By Julianna Kettlewell
BBC News science reporter
Communities of creatures living in Arctic lakes are undergoing dramatic changes in response to climate warming, according to Canadian experts.
The team looked at 55 lakes, covering five circumpolar countries extending halfway around the world
Groups of aquatic organisms in the Arctic show patterns of change over the last 150 years that are consistent with human-induced effects, they claim.
Shifts in the Arctic are likely to be indicative of wider reaching changes around the world, it is claimed.
Details appear in Proceedings of the National Academy of Sciences journal.
In their research paper, the team described the Arctic as the "canary of environmental change".
"Polar regions are expected to show the first signs of climatic warming, and are therefore considered sentinels of environmental change," said co-author Alexander Wolfe, from the University of Alberta, Canada.
"Unfortunately, long-term monitoring data are generally lacking in these areas which makes it difficult to determine the magnitude of past environmental changes."
Back in time
The Canadian-led team discovered what they call the "maybe irreversible effects of Arctic warming" in 55 lakes, covering five circumpolar countries extending halfway around the globe.
By analysing the sediments at the bottom of the lakes, they were able to "look back in time" to see what aquatic communities of algae, water fleas and insect larvae were like hundreds - sometimes thousands - of years ago.
The organisms the team were analysing are the bedrock of the aquatic food chain
"Lakes slowly accumulate sediments over time, so they are like a history book," said lead researcher John Smol, from Queen's University, Canada.
"We can reconstruct environments from the past when no one was actually measuring anything."
What they found was rather striking: the communities remained almost stable until the mid-1800s, when changes began to take hold. The most dramatic shift occurs in the last 30 years, the team claims.
"The timing of the changes is certainly consistent with human interference, and one of the major avenues is through climate warming," said biologist Dr Kathleen Ruhland, of Queen's University.
"This is another example of how humans are directly and indirectly affecting global ecology."
The actual nature of the changes varies, depending on the size of the lake and where it is located.
For example, some lakes have a richer biodiversity because they spend more time each year ice-free.
"In the higher Arctic, some of these lakes were frozen solid for 11 months of the year, but now they are only frozen for 10 months - so that is a 100% change," explained Professor Smol. "It is not surprising this has a dramatic effect on aquatic organisms."
Other deep lakes, which usually only thaw in shallow parts in the summer, are now thawing deeper. This means that organisms normally associated with deeper water are appearing for the first time.
Although the team is confident nobody can deny the changes are happening, they are aware some people might be unconvinced global warming is the cause.
For instance, pollution is also increasing, as is UV radiation - perhaps these factors could be responsible.
However, Professor Smol believes he can rule out these possibilities.
Not all Arctic areas are warming up. In fact, some places, like northern Quebec in Canada have remained remarkably stable, according to Professor Smol.
"In lakes in these areas, we don't see much change," Professor Smol told the BBC News website. "And this is a major card.
The researchers are aware some people might be unconvinced global warming is the cause
"Here we have a bunch of lakes that don't show the 'hockey stick' type changes we see in other lakes. If the changes were due to pollution, for example, you would also see them in these places. This makes a compelling case."
The organisms the team were analysing are the bedrock of the aquatic food chain in Arctic lakes. Therefore, Professor Smol believes, it is likely that there will be knock on effects higher up the food chain.
"All these organisms are food for other organisms higher up the food chain," he explained. "But unfortunately it is more difficult to study that because fish don't make good fossils."