Our initial dives on the coral reefs of Pohnpei highlighted a remarkable abundance (and likely diversity which is definitely on the wish-list to investigate) of marine algae. Typically, a large algal abundance is associated with human-induced eutrophication (high nutrient input from e.g., sewage, fertilizers from land run off etc.), and thus an indication of a decline in coral-reef health. However, here at Pohnpei and Ant Atoll, the reefs that we are exploring do not appear to be subject to such detrimental human impacts, in fact they are gorgeous and breathtaking!
Naturally, more research would be necessary to support our observations – yet it is heartwarming to witness what appears to be reefs as nature intended. It is no surprise that these reefs are so prolific considering the remarkable light penetration – photosynthesis is most definitely favoured.
Specific key benthic components populate the reef at any given depth. From our very first deep dive what struck me most was the immense water clarity. Photosynthesis (the conversion of light energy to chemical energy) is the name of the game on coral reefs. Both algae and benthic invertebrates are renowned for their battery of chemical warfare which enable them to commandeer space on the reef; reefs in shallow water are real estate being closest to the sun’s precious energy source for reproduction and growth. This then lends the question, why are we also seeing so much biodiversity and abundance on twilight reefs? Here it may simply be a combination of increased light penetration coupled with a lack of disturbance, itself nested within our bathymetric “habitat persistence” hypothesis.
Capturing light energy requires photosynthetic pigments with different types capturing specific wavelengths of light, which typically give the algae their colour. The characteristic pigments of red algae (Rhodophyta) for example, are particularly suited to twilight reefs as they absorb the deep penetrating blue light and reflect red light.
Interestingly, at depth we find many red and green algae with some brown. Although some green-looking algae may in fact be red – ah nature! – more intriguing patterns to unravel. Even more confusing is the presence of soft corals masquerading as gorgonian sea fans such as Siphonogorgia. There are tons of these at both Pohnpei and Ant Atoll, more than I’ve seen anywhere previous. An arborescent branching structure is definitely a favourable shape to be if your capturing particles in the water column. Whether this is a structure-function relationship and/or phylogeny is still under investigation – there is simply so much we don’t yet know, however, I strongly suspect our twilight specialists hold some of the answers.
The calcareous groups such as Halimeda and crustose coralline algae populate the sea bed from the shallows to beyond 135 m (443 ft)! Together with the previously discussed mystery forams (current status pending: Marginopora vertebralis vs. Cycloclype
Once again I digress, nevertheless what is evident in these crystal clear waters is that zooxanthellate gorgonians and other photosynthetic organisms are able to penetrate deeper along the reef slope likely due to light availability. I’m still blown away to find Briareum at 75 m (246 ft) and Rumphella at 48 m (157 ft)!
I delight every time we descend into a labyrinth of sea fans, appearing to increase exponentially with depth and the often somewhat bracing thermoclines. The intrusive sensation is gone in a cipher and I wonder how ‘peaceful’ the relationship really is between the algae and the gorgonians; are they creating their own disturbance mechanisms through competition in an evolutionary arms race for benthic space? What secondary metabolites are being leveraged, created and evolved as a consequence of such chemical warfare or are there harmonious collaborations amongst taxa? Whatever the case the fish love it, therefore our fish and gorgonian-centric research team are a perfect match. It is no accident that the boys frequently find some of their best catches and/or footage amidst the gorgonian fans.
On return to Hawai’i we rest, write and move to the next phase of specimen processing and analyses. We hope that our initial findings have been as fascinating to you as they are to us, and gain further support from the current and/or pending funds of the Bishop Museum, the National Geographic Society, and the Seaver Institute. No blog would be complete, however, without acknowledgement of three key players who made the trip happen: Brian D. Greene, Richard L. Pyle and Joshua Copus. Much work is yet to be achieved in this little, if ever, studied ecosystem at Pohnpei and its neighbouring atolls, we hope to be back there very soon.
Kalahngan and Mahalo!