Some regions of the deep ocean floor support abundant populations of organisms, despite being overlain by water that contains very little oxygen, according to an international study led by scientists at the United Kingdom's National Oceanography Centre, Southampton. But global warming is likely to exacerbate oxygen depletion and thereby reduce biodiversity in these regions, they warn.
The sunlit surface waters tend to be well oxygenated as a result of their connection with the atmosphere. Here, tiny marine algae called phytoplankton thrive. When they die and sink, they are degraded by bacteria, using oxygen from the water column.
In regions of high plant growth, this can result in the natural development of mid-water oxygen minimum zones (OMZs), especially where oxygen is not replenished by mixing of the water column. Where they touch the continental slope, OMZs create strong seafloor oxygen gradients at depths between 100 and 1000 m.
In addition to low oxygen, sediments within OMZs often contain large amounts of organic matter. As a result, species of animals and protozoans (foraminifera) that can tolerate low oxygen may flourish, despite the stressful conditions. However, if oxygen depletion is very severe, many types of animals disappear. Where animals are present, OMZs provide a variety of habitats created by steep gradients in oxygen and sulphide concentrations, different seafloor types, and variations in acidity and nutrient availability.
In this new synthesis, based on a survey of published and unpublished data, the team of researchers analysed the habitats and biodiversity of well-developed OMZs in the Arabian Sea, eastern Pacific and Bay of Bengal.
"Oxygen seems to be the overriding factor controlling biological diversity and seabed community composition within OMZ core regions," said Professor Andrew Gooday of NOCS: "Where oxygen levels increase, the strong seafloor gradients create variety that
|Contact: Dr. Rory Howlett|
National Oceanography Centre, Southampton (UK)