An iceberg more than four times the size of Greater London is damaging marine wildlife off the coast of Antarctica by blocking sunlight to a huge expanse of ocean, Nasa scientists said.
The iceberg, named C-19, is almost 20 miles wide and 124 miles long. It has created a build-up of sea ice that has killed the tiny marine phytoplankton - a key element in the Antarctic food chain.
Nasa has monitored a vast build-up of sea ice off the Antarctic coast since the iceberg was first detected. It began to break away from the Ross Ice Shelf last year. Kevin Arrigo of Stanford University said the "calving" of icebergs was largely responsible for reducing the plants available to the food chain.
Phytoplankton turns sunlight into food for smaller marine life, notably the shrimp-like krill which is an important source of food for fish, which are in turn eaten by seals and penguins. Sea ice stops sunlight getting to the shallower depths of the sea where phytoplankton grow. The result is that the C-19 iceberg has created the equivalent of a desert.
"Calving events over the last two decades indicate reduced primary productivity may be a typical consequence of large icebergs that drift through the ... Ross Sea during spring and summer," Dr Arrigo said.
C-19 is the second largest iceberg recorded in Antarctica and, with the bigger berg B-15, it has prevented sea ice from being blown clear of the coast. Nasa used a satellite instrument to measure the amount of chlorophyl - the green pigment used by plants to carry out photosynthesis - present in the sea.
Dr Arrigo said another large crack had appeared in the Ross Ice Shelf, meaning that a further iceberg was being formed. It is hard to say when it will break free.
Last month, the largest ice shelf in the Arctic - a feature that has existed for at least 3,000 years - split in two. Scientists said the disintegration of the Ward Hunt shelf ,off Ellesmere Island in Canada, was almost certainly due to global warming.