Long-term quantitative datasets in marine tropical areas are rare, yet needed for comparisons to present community states to identify temporal patterns and inform management about the status of culturally and ecologically important species. From 1966 to 1969, M. Doty and colleagues (Doty 1969) intensively surveyed the biomass and species composition of macroalgae, or limu, in 12 plots from 0 to 230 m offshore of the Waikīkī Natatorium War Memorial, south O‘ahu. In conjunction with the undergraduate research program Our Project in Hawaii’s Intertidal (OPIHI), our goal was to resurvey three of these plots (10, 110, and 210 m from shore) using the same methodology as Doty 1969 to identify changes in the species composition and abundance of macroalgae over the past ~50 years.
The loss of culturally-important native species, such as such as Sargassum spp. and D. plagiogramma, and increase in invasive species suggests this area has undergone a dramatic shift in community assemblage structure, with unknown ramifications for this area’s food web dynamics, fisheries habitat function, and overall ecology.
All algae were collected from a 0.45 m diameter quadrat at 10 randomized points within zones 10, 110 and 210. Algae were then sorted to the species level, and weighed (wet weight) according to Doty 1969.
The original zonation of the Doty 1966-1969 project, denoting twelve 20-meter plots extending west from the Natatorium War Memorial in Waikiki, O‘ahu, Hawai‘i. Areas outlined in red represent the zones being sampled in this recreation of the study.
In 1967, the most abundant nearshore (10 m) species were Ulva spp., Dictyosphaeria cavernosa, Valonia aegagrophila, and the invasive Acanthophora spicifera, while the offshore sites (110 and 210 m) were dominated by a high abundance of the native brown algae Sargassum spp. (limu kala), Dictyopteris plagiogramma (limu lipoa), and Padina sp. The species composition in 2018 dramatically shifted to a high abundance of the invasive species Gracilaria salicornia and A. spicifera, and an absence of large brown algae such as Sargassum spp. and D. plagiogramma.
|TOTAL wet weight (g)||9029||480||-95%|
We thank the University of Hawai‘i Sea Grant College Program for their support of the OPIHI program. Many thanks to Byron Inouye for his patience and assistance with the graphic design.