Warming 'destabilises aquatic ecosystems'
Future warming could have "profound implications" for the stability of freshwater ecosystems, a study warns.
Researchers said warmer water affected the distribution and size of plankton - tiny organisms that form the basis of food chains in aquatic systems.
The team warmed plankton-containing vessels by 4C (7F) - the temperature by which some of the world's rivers and lakes could warm over the next century.
The findings appear in the journal Global Change Biology.
"Our study provides almost the first direct experimental evidence that - in the short-term - if a [freshwater] ecosystem warms up, it has profound implications for the size structure of plankton communities," said lead author Gabriel Yvon-Durocher from Queen Mary, University of London.
"Essentially, what we observed within the phytoplankton (microscopic plants) community was that it switched from a system that was dominated by larger autotrophs (plants that photosynthesise) to a system that was dominated by smaller autotrophs with a lower standing biomass."
Dr Yvon-Durocher added that a greater abundance, but lower overall biomass, of smaller phytoplankton had "very important implications for the stability of plankton food webs".
"This meant that the distribution of biomass between plants and animals changed from a... situation where you had a large amount of plants and a smaller amount of animal consumers to an 'inverted pyramid' where you have a smaller quantity of plant biomass and a larger amount of animal biomass," he told BBC News.
"Systems that tend to have larger consumer biomass relative to the resource biomass tend to be less stable over time."
Dr Yvon-Durocher explained that phytoplankton played a key role in the food webs of oceans, rivers and lakes.
"An inordinate amount of the primary productivity and carbon draw-down in ocean and freshwater ecosystems are carried out by microscopic planktonic organisms."
Because the tiny plants are able to produce their own food by using energy from sunlight, they are an important food source for zooplankton - microscopic animals that are not able to synthesise their own food.
The zooplankton are also a vital food for other creatures higher up the the food chain.
"Understanding the dynamics of these communities is going to be crucial in understanding how marine and freshwater ecosystems will respond to changes in temperature."
For their experiment, the team of UK and Spanish researchers used 20 mesocosms, which are containers that allow scientists to study freshwater ecosystems in a controlled environment.
"We were able to, in a relatively small plot of land, have 20 replicated ecosystems - half of which we warmed, and the other half we kept at an ambient year-round temperature," explained Dr Yvon-Durocher.
"The great advantage of using the mescosm set-up is that it allows the manipulation of an entire ecosystem.
"There is an absolute wealth of literature on the effects of warming and climate change on single species, but we understand very little about what happens at a community level."
Commenting on their results, the team said: "These findings could provide some novel insights into how future warming might change the distribution of organism size and biomass in freshwater ecosystems.
"The size structure of plankton communities is a key driver of rates of carbon sequestration and nutrient cycling."
They added that warming waters could have an impact on a global scale.
However, Dr Yvon-Durocher said that it did not mean that the future for aquatic ecosystems was looking bleak.
"What it means is that the make-up of ecological communities are likely to profoundly change as a result of warming," he suggested.
"It may mean that the species' composition might change, but what we don't understand is how those changes are going to affect the functions of the ecosystems.
"That is the next step."