Nitrogen Polluting Lakes in Northern Hemisphere

Scientists claim to have found evidence that nitrogen from human activity has been polluting lakes in the northern hemisphere since the late 19th century.

In fact, the clear signs of industrialisation can be found even in very remote lakes, thousands of kilometres from the nearest city, according to their findings published in the 'Science' journal.

The research is based on studies of sediment from 36 lakes in the USA, Canada, Greenland and Svalbard, Norway.

The scientists have analysed how the chemical composition of the sediment has changed over the centuries. Twentyfive of the lakes all show the same sign -- that biologically active nitrogen from human sources can be traced back to the end of the 19th century.

The nitrogen analyses of the lake sediments show that the changes began around 1895. The results also show that the rate of change has accelerated over the past 60 years, which is in agreement with the commercialisation of artificial fertilizer production in the 1950s.

Sofia Holmgren, one of the scientists from Lund University in Sweden, said: "I have studied lakes on Svalbard, where the effects of the increased nitrogen deposition are clearly visible in the algal flora."

She explains that both the species composition and production of diatoms -- microscopic siliceous algae – have changed dramatically in the lakes on Svalbard since the start of the 20th century, with the most significant changes over the past decades.

Combustion of fossil fuels and use of fertiliser are the main sources of the increasing amount of nitrogen in the atmosphere.