guardian.co.uk, Wednesday 28 July 2010 18.02 BST
Phytoplankton might be too small to see with the naked eye, but they are the foundations of the ocean food chain, ultimately capturing the energy that sustains the seas' great beasts such as whales.
A new study though has raised the alarm about fundamental changes to life underwater. It warns that populations of these microscopic organisms have plummeted in the last century, and the rate of loss has increased in recent years.
The reduction – averaging about 1% per year – is related to increasing sea surface temperatures, says the paper, published tomorrow in the journal Nature.
The decline of these tiny plankton will have impacted nearly all sea creatures and will also have affected fish stocks.
Phytoplankton provide food – by capturing energy from the sun – and recycle nutrients, and because they account for approximately half of all organic matter on earth they are hugely important as a means of absorbing carbon.
"This decline will need to be considered in future studies of marine ecosystems, geochemical cycling, ocean circulation and fisheries," add the paper's authors, from Dalhousie university in Nova Scotia, Canada.
The researchers looked at measurements of ocean transparency and tested for concentrations of chlorophyll, which gives large numbers of phytoplankton a distinctive green sheen. They said that although there were variations in some areas due to regional climate and coastal run-off, the long-term global decline was "unequivocal".
The Nature article comes as climate scientists published what they said today was the "best ever" collection of evidence for global warming, including temperature over land, at sea and in the higher atmosphere, along with records of humidity, sea-level rise, and melting ice.