Atolls & Islands

The Island Hopper - for some it is arduous, for me, it is infinitely fascinating because we see reefs and islets of captivating beauty. As we travel between the atolls and islands (Majuro, KwajaleinKosrae, and eventually Pohnpei) we sense that these fascinating geological structures harbour a remarkable and individually unique level of biodiversity, the origins of which we, as yet, do not fully understand and are here to exploreMicronesian Atoll from the Island Hopper. Image: SJ. Rowley 2015

Figure 1. Aerial view of one of the atolls. Image by SJ. Rowley.

Atolls are essentially sunken volcanoes with an upward growth of hermatypic corals (reef-building coral, primarily scleractinians) and deposition of calcareous foraminifera that constitute the outer reefs and islets. The theory of coral atoll formation was first postulated by Darwin in 1835 during his time on the Beagle, and published thereafter in 1842 volume "The Structure and Distribution of Coral Reefs". In the 1950s his theory was officially confirmed by the US Navy whilst conducting nuclear testing operations on Enewetak Atoll, near the ill-fated Bikini Atoll. By drilling down to 1408 m (4620 ft) they were still finding limestone. This clearly demonstrated the presence and persistence of carbonate coral reefs over geological (deep) time.

Pakin_Atoll_110m_Screenshot 2017-09-28 13.57.09_BLOG-1.pngFigure 2. Ancient wave-cut ledge at 100 - 110 m (328 - 360 ft) depth, Pakin Atoll. Image by SJ. Rowley.

Exploration to mesophotic depths continue to reveal that the oceanic islands and atolls of the Pacific typically possess a wave-cut ledge at ~80 - 100 m depth (226 - 328 ft) with a subsequent shear drop to the deep ocean below. How does this distinct geomorphology influence the biodiversity throughout the tropical Pacific? Interestingly, as research exploration persists, distinct patterns of biodiversity and biogeography are being revealed. Most notable is from the biodiversity hotspot of the Coral Triangle, eastwards across the Pacific, the coral-reef biodiversity at shallow depths (0 - 30 m/0 - 98 ft) reduce. However, at deeper mesophotic depths (30 - 150 m/98 - 492 ft) the sheer diversity and abundance of biota persists and, therefore, is just as high as on the continental regions of the Coral Triangle. Why would this be? An increasing number of researchers and explorers are intrigued by this seeming phenomenon.

DSC04163_Ant_Shallow_Cave.jpgFigure 3. Shallow cave at 12 - 17 m (39 - 56 ft) depth, Ant Atoll. Image by SJ. Rowley.

The oceanic volcanic islands and atolls across the equatorial Pacific offer the opportunity for evolutionary novelty to develop in isolated enviroments. This allows for the persistence of forms that may typically be selected against on the more diverse communities of continental regions. The trail of atolls and islands, nonethelss, provide stepping stones for biodiversity export and import for specific taxa that can rely on the ocean currents for transport.

What is being increasingly revealed is that the shallow and mesophotic depths of these remote locations are subject to different bodies of water, and thus, will have different influences on the reef communities. In addition, eustatic sea level change over the millenia may also contribute towards these contrasting biogeographic patterns. As sea level changes over the millenia (~100 - 130 m/330 - 426 ft) the steep geomorphology of the oceanic islands and atolls gain (at low sea level because water is locked up as ice in the poles) and loose (at high sea level as water is released as melt-water pulses from the poles) habitat predominantly in the shallows. It is suggested that these shallower habitats will need to be re-populated with fauna and flora both vertically (from depths e.g., the deep-reef refugia hypothesis) and horizontally (from other islands and atolls - stepping stones if you will e.g., a bathymetric "habitat persistence" hypothesis), whereas the deeper mesophotic reefs are postulated to stay constant! Neither of these ideas have been successfully proved, however, what is clear is that a remarkable level of undescribed species exist at depth and with the hastening of environmental degradation and global climate change, such unknown treasures may never be revealed; burried under the weight of the onerous demands of the Anthropocene

DSC08943 copy 2[ps]_THUMBNAIL.jpgFigure 4. Ant Atoll, 135 m (443 ft) depth. Image by SJ. Rowley.                                       

We are here to help unravel this conundrum, explore, discover, and describe new taxa in novel environments, and ultimately, share these findings with the local communities. Our hope is that such awareness will provide convincing tools to help mitigate the formidable persistence of habitat degradation and loss in the face of economic policy.

Blog1_PlateSJR[s]_BLOG.jpgFigure 5. Large macroalgal patches in the shallow's and around the gorgonians into the mesophotic depths, Pohnpei. Images by SJ. Rowley.

At depth, two key groups dominate; gorgonian (sea fan) corals and fishes. It is the coral that is my primary area of reserach interest and, on this trip, our initial test dives did not disappoint. I see intriguing species and patterns emerge as we descend the reef. Small colonies of gorgonians exist in the shallows due to a dominance of algae, sponge, and hard coral; but also significant wave surge at many sites. The sea fans are positively correlated with depth: increasing in size as I decend. Some of these gorgonian taxa are groups that I regularly encounter throughout the Indo-Pacific. Others, however, are new!

DSC04965_Blog_Pohnpei_Lagoon_P-Pass.jpgFigure 6. The view from the sea bed filled with sharks, rays, fishes in a clear blue ocean, Palikir Pass, Pohnpei. Image by SJ. Rowley.

What intrigues me most is, will we get to Pohnpei's neighbouring atolls Ant and Pakin? Both are marine protection priorities with the Conservation Society of Pohnpei (CSP) and Hawley family successfully achieving the Ant Atoll biosphere reserve through the UNESCO Man and Biosphere program. I am, nonetheless, intrigued by Pakin Atoll, NE of Pohnpei and considered to contain the most pristine coral reef environments of the entire Pacific! Not to mention the largest abundance and size of sea fan corals known throughout the region! Perhaps for another trip as we work with local communities to find ways to maintain these precious resources.

This research was generously supported by: 

The Seaver Foundation


Adapted from Rowley SJ. 2014. Atolls & Islands. Bernice P. Bishop Museum. 2nd July 2014.