Mapping and biomass estimation of the invasive brown algae Turbinaria ornata (Turner) J. Agardh and Sargassum mangarevense (Grunow) Setchell on heterogeneous Tahitian coral reefs using 4-meter resolution IKONOS satellite data

Turbinaria ornata and Sargassum mangarevense (Halophyte, Sargassaceae) are two Fucales that have strong biotechnological potential for the cosmetic industries. To plan for the harvesting of these two species on Tahiti (French Polynesia, South Pacific Ocean) reefs, their total biomass is estimated for three morphologically different reefs using a combination of field data and three 4-m resolution IKONOS satellite images. Fieldwork provided mean algal cover for each of the main habitats of the reefs and a ubiquitous relationship linking percent cover and biomass. Images were used to map the spatial extent of the habitats. Image classification resulted in an overall habitat map accuracy of 70%. For the three reefs, the total biomass was 153,565 +/- 73,441, 561,718 +/- 192,956, and 215,203 +/- 75,012 tons of dry matter, which yielded a mean areal biomass of 0.173 +/- 0.083 kg.m(-2),0.133 +/- 0.046 kg.m(-2), and 0.193 +/- 0.067 kg.m(-2) (dry matter). The different total and areal biomass reflect different reef structures and the abundance of suitable substratum for algal settlement. Images reveal the spatial distribution of the algae, mostly located on the outer edges (crest and dense back-reef) of the reefs. Since percent cover data were collected during the cool season when algal densities are at their highest, the computed biomasses are maxima.