxonomy of the majority of the Cyanophyceae,
they are excluded from the analysis, although they are widespread at most stations.
Comparison of stations reveals marked differences in the occurence of red algae, although
the green algae have a more constant distribution. There is also a marked north-south
distribution pattern of certain species of Chlorophyta, especially Halimeda and Caulerpa
(Fig. 21 1). The latter trend may reflect differences in the habitats of opposite coasts on the
island, as discussed below.
Discussion
North-South Distribution Patterns
As can be seen from Fig. 21 1, there is marked difference in north-south distribution of
Rotuman algae, especially with respect to the genera Halimeda, Avrainvillea, Rhipilia,
Melanamansia and Caulerpa. This could be attributed in part to the different habitats on
opposite sides of the island, which are predominantly sandy and sheltered to the north
(favouring Halimeda and Melanamansia growth) and relatively rocky and exposed to the
south (favouring more cryptic, robust species).
The Caulerpa distribution is, however, the opposite, with sites on the north coast (e.g.
Lopta) being rich in clumps of relatively resistant Caulerpa racemosa var, uvifera, whereas
the sandy and protected area of Hapmafau Bay at the Motusa Isthmus favours growth of
larger and less robust Caulerpa species (Caulerpa racemosa var. peltata, Caulerpa
cupressoides, Caulerpa serrulata). It should be stressed that Hapmafau Bay is not a typical
southern station, most of which support robust forms of Caulerpa serrulata (e.g. at Tuakoi,
Savlei).
A possible explanation for these distribution patterns could be the orientation of the
island, which lies approximately in an east-west line. The southern coast is therefore more
exposed to the south-east trade winds predominant most of the year in this part of the Pacific,
resulting in significantly more wave-washed and eroded habitats. The northern coast, on the
other hand, is-relatively protected and this has allowed the accumulation of sandy deposits
and the formation of small lagoons and tidal pools, favouring growth of such species as
Halimeda, Avrainvillea, Rhipilia and Melanamansia.
Rotuma in the Context of Other South Pacific Islands
Probably the most distinctive feature about Rotuma is its isolation, as it is not
geographically connected to any other island or island group. In this aspect, it resembles
Lord Howe Island or Easter Island, both high volcanic islands. In size, it is much smaller
than its nearest neighbouring islands in Fiji, and its combination of equatorial climate and
extremely fertile volcanic soil makes it a far more productive island than most of the
predominantly coral atolls to the north such as Tuvalu and Kiribati.
The small size of the island, coupled with its mostly exposed fringing reef structure,
severely limits the types of algal habitats found there. There are thus marked floristic
differences to be expected when comparing the island with such locations as the Solomons,
Samoa, New Caledonia or Fiji, all of which have wide ranges of algal habitats. In particular,