Where on Earth...? MISR Mystery Image Quiz #16: Maldives and Tuamotu

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These natural-color images of the Maldives and Tuamotu, two coral atoll ecosystems located in different parts of the globe, were acquired by the Multi-angle Imaging SpectroRadiometer (MISR) during October and November 2003. The two images represent about 310 kilometers x 141 kilometers (left panel) and 310 x 184 kilometers (right panel). Answers to the questions are provided.

1. Three of these four statements about the geomorphology of the coral reefs are true. Which one is false?
   1. Their modern shape has been influenced by sea level changes over geologic timescales.
   2. The reefs have grown over millions of years atop ancient volcanic mountain ranges.
   3. Unless there is too much sediment present in the water, some of the reefs can grow at depths of up to 170 meters.
   4. The reefs tend to grow in density and thickness where the water flow is greatest due to prevailing ocean currents.

      • Answer: C is false
        Today's coral reefs were formed over the last 500 million years and are influenced by climactic and geologic processes. Coral reefs can typically only accumulate at a rate of several millimeters per year. If the Earth's crust is subsiding or sea level rising slowly enough, reefs are able to construct their protective calcium carbonate encasements at a rate fast enough to keep abreast of the diminishing coastline. Volcanoes that form near mid ocean spreading centers gradually subside as the crust on which they stand cools and moves away, and the coral reefs that fringe their edges grow upward as the volcanic rock sinks, leaving behind coral atolls such as those pictured here. Reef-building corals benefit from a symbiotic relationship with photosynthetic algae, and since the algae rely on sunlight, reefs do not grow at depths greater than 70 meters below the ocean's surface. Coral reefs are also restricted to the relatively nutrient-poor tropical oceans, where their growth is affected by prevailing ocean currents. The currents agitate the water and provide the coral and their algal partners (zooxanthellae) with more calcium, nutrients, and food particles. This stimulates faster coral growth on one side of the outer reef, so that the reefs become wider and denser over time near the locations where the water flow is the greatest.

2. Three of the following four statements accurately describes coral atoll ecosystems. Which one does not?
   1. Other living organisms besides coral help to build the atolls through calcium carbonate production.
   2. Symbiotic zooxanthellae provide reef corals with the calcium and nutrients they need to grow.
   3. Atolls can be very open, entirely closed, or have a number of entry points by which a lagoon area is open to the ocean.
   4. The diversity and abundance of corals is greater along the protected inner atoll and lagoon areas than along the outer reef slopes.

      • Answer: D is false
        Various types of corraline algae help to produce calcium carbonate, including zooxanthellae which live in the tissues of modern reef-building corals. Zooxanthellae are intimately related to the rate of reef building in that they help these coral to produce calcium carbonate encasements faster during the day when photosynthesis occurs. Hard corals obtain calcium from the seawater and carbon dioxide from cell respiration, and bring these products into the internal tissues of the coral polyp. The zooxanthellae provide nutrients for the coral community via photosynthesis and assist in calcium carbonate formation by exchanging the products of photosynthesis and cell respiration. Atoll shapes are extremely varied, and range from the classic closed ring and inner lagoon, to a completely open reef rim. The diversity and abundance of corals is greater along the outer atoll edges, where the flow of relatively nutrient-rich water is greater.

3. Three of these four statements about the atolls of the left-hand image are true. Which one is false?
   1. The many small saucer-shaped reef structures within the lagoonal areas make this reef system different from all others.
   2. The word "atoll" is a term that comes from a language native to this region, and has been expanded for use worldwide.
   3. The two largest cities in these atolls are home to about 8000 people each.
   4. At one atoll, construction of an artificial reef began in November 2001.

      • Answer: C is false
        The many small saucer-shaped closed oceanic atolls that occur within the larger lagoonal areas are called "faros" and are part of the nation of the Maldives. These atolls grew upon a submerged mountain range named the Laccadives Chagos Ridge, which runs north-south along 2000 kilometers of the Indian Ocean. While "faro" is simply the Maldives name for a reef with a central enclosed lagoon or depression, the processes that created faros in the Maldives are different to the processes that created atolls worldwide. The native language of the region is Dhivehi and the word "atoll" comes from their word, atholhu. The capital city, Male, only covers about 2 square kilometers, but is home to about 70,000 people. The maximum elevation of the atolls is only 5 meters above sea level, and the very existence of these atolls and their inhabitants depends on healthy coral reefs. In the last few decades the reefs have been damaged by higher than average sea-surface temperatures and unsustainable resource management practices. One method used to restore damaged reef structures is to construct artificial reefs and artificially stimulate the coral growth (e.g., the Vabbinfaru Lotus Structure on North Male Atoll).

4. Three of these four statements about the atolls of the right-hand image are true. Which one is false?
   1. In this region the prevailing ocean current, or swell, comes from the south.
   2. The chemical character of the inner lagoons is not significantly influenced by the size of the openings in the reef rims.
   3. Within the image area there is a designated biosphere reserve and breeding grounds for at least 3 species of sea turtle.
   4. The most extensive freshwater is found at the small, round, green atoll that appears along the left-hand image edge.

      • Answer: either A or B accepted as false
        The right-hand image includes (in whole or part) 11 of the 77 atolls that make up the Archipelago of Tuamotu in French Polynesia. The Tuamotus are located within the ocean current known as the South Pacific Gyre, and the surface hydrological characteristics here are particularly stable, warm, clear and salty. Although the dominant wind and ocean currents are from the east, the dominant wave direction (swell) is from the south. Since wave direction (or swell) and ocean current (or circulation) are different attributes, statement (A) is accepted as false. Statement (B) is also false since the openings of the atoll rims enable, to a greater or lesser degree, water exchange between the lagoon and the open sea, and thereby influence the chemical character of the lagoon waters. Taiaro Atoll (the small circular atoll at the right-hand image edge) was designated as a biosphere reserve in 1977. There are three turtle species (green, hawksbill and leatherback) at Taiaro. The most extensive fresh-to-brackish-water marshes in the Tuamotus are thought to be at Niau, the small green atoll at the left-hand image edge.

MISR was built and is managed by NASA's Jet Propulsion Laboratory, Pasadena, CA, for NASA's Office of Earth Science, Washington, DC. The Terra satellite is managed by NASA's Goddard Space Flight Center, Greenbelt, MD. JPL is a division of the California Institute of Technology.

Image credit: NASA/GSFC/LaRC/JPL, MISR Team.
Text acknowledgment: Clare Averill, MISR Team, Geographic Interpretation and Science Outreach, c/o Raytheon, Australia.