Plankton - Ascension Island Read More

These organisms include phytoplankton, microscopic bacteria and algae that convert the sun’s energy into food via photosynthesis, and zooplankton, small animals that feed on other plankton. Some of the eggs and larvae of larger marine organisms, such as fish and crustaceans, are also classed as zooplankton. Hundreds of zooplankton, measuring a fraction of a millimetre in size, can be found in a single cubic metre of seawater. As well as being an important means by which the juvenile forms of many animals may disperse over large distances during their development into adults, zooplankton are a key food source for larger animals, supporting the entire marine food web. For example, flying fish, which eat mostly plankton, are an important part of the diet of large predatory fish and seabirds. Even some of the largest fish species in the world, such as manta rays and whale sharks, are planktivorous, feeding almost exclusively on plankton.
As part of its marine and fisheries research programme, the Ascension Island Conservation Department collects and analyses zooplankton samples from throughout Ascension’s waters, encompassing the island’s shallow coastal zone and its vast offshore territory. Examination of these samples under the microscope reveals a multitude of strange and wonderful organisms, dominated by tiny crustaceans known as copepods that spend their entire lives in the plankton, but also including gelatinous species (like salps and siphonophores) and juvenile forms of shrimps, crabs, barnacles, worms, starfish and shelled molluscs, in addition to the eggs and larvae of fish species inhabiting Ascension’s underwater environment. Notably, the larval stage of Ascension’s most charismatic native land animal, the land crab Johngarthia lagostoma, may be encountered in coastal waters following seasonal mass spawning events, during which hordes of adult land crabs migrate down to the sea to release their eggs.

There is a huge amount that we can learn about the functioning of marine ecosystems by studying plankton communities. Through this research, Ascension Island Conservation Department aims to build a better picture of the food webs that characterise the island’s unique underwater habitats, which support ecologically and economically important species. Perhaps even more importantly, monitoring the abundance and diversity of plankton over time can inform predictions of how marine ecosystems will respond to changing environmental conditions, not only within Ascension’s waters, but also throughout the wider Atlantic.

Text by Dr Robert J. Mrowicki, Conservation Department, AIG.