Widening Our View of the Reef
By Gareth J. Williams
We are hundreds of miles away from civilization in the middle of the south Pacific Ocean when a tiny thin speck of land comes into view. It is Starbuck Island in the Southern Line Islands. The bright white sand reflects the morning sun and a few lone palm trees huddle together. As we jump in the water on our first day I am faced with a very different looking reef to that of Vostok Island just a few days ago.
The reefs here are almost entirely dominated by macroalgae; corals are rare and scattered and appear surrounded on all sides by their encroaching algal competitors. My goal on this research cruise is to capture large (100s m2) continuous images of the reef floor (which we call photomosaics) to document the spatial patterning of benthic communities at scales rarely ever captured on coral reefs. The process involves swimming a customized camera system over the reef and taking thousands of individual images that are later stitched together using image recognition technology. Unlike at Vostok, most of my images here appear bright green as I traverse a landscape of Halimeda, a calcifying macroalgae. There are reports of other such macroalgal-dominated reefs at remote islands in the Pacific, such as in parts of the Northwestern Hawaiian Islands; our goal here is to determine why Starbuck Island appears predominantly an algal reef rather than a coral reef.
This morning we set out down into the depths and I shot a continuous photomosaic from 30m right up the reef slope into the shallows; there appeared little zonation to speak of and Halimeda remained dominant across the entire depth range. One may have thought that the lack of high coral cover would lead to an unhealthy ecosystem. However, the fish at Starbuck are plentiful and the biomass is dominated by predatory species, such as snapper and sharks. To me Starbuck Island provides a nice example of how coral reefs can reorganize and appear vastly different, yet still remain functionally intact. Such reorganizations are likely to be important as climate change and human impacts continue to affect coral reefs worldwide. It will be fascinating to see whether the next island on our journey (Malden) maintains algal dominance, or whether the corals will once again be the stars of the camera.
Photos: Top: Dr. Gareth Williams (Scripps) armed with a customized dual SLR and GoPro camera set-up that is used to capture benthic photomosaics on coral reefs. Middle: Example image captured by the cameras. A lone hard coral (Porites) wards off encroaching macroalgae (Halimeda) in a competition for space on the reef floor at Starbuck Island. Bottom: Diver swimming along a drop-off at a Halimeda-dominated reef at Starbuck Island.