15. Jun, 2019

How coral island hotspots developed in an oceanic desert

In his book "The voyage of the Beagle", Charles Darwin set out the framework of a theory for the formation of lagoon-islands, atolls, and reefs. It started with a coral reef formation around an extinct volcanic island, growing from a fringing reef to a barrier reef to an atoll,  as the island and ocean floor subsided. Since Darwin's discovery, many have wondered how rich and diverse ecosystems teeming with fishes like coral reefs could grow in the marine equivalent of a desert.  Some even called it the ‘Darwins paradox’. But is it really a paradox? Evolution basically describes how complex and rich forms of life developed from very elementary organisms and mechanisms.  And even an empty ocean may not be the barren desert some take it to be.

The basic mechanism of coral growth is the symbiosis between the coral polyps and symbiotic algae called zooxanthella stocked in the tentacle of the coral polyps. The algae provide the carbon necessary to build up the skeleton of the coral. The coral polyps also extract nutrients like nitrogen and plankton from the seawater to provide the algae carbon dioxide and phosphates. Some marine biologists believe that the phytoplankton may be the primary source of the development of coral ecosystems. The enhancement in phytoplankton near an island-reef ecosystem is also called the  Island Mass effect (IME, see insert).  

This paradoxical enhancement in phytoplankton near an island-reef ecosystem was discovered about 60 years ago by Doti and Ogury, and recently confirmed by Jamison Gove. He and his team showed that IME is an important feature among a majority of coral reef ecosystems surveyed, creating near-island ‘hotspots’ of phytoplankton biomass throughout the upper water column. The team supervised by Jamison found phytoplankton coral island hotspots surrounded by barren oceans landscapes were nearly everywhere the team looked. Apparently, corals have an astonishing capacity for storing scarce nutrients for their survival. They pick up phytoplankton from the surrounding currents or rising from deeper colder layers around the island to feed the algae that that provide nutrients for the growth of corals using photosynthesis.

A recent study by  Simon Brandl published in Science suggests that the ocean’s smallest vertebrates, called cryptobenthic reef fishes, promote internal reef-fish biomass production through exceptional larval supply from the pelagic environment. In addition to the phytoplankton, these tiny fishes and larvae form an important nutrient for larger fishes such as smaller coral fishes and even pelagic predators of the coral reef.  So they could also provide (indirectly) the energy and fuel for species on the reef higher in the food chain. Finally, sponges and their excrements also seem essential for the energy cycle,  providing ‘manure’ for the coral reefs as well as food for smaller species like crabs, worms, and snails.