Extensive explorations from Bank 11 to the southeast in the NWHI had so far not discovered any deep-sea coral reefs, even though both submersible dives and ROV explorations had included the same depth zones at several different islands and seamounts11. Indeed, it was thought to be improbable that scleractinian reefs would occur in the North Pacific due to carbonate chemistry that is expected to make reef formation and accumulation challenging. The carbonate dissolution rate in the North Pacific peaks between 400–600 m then decreases rapidly with depth13. This depth range overlaps with the relatively shallow range of the ASH in the North Pacific (50–600 m). Thus, theoretically, the most challenging depth range for reef formation in the North Pacific would begin somewhere around 600 m depth, and continue deeper. Yet, here we document observations of reefs at six seamounts in the NWHI and ESC at depths of 535–732 m.

This begs the question, how is it that the reefs can occur at these sites? One potential insight comes from a closer examination of the ASH depth in Feely et al.13, which indicates that the ASH becomes deeper moving to the northwest along the NWHI (confirmed by our results in Fig. 4). This suggests that, if the reef-forming species have a narrow depth range tolerance, the likelihood of finding reefs would increase moving to the northwest along the NWHI. That is indeed what we found, reefs were observed on every feature explored to the northwest of Academician Berg except Bank 11. However, if the ASH were the main factor governing the distribution of these corals, we would expect their depth range to stay the same or deepen moving northwest along the chain, paralleling the ASH. Instead the observed depth range becomes shallower. Further, a number of reef sites also occur below the ASH, with measured Ω arag values of 0.71–1.33, together these suggest the ASH is not the primary controlling factor for the distribution of reefs.

Although the ASH would be expected to be a limiting factor for calcifying deep-sea coral species that produce aragonite5, results are mixed with respect to the response of calcifying species to low Ω arag and pH. Supporting the idea that the ASH would be limiting, laboratory CO 2 exposure experiments have shown that calcification rates of deep-sea corals decrease with decreasing Ω arag 16,17,18,19,20, that deep-sea corals produce skeletons that are more susceptible to erosion under low Ω arag conditions21, and that net dissolution occurs in live corals in undersaturated (Ω arag < 1) waters18,19,20. However, other experimental studies have shown no response in calcification and respiration rates to changing Ω arag 19, 22, 23, and it has been well documented that deep-sea corals can live and calcify in undersaturated waters24, 25. For example, Thresher et al.24 also found a number of deep-sea corals on seamounts south of Tasmania living below the ASH and calcite saturation horizon. An important exception that they note though is that the reef-forming scleractinians, including Enalopsammia and Solenosmilia, were limited to depths “saturated or near saturated” with respect to aragonite. In contrast, we find scleractinian reefs in waters with Ω arag well below 1 in the NWHI and ESC.

In fact, a number of studies have suggested that some deep-sea coral species have physiological mechanisms to compensate for undersaturation and to maintain their calcifying processes and internal pH25,26,27. For example, Thresher et al.24 postulated that the non-reef forming scleractinian corals below the ASH might be able to survive due to high regional productivity resulting in an abundant food supply. This food supply could provide the excess energy needed for calcification in undersaturated waters. In contrast, Maier et al.18 found that feeding in laboratory experiments did increase calcification rates of the Mediterranean deep-sea coral Madrepora oculata at ambient Ω arag , but feeding had no effect on calcification rates under low Ω arag conditions. While the authors attribute this to the small fraction (1–3%) of the total metabolic energy demand required for calcification in Madrepora oculata, this does not explain why feeding appears to have enhanced calcification at ambient Ω arag . Georgian et al.28 found that net calcification, respiration and prey capture rates of Lophelia pertusa from the Gulf of Mexico decreased with decreasing pH and Ω arag but, in the same species from Norway, respiration and prey capture rates increased, and calcification only decreased slightly with declining Ω arag . These results suggest that local environmental conditions, including food supply, could result in regional differences in the ability of deep-sea corals to adapt and/or acclimate to ocean acidification.

While the main Hawaiian Islands and much of the NWHI are located in oligotrophic waters, there is a transition to higher chlorophyll waters moving to the northwest, characterized by a front referred to as the Transition Zone Chlorophyll Front (TZCF). The position of this front varies seasonally and annually, crossing the Archipelago somewhere near 180° longitude in the years it reaches its maximum extent29, very close to our southeastern most site with reefs, Academician Berg at 178.84°W. Figure 6B shows the annual mean surface water chlorophyll concentration across this region averaged over the period 2008–2016. Sites with reefs have higher mean annual chlorophyll than those without. Thus higher chlorophyll, while not an explanation for why the corals occur shallower, could at least be a plausible explanation for why these reefs occur from Academician and further northwest. Consistent with this, PCA analyses indicates that particulate organic carbon (POC) and Chlorophyll a (Chl-a) were most strongly correlated with PCA axis 1 (Fig. 5, Table 3). However, sites with scleractinians were distributed broadly across the PCA 1 axis, suggesting that potential food supply alone cannot explain the distribution of these reefs.

Of the five PCA axes, transects with scleractinian reefs occupied a very narrow range of PCA axis 2 when graphed relative to the four other axes. PCA axis 2 was most strongly correlated with sound velocity and the east-west component of surface currents. This suggests there is a very narrow range of current velocity needed for the survival of the deep-sea scleractinian reef-forming species. That currents might be tied to the distribution of deep-sea coral reefs is not at all surprising because the occurrence of corals near topographic highs with maximum current velocities has been recognized since some of the earliest work on seamounts30. However many other transects without reefs also fell within the same range of the PCA 2 axis as the scleractinian reef sites, suggesting currents are also not the only factor critical for reef occurrence. In addition, surface current data may not necessarily represent what the corals are experiencing at depth and alone could not explain why the depth distribution of reefs decreases to the northwest.

More research is clearly needed to explain the distribution of these reefs. Ideally we would use species distribution modeling to analyze the factors most correlated with the distribution of these species as well as to determine the locations of possible areas of suitable habitat14, 15, 31, but we currently do not have high enough resolution data for key parameters that would go into such modeling, in particular backscatter, in situ currents, and other important factors.