Consider it the ecological equivalent of the meek inheriting the Earth: in response to global warming, aquatic creatures are becoming smaller, according to a new scientific study.

Researchers in France and Germany looked at a number of species of fish, bacteria and phytoplankton in Europe and found that as temperatures rose, the mean body size of the organisms dropped significantly.

Previous research had shown that within a species, creatures with smaller body size tended to thrive in warmer environments. But Martin Daufresne of the Cemagref Public Agricultural and Environmental Research Institute in Lyon, France, and his colleagues are the first to consider the importance of this relationship to the larger trend of global warming.

"Understanding the ecological impacts of climate change is a crucial challenge of the 21st century," they wrote in the Tuesday issue of the scientific journal Proceedings of the National Academy of Sciences.

"Our study provides evidence that reduced body size is the third universal ecological response to global warming in aquatic systems besides the shift of species ranges toward higher altitudes and latitudes and the seasonal shifts in life cycle events," they wrote.

Fishing not to blame, researchers say

The researchers looked at a long-term survey of freshwater fish in a number of rivers in France, the North Sea and the Baltic Sea, with time frames of study ranging from 14 to 31 years. They also simulated environmental conditions indoors when testing the responses of phytoplankton and bacteria communities.

The shift in body size for fish species was due to a number of factors: a shift in favour of small-sized species, an increase in the juvenile population within a species and a significant decrease in the size of adults, particularly for herring and sprat populations in the Baltic Sea.

Since many of the fish they looked at were not targets of fisheries, the researchers said fishery activity could not fully explain the decline in fish size.

The findings add another factor when attempting to understand the implications of climate change, they wrote, but further research is needed to understand at what temperatures the changes in evolutionary responses are triggered.