Spatio-temporal variability in macroalgal assemblages of American Samoa

Abstract

Tribollet A.D., Schils T. and Vroom P.S. 2010. Spatio-temporal variability in macroalgal assemblages of American Samoa. Phycologia 49: 574–591. DOI: 10.2216/09-63.1

Spatial and temporal variability of relative abundance of macroalgae at the genus and functional group levels was examined at 14 m depth across the six islands of American Samoa between 2004 and 2006. Diversity of common macroalgae was high with 54 taxa (50 genera and four algal functional groups) identified from sampled quadrats. The highest taxon richness (33) was found at the largest island, Tutuila. Crustose coralline algae, mixed turf algal assemblages, and the chlorophyte Halimeda were ubiquitous across all islands. Other algal genera or functional groups were, however, representative of specific areas. For instance, the chlorophyte Microdictyon was only found at Rose Atoll and Swains Island (the only low open-ocean atoll systems examined). Similarly, the siphonous green alga Rhipilia only occurred at Swains Island, where it was very abundant. Multivariate analyses revealed that macroalgal assemblages differed significantly among sites around a single island and among islands. All islands were significantly different from each other with Swains Islands having the most important dissimilarity with other islands. We hypothesize that Swains Island differs from the other islands because of its geographic isolation. Statistical analyses also highlighted significant differences in the relative abundance of macroalgae between years. The interaction between spatial and temporal variability was also significant, indicating that temporal changes in algal assemblages are island dependant, and spatial differences among islands are a function of the year in which surveys were conducted. Changes in algal assemblages between the 2004 and 2006 surveys were apparent for all islands except Swains Island and Tutuila, probably because of intense cyclone activity in the region during 2005 (Cyclone Olaf). This study contributes to the overall understanding of coral reef diversity and function in American Samoa and forms a baseline for future algal monitoring surveys.

Key Words::

• American Samoa
• Coral reefs
• Macroalgal abundance
• Spatio-temporal variability

ACKNOWLEDGMENTS

We would like to thank Kimberly Page (CRED), Bonnie DeJoseph (CRED), and Meghan Dailer (UH Botany) for their help in collecting algal field data. Special thanks to Captains Ken Barton and Scott Kuester and the crews of the NOAA ships Oscar Elton Sette and Hi'ialakai. Funding to PIFSC-CRED for scientific expeditions to American Samoa was provided through the NOAA Fisheries Office of Habitat Conservation, as part of the NOAA Coral Reef Conservation Program. The observations and collections of algae carried out in American Samoa complied with the current laws of the country.