Dr. Eleanor Sterling — World War II Still Shapes Atoll’s Ecosystem

Palmyra Atoll has a north-south causeway built by the United States military during World War II. Photo credit: Felicity Arengo

Crossposted from adjunct faculty member Dr. Eleanor Sterling’s NYTimes “Scientist at Work” blog

I often wonder what life was like for military personnel who served on Palmyra Atoll during World War II. We see ghosts of their activities — whether abandoned buildings and ancient, odd-shaped light bulbs or structural changes to the islets and the lagoon. The military occupation there was brief, but the effects, like causeways built across the waterways, are long lasting. Some of the modifications cut off the flow of water and turned the lagoons into warm, silty bathwater, which influences what can grow and live there.

The United States Fish and Wildlife Service and Palmyra Atoll Research Consortium institutions hope to restore the lagoon system. Researchers have mapped changes in the atoll through time and are measuring water flow and the amount of silt suspended in the water to determine how these factors affect biodiversity. At the American Museum of Natural History we are documenting how sea turtles use the lagoon areas for foraging or resting and how changes in water flow would influence the resources produced there that the turtles rely on.

The sediment in the water is so thick in places that you can hardly see. We know this firsthand from diving to retrieve the data from our acoustic receivers that track turtle movements. At the lower depths the water is so murky we can’t even read our dive computers much less see each other or the receivers. Consequently, we try to descend at exactly the right spot using GPS coordinates and string a rope between divers to communicate via prearranged tugging signals. It is worth the effort, though. The murkiest of our sites (called, fittingly, Turtle Hole) yielded a cache of information on turtle activities over the past year. Our receivers also recorded shark and bonefish movements. The bonefish team comes in September and ideally will have turtle data on their receivers.

We anticipate that adult sea turtles on Palmyra are primarily vegetarian and rely on algae, though some of the smaller animals may still maintain their youthful carnivorous diet. Our team is undertaking chemical analysis of slivers of the turtle shells to determine the diet of different-sized animals. We are also documenting what algal food the turtles choose in comparison with what is available. This involves identifying and quantifying the algae in plots that extend across different habitats, and working to understand how changes in temperature or sedimentation rates might affect algal growth.

Other researchers at Palmyra are working to understand the ecological mechanisms that contribute to reef resilience — the ability of reefs to recover from major disturbances. The reefs are an important factor in turtle habitat as the turtles not only feed there but rest there also, nestled between coral heads or under a ledge. To track one aspect of resilience, coral growth, these scientists have placed terra-cotta tiles around the atoll and measure how many coral larvae settle, how well baby corals survive, and how fast they grow in different locations and habitats. They count the baby corals and map them so that their fates can be followed through time. The researchers are characterizing the ecological communities of algae and invertebrates on the tiles, as well as fish and bottom organisms at the field sites, so that they can relate the coral settlement and survival rates to community dynamics at both tile and site scales.

So far, ongoing analyses amplify what we know from other reefs — that grazing fishes, especially parrotfishes that scrape the surfaces of the reef and keep it mostly clear of fleshy and turfy seaweed, are important contributors in providing habitats for baby corals. In addition, we’re also learning how baby corals depend on even finer-scale ecological dynamics — tiny patches of dead algae and even individual bite marks from parrotfishes — that create toeholds among their algal and invertebrate competitors on reef surfaces. Eventually, after they complete their analyses, the researchers will compare their results with those from more degraded reefs elsewhere to determine how coral dynamics may differ in key details on healthy reefs like Palmyra.

Consortium researchers are also studying climate data in corals, documenting change over several centuries, as well as assessing the effect of ocean acidification on corals. Palmyra is an ideal place to study the potential effects of climate change on marine systems, as it undergoes natural variations in pH, partly due to its location at the junction of northern and southern trade winds and at the boundary between the eastern cool tongue and western “warm pool” of the Pacific. By understanding the range of conditions under which different species thrive, we can model future scenarios to use when considering the best conservation plans, as we have been doing for sea turtles.

Green sea turtle. Photo credit: Kevin Frey

In general, Palmyra Atoll is ideal for research on the effect of humans on marine systems, both through better understanding of the outer reef system, which can serve as an important reference point for restoration activities globally, as well as lessons learned while restoring function in the heavily modified lagoon and terrestrial environments.

While we are winding down our research, we still have plenty to do as we pack up and make way for the next group of scientists, whose flight in we will take back to Honolulu. During the day we take time out to help the Fish and Wildlife Service staff conduct a survey of rare bristle-thighed curlews, birds that migrate between the Pacific and Alaska. Our evening activities have switched from late hours in the lab to activities like a Ping-Pong tournament and a “triathlon” relay (biking on the runway, swimming from the runway to the kayak shack, kayaking around no-wake buoys and back to the dock). We look forward to analyzing the data in the coming year and thus remaining part of an ecosystem of scientists and managers who work to understand and conserve the ecosystem of Palmyra.

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