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When yesterday's agriculture feeds today’s water pollution [umontreal.ca]

23 watersheds studied

Focusing on 23 watersheds feeding the St. Lawrence River in Quebec, the researchers reconstructed historic land-use practices in order to calculate how much phosphorus has accumulated on the land over the past century.

The two main sources of phosphorus to watersheds, the land adjacent to tributaries, come from agriculture (fertilizers and animal manure) and from the human population (through food needs and sewage).

Using Quebec government data, the researchers matched the estimated accumulation with phosphorus concentrations measured in the water for the last 26 years. Since the watersheds they studied had different histories – some had been used intensively for agriculture for decades whereas others were forested and pristine – this method allowed the researchers to establish a gradient of different phosphorus accumulations among sites. In so doing, they were able to see at what point the watershed “tipped” or reached a threshold and began to leak considerably more phosphorus into the water.

“Think of the land as a sponge,” Maranger said. “After a while, sponges that absorb too much water will leak. In the case of phosphorus, the land absorbs it year after year after year, and after a while, its retention capacity is reduced. At that point historical phosphorus inputs contribute more to what reaches our water.”

Until now, no-one had been able to put a number to the amount of accumulated phosphorus at the watershed scale that’s needed to reach a tipping point in terms of accelerating the amount of the mineral flowing into the aquatic ecosystem.

Original Submission