Seminar Series 2003

The Role of Buoyancy And Material Properties In The Dispersal Of The Macroalga Turbinaria Ornata Across Reefs Throughout French Polynesia

Stewart, H. L. Department of Integrative Biology, University of California at Berkeley, Berkeley, CA 94720; hstewart@socrates.berkeley.edu

French Polynesian reefs have experienced a proliferation of macroalgae since the 1980s. Affected areas, mainly located in shallow (< 10 m depth) water around volcanic high islands, have experienced shifts from mixed coral and algae communities to domination largely by the frondose macroalga, Turbinaria ornata. T. ornata’s distribution has also been expanding across the Society Islands, and recently to remote atolls of the Tuamotu archipelago. Buoyancy plays a key role in the successful dispersal of T. ornata, dislodged buoyant thalli forming large mats that can drift 100s of km and colonize new areas. Thalli from high-energy forereef locations lack pneumatocysts and are negatively buoyant, while thalli from calm lagoon sites are buoyant with up to 85% of their blades containing gas-filled pneumatocysts. Thalli transplanted from forereef to lagoon sites developed pneumatocysts and became positively buoyant, indicating that pneumatocyst production is a plastic trait. In lagoon thalli, buoyant force increases and breaking strength decreases with percent of thallus weight made up of reproductive material. Thus, as thalli become reproductively mature, they are more susceptible to dislodgment from hydrodynamic forces, and the result is large floating rafts of reproductively mature and fertile thalli that help to disperse T. ornata throughout the South Pacific.