Stephen Carpenter of the University of Wisconsin, US and Elena Bennett from McGill University, Canada, have found that current conditions exceed all planetary boundaries for phosphorous in freshwater.

In 2009 Rockström et al introduced the concept of planetary boundaries to define a safe operating space for humanity on Earth. They categorized a planetary boundary as a human-determined acceptable level of a key global variable.

"But the phosphorous boundary introduced by Rockström et al was based on oceanic conditions not freshwater," Carpenter told environmentalresearchweb. "The research found that the world was substantially below the oceanic planetary boundary, and this motivated us to look at freshwater. We were not surprised when we found that we are substantially past the boundary for freshwater."

Carpenter and Bennett calculated the boundaries by using target phosphorous concentrations for adequate freshwater quality and determining the global rate of transport of phosphorous from land to freshwaters. This was worked out using various factors, including the annual input of phosphorous in fertilizer and manure to erodible soils, and the mass of phosphorous in terrestrial soils.

"We found that, globally, we are already substantially past the boundaries for phosphorous in freshwater," said Carpenter. "This is despite the fact that in some areas of the world we have a phosphorous shortage."

Carpenter believes that there are some simple solutions to prevent eutrophication. "Don't apply phosphorous to soils that don't need it and develop technology for recycling phosphorous," he said. "I do think it is possible to undo what has been done – there are places in the world where manure digesters are being used to extract phosphorous, for example. But phosphorous is currently too cheap and this means that there is not enough of an incentive to recycle it on a larger scale."

However, this situation is set to change in coming decades, with phosphorous reserves running low and minable phosphorous limited to just a few locations such as Morocco, the US and China. "The growing monopoly of these countries will have a severe impact on the market," warned Carpenter.

Carpenter and Bennett believe that the solution is widespread adoption of better practices for conserving phosphorous in agricultural ecosystems, so that phosphorous is cycled effectively among soil, crops, livestock and people without contributing to the eutrophication of surface waters.

The researchers published their work in Environmental Research Letters (ERL).