Scripps Institution of Oceanography

Spatially broad and long-term monitoring studies are lacking in tropical intertidal systems yet are necessary to test predictions regarding community assembly. To fill this gap, we examined spatial and decadal temporal patterns in benthic community structure at rocky intertidal sites along the main islands of Hawai‘i. Quantitative community surveys done in 2017 across nine sites and five islands showed that organismal composition differed by site, substrate type, and island. Secondly, we leveraged an earlier dataset collected using the same methods and analyzed intertidal communities at five sites on three Hawaiian islands for temporal changes in organismal abundance and composition from 2006 and 2007 vs. 2016 and 2017. Overall community structure differed significantly across years and decades. Most decadal differences were site specific, such as the fivefold increase in turf algae at one site. Crustose coralline algae and Turbinaria ornata increased significantly across five sites; both are physically resilient algae and similar increases in their abundances have been observed in tropical systems worldwide. This increase in physically resilient macroalgal species is potentially caused by global drivers, such as rising temperatures and changing land uses. In conclusion, there is evidence that both local and regional factors contribute to structuring tropical intertidal communities.