Abstract
The Northwestern Hawaiian Islands (NWHI) are considered to be among the most pristine coral reef ecosystems remaining on the planet. These reefs naturally contain a high percent cover of algal functional groups with relatively low coral abundance and exhibit thriving fish communities dominated by top predators. Despite their highly protected status, these reefs are at risk from both direct and indirect anthropogenic sources. This study provides the first comprehensive data on percent coverage of algae, coral, and non-coral invertebrates at the species level, and investigates spatial diversity patterns across the archipelago to document benthic communities before further environmental changes occur in response to global warming and ocean acidification. Monitoring studies show that non-calcified macroalgae cover a greater percentage of substrate than corals on many high latitude reef sites. Forereef habitats in atoll systems often contain high abundances of the green macroalga Microdictyon setchellianum and the brown macroalga Lobophora variegata, yet these organisms were uncommon in forereefs of non-atoll systems. Species of the brown macroalgal genera Padina, Sargassum, and Stypopodium and the red macroalgal genus Laurencia became increasingly common in the two northernmost atolls of the island chain but were uncommon components of more southerly islands. Conversely, the scleractinian coral Porites lobata was common on forereefs at southern islands but less common at northern islands. Currently accepted paradigms of what constitutes a “healthy” reef may not apply to the subtropical NWHI, and metrics used to gauge reef health (e.g., high coral cover) need to be reevaluated.
Highlights
The Papahanaumokuakea Marine National Monument (PMNM) is among the largest marine protected areas in the world [1] and provides unprecedented opportunities to study intact subtropical reef ecosystems
Research in recent years has documented that reefs in the PMNM differ from many truly tropical, equatorially-situated reefs that typically contain island-wide averages of over 30% live coral cover [2,3,4] and instead are characterized by benthic communities containing patches of dense coral interspersed amongst expansive stretches of hard-bottomed habitats dominated by algal functional groups [2,5]
The Northwestern Hawaiian Islands represent minimally impacted subtropical reef ecosystems that receive both federal and state protection [1] and are dominated by algal functional groups [2,38,48]. Such algal functional groups are vitally important for healthy reef function [2] and, contrary to currently accepted models of reef health [49], historical reports [50,51,52] indicate many near-pristine, Hawaiian, subtropical reef systems naturally contain expansive populations of macroalgae, with scleractinian coral species usually only being dense in localized areas
Summary
The Papahanaumokuakea Marine National Monument (PMNM) is among the largest marine protected areas in the world [1] and provides unprecedented opportunities to study intact subtropical reef ecosystems. Normal oscillations in reef assemblages would have occurred due to changing sea levels and warming/cooling events over geologic time scales [22], and it cannot be assumed that one constant state of ‘‘reef health’’ was maintained over thousands of years [23] Because of these issues, accurately defining pre-human reef attributes is difficult, but gaining current comprehensive, multidisciplinary, species-level surveys of benthic communities in the Northwestern Hawaiian Islands (NWHI) provides a critical baseline of the present state of NWHI reefs and will allow future researchers to determine if benthic communities change over time in the face of rapidly changing sea surface temperatures [24] and ocean acidification [25,26]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
