Coral assemblages and reef growth in the Commonwealth of the Northern Mariana Islands (Western Pacific Ocean)

Reef development varies considerably around the high, raised-limestone islands of the Commonwealth of the Northern Mariana Islands (CNMI). Here we examine the modern assemblages at 30 sites for coral composition, colony density, colony size, and fidelity. We defined four reef types and hypothesize the presence of environmentally driven ecological stasis, whereby the environment continuously selects for coral species membership, defines colony sizes, and over time creates the noted reef types. Our results show that constructional spur-and-groove reefs supported significantly larger coral-colony sizes and higher coral species richness compared with high-relief interstitial framework, low-relief incipient, and non-constructional coral assemblages. Non-constructional reefs supported much smaller coral colony sizes, despite similar population densities, and were consistently found in association with high wave exposure. The distinct coral assemblages found on interstitial framework and low-relief incipient reefs were not affiliated with any wave exposure regime, but were located adjacent to large watersheds and on islands with unique geological history. These assemblages were nested within the spur-and-groove species pool. Overall, modern coral cover was well predicted by bathymetric slope and watershed size, while species richness was additively influenced by two proxies of pollution, suggesting the latter is better suited for establishing management targets. In contrast with previous studies that suggested modern assemblages were biologically controlled in the CNMI, we show reef assemblages and reef development are highly influenced by long-term environmental forcing.