Hawai'i conservation alliance

2009 Runner-up Best Student Oral Presentation

Deepwater Halimeda Meadows in Hawaii: The Ohia of the Ocean

Heather Spalding
University of Hawai‘i at Mānoa, United States

The green alga Halimeda kanaloana forms expansive meadows over soft sediments, but little is known about its ecology in Hawaii. We used technical diving, ROVs, and submersibles to describe spatial and temporal variation in distribution, abundance, demography, and growth of H. kanaloana. We found H. kanaloana meadows occurred to 90 m and covered a substantial area of the ocean floor, linking soft sediments to coral reefs. The meadows formed a unique habitat for cryptic organisms, and were used as a hunting ground for large, predatory fish and Hawksbill sea turtles. Halimeda were long-lived (> 27 months), but fluctuated greatly in segment number and height over time. Densities peaked at 20 m (342 ± 13 SE individuals per m2), but varied seasonally and among locations. Halimeda growth was rapid (9.8% ± 1.4% SE new growth per plant per week) and generally decreased with increasing depth. Episodic abundances of other green algae (e.g., Caulerpa filicoides) and cyanobacteria (Lyngbya majuscula) were observed overgrowing Halimeda. Manipulative clearing experiments (mimicking observed anchor scars) showed Halimeda could quickly regrow from the intact holdfast, but was slow (> 20 months) to recolonize areas cleared of both holdfast and thallus. The perennial nature and rapid growth rates of H. kanaloana appear to contribute toward the broad success of this species and serves to inform management of deeper reefs. As an example, disturbance removing entire individuals over a large area, e.g. repeated cruise ship anchoring, would require years for recovery.