A World of Coral

A World of Coral

By Stuart Sandin

We have arrived at Flint Island in the Republic of Kiribati. Flint is a small and elongated island, approximately 4km north-to-south and about 500m across at its widest. The island is uninhabited, though in the past groups of people lived here to mine guano (seabird droppings used as fertilizer) and to grow copra (the meat of coconuts used principally to make coconut oil). The profile of the island reflects its history, with the native vegetation filling in the gaps between the coconut palms that were introduced in the late 1800s. Despite the temptation to explore the land, we are here to focus on the coral reefs that fringe the island.

From our past work here done with friends from National Geographic, we know that the reefs are among the most coral-laden that any of us have visited. We estimate the configuration of the growing reef in units of percent cover – what proportion of the bottom is composed of stony corals, of encrusting algae, of fleshy turf algae, etc. The first of these, the stony corals, are the most notable of the framework builders of the reef environment, and thus we use percent cover of coral as a metric of reef health. A recent study estimated that across Pacific coral reefs, the average coral cover is just over 20%, which you can imagine as coral composing the area of a typical welcome-mat within every square meter (which is about a square yard) of reef surface. Well, here at Flint the average coral cover is 80%. So flip your welcome-mat image around and consider this 20% area on Flint is the only non-coral surface. There is an astounding amount of coral on this island!

While it is certainly impressive to swim around these ‘coral infested’ reefs, our goal here is to understand why there is so much coral and how this influences the working of the island’s reef ecosystem. We believe that corals are winning the battle for space because the water around Flint has very few nutrients. As such, fleshy algae have little food to help them to grow. Corals, on the other hand, are creative animals that catch plankton as food (and thus are a source of nutrients) and also house single-celled algae inside of their bodies to help them to gather energy through photosynthesis. Without the ability to harvest nutrients as effectively as corals, fleshy algae cannot gain a significant competitive foothold.

All of these ideas are swirling around the team as we set up our experiments and complete our collections around Flint.

  • The benthic team is deploying tents to track this photosynthesis and other processes on the reef.
  • The invertebrate team is searching through the rocks and rubble to see who is crawling within the reef matrix.
  • The microbe team is tracking the flow of energy through the bacterial and viral communities in the water column.
  • The fish team is trying to understand what the fish eat when there is little to no algae fueling the food chain.

If only we knew as much about the history of Flint’s reefs as we know about its land. Well, that is why we are here, to see what we can work out…