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.

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.

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.