Coral sampling in St. John, USVI
Ross Cunning

I spent the last two weeks of July conducting fieldwork with Dr. Peter Edmunds and his team in St. John, USVI. We operated out of the Virgin Islands Environmental Resource Station (VIERS) in Great Lameshur Bay, which is in the Virgin Islands National Park on the south coast of St. John. This was an incredible place to work, which is why Dr. Edmunds has been working here for the past 30 years, collecting long term ecological data on the beautiful and abundant coral reefs. Since most of my experience in the Caribbean has been in Florida where the coral cover is much lower, I was amazed by the relatively high coral cover on the reefs of St. John. Of particular interest was the high abundance of the elkhorn coral (Acropora palmata) and the pillar coral (Dendrogyra cylindrus), which are listed as threatened under the Endangered Species Act, and which are nearly absent from the reefs on Florida where I have worked before. Another threatened species, the boulder star coral (Orbicella annularis), was also common and dominant across large areas of fringing reef. Some of the most conspicuous residents of these reefs, however, were soft corals, or Gorgonians, commonly known as sea fans, sea rods, and sea whips, which were so dense in some areas that they formed forest-like canopies a meter or more above the reef.

What I traveled here all the way from Hawaii for, however, were the small single-celled organisms that live inside the corals and feed them through photosynthesis -- they are called Symbiodinium. Though you can't see them individually with the naked eye, they are packed so densely inside the corals that they give them their characteristic brown coloration. In fact, when these symbiotic algae leave the coral, which happens when corals are stressed by warm water, the corals turn white, in a phenomenon known as coral bleaching. Bleaching is occurring more frequently in association with climate change, and in fact is predicted to occur in many places around the world in the coming months. There are many different types of Symbiodinium, many of which form symbioses with corals, and some of which provide better nutrition to the corals, or better thermal tolerance. Symbiodinium also live in the water column and the sediments, as well as in sponges, soft corals, and other reef animals. The assemblage of Symbiodinium in a coral may depend on the assemblage found in nearby hosts, or in the surrounding seawater in sediment. In this way, many pockets of Symbiodinium existing in different reef compartments may influence each other as interacting communities—or a metacommunity. This idea is what I went to St. John to explore. Are communities in corals more similar to the communities in other nearby corals? Do the types of Symbiodinium present in the water or the sediment influence the types present in the coral, or vice versa?

Answering these questions will help us understand how Symbiodinium communities in corals are structured, and how they may change over time. The dynamics of these communities plays a large role in determining how these corals will be able to survive and grow in a rapidly changing ocean.

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