The Greene Green Grass of Pohnpei!

Our initial dives on the corals reefs of Pohnpei highlighted a remarkable abundance (and likely diversity) of marine algae. Typically, a large algal abundance is associated with human-induced eutrophication (high nutrient input from e.g., sewage, fertilisers 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 (but see Rowley et al. 20182019) to be subject to such detrimental human impacts, in fact they are gorgeous and breathtaking!

DSC07758_8m[PS]_Acropora_banner.jpgFigure 1. Beautiful colony of Acropora sp. at 8 m (26 ft) depth at Ant Atoll. Image by SJ. Rowley.

Naturally, more research would be necessary to support these observations, yet it is heartwarming to witness what may appear to be reefs as nature intended. It is no surprise that these reefs are so prolific considering the remarkable light penetration - whereby photosynthesis is most definitely favoured. 

Porites_Rumphella_BLOG.jpgFigure 2. Zooxanthellate corals - bearing single-celled algae Symbiodinium. The scleractinian coral Porites lutea Edwards & Haime, 1851 (left) and the gorgonian Rumphella sp. (right) colonies found on shallow reefs throughout Pohnpei at 10 - 20 m (33 - 66 ft) amidst the calcareous green algal tufts of Halimeda spp. and generous quantities of crustose coralline algae. Images by SJ. Rowley.

Specific key benthic components populate the reef at particular depths (see Rowley et al. 2018, 2019). 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 invaluable energy source for reproduction and growth. This then lends the question, why are we also seeing so much biodiversity and abundance on mesophotic reefs? Here it may be a simple combination of increased light penetration coupled with a lack of disturbance, and/or other combinations of events. 

DSC05386_75m[PS]_blog.jpgFigure 3. The gorgonian coral Astrogorgia sp., with Halimeda spp., crustose coralline algae, with scattered small colonies of the mesophotic-specialist scleractinian Leptoseris spp., and other benthic flora and fauna, 75 m (246 ft) depth at Pohnpei. Image by SJ. Rowley.

Capturing light energy requires photosynthetic pigments. Different types of pigments capture specific wavelengths of light, and it is these pigments, which typically - but not always, see below - give the algae their colour. Nonetheless, the characteristic pigments of red algae (Rhodophyta), for example, are particularly well suited to mesophotic reefs as they absorb the deep penetrating blue light and reflect red light. DSC05962_90m[PS]_blog_SMALL.jpgFigure 4. An unusual, delicate Acanthogorgiidae gorgonian with Halimeda sp. at 90 m (295 ft), Pohnpei. Image by SJ. Rowley.

Interestingly, at depth we find many red and green algae with some brown. Although some green-looking algae may actually be red! Even more confusing is the presence of soft corals masquerading as gorgonian sea fan corals, such as Siphonogorgia Kölliker, 1874. There are plenty of these soft corals at both Pohnpei and Ant Atoll, more than I've seen anywhere before. An arborescent branching structure is definitely a favourable shape to be for capturing particles in the water column. 

DSC06715_107m_Siphonogorgia_blog.jpgFigure 5. The soft coral Siphonogorgia sp., masquerading as a gorgonian at 107 m (35 ft) depth, Palikir Pass, Pohnpei. Image by SJ. Rowley.

The calcareous algal (where the plant body, the thallus, contains biochemically precipitated calcium carbonate (CaCO3)) groups such as Halimeda and the non-geniculate crustose coralline algae populate the sea bed from the shallows to beyond 135 m (443 ft)! Together with the foraminifera Cycloclypeus carpenteri Brady, 1881, these calcareous phototrophs can be considered reliable indicators of ocean chemistry in this area. If too acidic, these and other calcareous organisms such as hard corals and clams, would not be able to survive due to calcium carbonate dissolution as seen at other locations. But what of the gorgonians, how would they fare up in the advent of ocean acidification? It may be that they are less susceptible to ocean acidification, particularly as the proteinaceous material 'gorgonin' characteristic of the gorgonians, is resistant to acidic environments. Taken together, these animals are of significant interest, particularly in comparison with other benthic groups across the tropical Pacific. Definitely captivated with the challenge! 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 taken aback with finding Briareum at 75 m (246 ft) depth and Rumphella at 48 m (157 ft)!

3_Gorgonians_Twilight_Reefs_Pohnpei_BLOG.jpgFigure 6. A riot of azooxanthellate benthic cnidarians from gorgonians, soft corals, and zoanthids at 135 m (443 ft) depth, Ant Atoll. When faced with this biodiversity, where does one begin in such a brief window of time? Image by SJ. Rowley.

I delight every time we descend into a labyrinth of sea fans, appearing to increase exponentially with depth and the often bracing thermoclines [note that later we found that these temperature differences were, in fact, due to internal waves - see Rowley et al. 2019]! 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, if at all, are being leveraged; created and evolved as a consequence of such chemical warfare or are there harmonious collaborations between taxa? Whatever the case the fish are still in both high abundance and diversity, and it is no accident that the fish biologists frequently find some of their best catches and/or footage amidst the gorgonian fans. 

DSC06316_80m[PS]_BLOG.jpgFigure 7. The conspicuous and ubiquitous Annella gorgonian coral, found throughout the tropical Pacific. Pictured here at 80 m (262 ft) depth with Halimeda and crustose coralline algae, SW. Pohnpei Island. Image by SJ. Rowley.

On return to Hawai'i it is time to rest, write, and move to the next phase of specimen processing and analyses. It is hoped that these observations and findings will be as fascinating to others as it is to us, and to gain further research and funding support. Much work, however, is yet to be achieved in this little, if ever, studied ecosystem at Pohnpei and its neighbouring atolls Ant and Pakin.


This research was generously supported by: 

The Seaver Foundation


Adapted from Rowley SJ. 2014. The Greene Green Grass of Pohnpei! Bernice P. Bishop Museum. 27th July 2014.