The geological and economical importance of non-geniculate coralline algae (Rhodophyta, Corallinaceae) in coral reef systems is generally well known and documented. However, virtually nothing is known about the biology and ecology of non-geniculate coralline algae (NCA) on one significant coral reef system, the Great Barrier Reef (GBR). This study comprises investigations on the taxonomy, general patterns in recruitment, growth, community development and ecological aspects on NCA carried out on Heron Reef (GBR).
The first part of the study comprised the first comprehensive account of non-geniculate coralline algae (Corallinales, Rhodophyta) occurring on Heron Reef (GBR). Species were identified using reproductive and vegetative anatomy as diagnostic features. Eleven different species were identified. Although none of the species are new to science, one is newly recorded for Australia (Hydrolithon reinboldii) and five are newly recorded for the Great Barrier Reef region (Spongites fruticulosus, Lithophyllum frondosum, L. pustulatum, Mastophora pacifica and Mesophyllum erubescens). Illustrations of each species, a tabular key and a field guide are provided to facilitate NCA identification on Heron Reef. Information on the distribution and growth forms for each species is provided, along with comparisons to findings from other tropical reef systems.
Although to date it is not known how many NCA species occur within coral reef systems, taxonomic comparisons at species level show that many species are widely distributed among tropical reefs. . It is not possible however, to make general ecological comparisons of different species and their growth forms because it appears that the various species occur within different habitats and under different environmental conditions at various geographical locations.
In situ investigations on recruitment patterns, growth rates and growth forms, development of conceptacles and calcification in newly recruited NCA were made at temporal scales of 2, 4, 6 and 8 weeks for a one year period. NCA recruited year round, with a peak between March and June. They colonised, calcified and produced conceptacles within 14 days of immersion of an artificial substrata. Their growth rates (diameter of the plants) ranged between 2.8-3.2 mm after 4 weeks exposure, 3.9-4.5 mm after 6 weeks exposure, and 4.4-5.1 mm after 8 weeks exposure. Three species (Hydrolithon farinosum, H. onkodes, Lithophyllum pustulatum) could be identified clearly after 8 weeks and each had developed a particular growth form. For all parameters measured (growth rates, development of conceptacles and calcification) there was substantial variability within and among sites and it appeared that recruitment was independent of water movement.
In situ growth rate measurements of established crustose and branching non-geniculate coralline algae showed that branching NCA (bNCA) grow faster (gained more weight over time) than crustose NCA (cNCA). These findings have been corroborated by two other investigations. First, faster rates of growth of the two branching species (Lithophyllum tamiense and Mesophyllum erubescens) compared to the crustose species Hydrolithon onkodes and second, higher growth-rates of branching than crustose recruits.
Both growth forms successfully survived transplantation to a different environment provided they were not grazed; their rate of growth did not change significantly from that at their site of origin following transplantation. Growth rate and survivorship of both forms appeared to be independent of water movement, and were more likely to be influenced by grazing. Survivorship of NCA therefore appears to be more sensitive to biotic disturbances rather than abiotic changes in the environment. The gained results on recruitment, growth and survivorship of NCA highlighted that they have the potential to shift a coral dominated community into a coralline algae dominated community following any kind of disturbances. Furthermore, it appears that NCA are less sensitive to changes in the environment, in terms of survivorship, than corals, possibly one factor explaining their overall presence through geological time. Investigations on community composition, distribution of growth forms and abundance of NCA in the natural environment and on inert substratum were correlated with water movement and/or sedimentation. The results from this study suggest that water movement is an important post-settlement factor (post-recruitment) i.e. once non-geniculate coralline algae (NCA) with the potential to develop branches have settled, then water movement is a critical factor in determining the distribution and abundance of growth forms. However, water movement is not the ultimate or only factor determining settlement and recruitment of NCA. In general, it appears that community development and development of NCA growth forms will be explicable in terms of local (small scale) rather than regional processes.
Overall, within this study II species occurring on Heron Reef were identified, general patterns of recruitment and growth have been revealed and ecological aspects have been investigated. It appears that most tropical NCA species are generalized species with broad tolerances to environmental conditions and that the distribution patterns on particular reefs or within particular regions may be caused by varied local abiotic/or biotic conditions.
As this work represents the initial study on the above mentioned aspects of NCA on the GBR, further studies are needed to evaluate the significance of these results in comparison with other reefs in the GBR region and with other coral reef systems throughout the world.