Written by Chester Sands and David Barnes, benthic team, British Antarctic Survey
Seamounts in tropical oceans provide oases of life in the shallows surrounded by sparser deeper waters. The physical feature of the underwater mountain affects currents such that cool, nutrient rich waters are forced up and mix with the warm surface waters providing an environment for the basic building blocks of ecosystems – the microalgae – to flourish.
These tiny single celled plants, also called phytoplankton, draw carbon dioxide from the atmosphere, and together with the nutrients from the deep sea turn the carbon dioxide into starch, a kind of sugar which is biological energy, and oxygen which is released back into the water and then the atmosphere, in the same way as leaves on the trees in forests.
Microalgae are the staple diet for a host of tiny animals termed zooplankton that live in the deeper waters where they are hidden from predators in the dark, but move into the shallower waters to feed at night. Zooplankton are a foodweb of tiny animals eaten by ever larger animals and so the energy that started as carbon dioxide and sunlight is passed through the ecosystem, eventually being accumulated by the giants of the sea such as sharks, turtles and whales, and to the less obvious predators attached to the summit and sides of the seamount; the corals, sea stars and crabs just to name a few.
And so seamounts are vibrant and diverse ecosystems all because of the possibility of draw down of carbon dioxide from the atmosphere and its conversion to energy that is passed along the food chain. We can use diverse scientific apparatus, such as sound (echosounders – see image below), nets, cameras and water samplers to investigate this plankton and its predator foodweb. But we have not reached the end of the carbon story. Many of the animals that live on the sea floor not only use the carbon as energy, but also convert it into shells for protection.
A coral reef clinging to a seamount is largely a store of carbon that has been drawn out of the atmosphere in the form of carbon dioxide, converted to starch bio-energy, then a calcium carbonate casing for the coral to live in. In fact, most animals you find on the sea floor, and most that are swimming in the water column, have outer shells that made to some degree from carbon. So not only is a seamount an oasis of life, but a valuable “ecosystem service” for drawing out carbon dioxide, a potent green house gas, from the atmosphere and storing it in the hard shells of animals. Just how helpful this form of carbon storage is in the long term is one of our areas of interest http://onlinelibrary.wiley.com/doi/10.1111/gcb.13157/full. Much of the carbon in shells is eventually released back into the water, but if it is buried, it may be an example of a very long-term natural carbon storage system – so called ‘blue carbon’.
We took the opportunity to sample the very bottom of the seamount – making it a very rare and precious sample – where the steep slopes suddenly flatten out into the abyssal planes, with the thought that this may be where a lot of dead carbonate animals, broken bits of dead coral for example, end up and could be potentially buried in the usually soft muddy sediments that are typical of these vast regions.
Deep trawls like this are unusual as they take a long time (nearly 4 kilometers of cable is slowly let out by the winch) and usually come up with very few samples. Three hours after deployment our trawl reached the surface and was hauled on deck. We were expecting about tens of kilos of mud which we would have to carefully sieve to find what was living down there, but rather than mud, we found many hundreds of dead shells, mostly of a kind of sea snail called a pteropod, or sea angel.
The sea floor is covered by shells containing carbon, right in the area where it may be buried in the mud. Although we predicted this may be the case, we were not at all expecting such a huge catch of these sea shells. It would be fascinating to come back one day and sample columns of mud here to see just how much of the carbon in the shells remains captured and locked away in the sea floor – so called sequestration.
The Pristine Seas team is currently conducting an expedition to the remote island of Ascension, in partnership with the Ascension Island Conservation Department, the British Antarctic Survey, the Royal Society for the Protection of Birds, and The Blue Marine Foundation.