The Island Hopper – for some it is arduous, for me, it is infinitely fascinating because we see reefs and islets of tantalizing beauty. As we travel between the atolls and islands (Majuro to Kwajelain, to Kosorae, and to Pohnpei) it is clear to me that the formation of these geological structures are encased in a mystery that we, as yet, do not fully understand, and are here to explore.
Atolls are essentially sunken volcanoes with an upward growth of hermatypic corals (reef-building coral, primarily scleractinians) maintaining the outer reefs and islets. First postulated by Darwin, it was officially confirmed by the US Navy in the 1950s, whilst running 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 clearly demonstrating the presence and persistence of carbonate coral reefs over geological time. Islands, on the other hand, typically constitute a larger structure of continental origin possessing a ledge of some sort ~ 80 – 100 m (226 – 328ft) deep, whereas atolls have a shear drop to the deep ocean below. How does this effect the biota, are there differences and if so, why? We have also noticed that throughout the tropical Pacific, oceanic islands and atolls not only have less biodiversity on shallow coral reefs, but also the biodiversity at twilight depths (30 – 150 m/100 – 492 ft) appears just as high as on continental reefs. Why would this be? We think that the answer may be very simple; as the sea level changes over the millennia (~100 m/330 ft) shallow reefs need to re-populate but deeper reefs are stable. So our bathymetric “habitat persistent” hypothesis is one of many things we are here to investigate.
Two key groups occupy twilight reefs, fishes and gorgonian (sea fan) corals. It is the corals that are my primary interest and our initial test dives do not disappoint. I see patterns already as we descend the reef and they begin to emerge; small colonies in the shallows due to large algae, sponge and hard coral dominance and larger more prominent sea fans with depth. These sea fans are groups I see most often throughout the Indo-Pacific. We bottom at 50 m (165 ft) and are greeted by a myriad of fish, sharks, rays and a manta ray.
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) successfully achieving the Ant Atoll biosphere reserve through the UNESCO Man and Biosphere program. Secretly, I’m 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! Perhaps another trip as we work with local communities to find ways to maintain these precious resources.