Future Landscapes

Globally, Phosphorus is a Bigger Problem than Nitrogen

In the first global analysis of freshwater quality, a paper published in Nature’s Scientific Reports shows that 31% of the world’s land mass surrounds rivers with poor water quality, affecting two billion people

Global map of periphyton growth limited by P and N

Our Land and Water chief scientist Professor Rich McDowell is the lead author of a paper published in Nature’s Scientific Reports that provides the first global analysis of freshwater quality. The paper shows 31% of the world’s land mass surrounds rivers with poor water quality– as measured by algal growth.

Algal growth in rivers is a problem worldwide, affecting the water used by at least two billion people for food, work and recreation.

The paper’s authors combined several databases to find global nitrogen and phosphorus concentrations for 1406 of the world’s largest rivers (those analysed between 1990 and 2016).

The research found that globally, phosphorus is a much greater problem than nitrogen.

Phosphorus enrichment was mapped to catchments dominated by agricultural land in North and South America and Europe, containing 1.7 billion people. Unwanted algal growth caused by nitrogen (dissolved and total) was mapped to areas of North Africa and parts of India and the Middle East, affecting 280 million people.

Phosphorus enrichment was mapped to catchments dominated by agricultural land in North and South America and Europe, containing 1.7 billion people.

“Even in New Zealand, when you just look at our huge catchments, they are largely phosphorus-limited,” explains Professor McDowell. A phosphorus-limited water body will grow periphyton (algae) in response to an excess of phosphorus.

“Of course, when we look in more detail, we know that substantial areas of New Zealand contain rivers with unacceptable periphyton growth controlled by dissolved nitrogen.”

“This tells us that New Zealand’s dual-nutrient approach is the right way to improve the quality of our rivers.”

“We can’t just focus on one nutrient, because we don’t know what’s around the corner that might tip the ratio of nitrogen and phosphorus off-balance, leading to algal growth.” For example, the problematic nutrient can change as a river runs from mountain to sea, or due to an unexpected event like an effluent spill.

The global analysis was restricted to very large catchments (stream order 6 or above). However, previous finer scale work on around 700 New Zealand rivers showed a similar pattern of P-limitation.

Despite this caveat, the results can be used internationally to guide regional or national-level actions that target the right nutrient and prioritise areas that need remediating.

“Once land stewards know that nutrients are an issue, they can identify the critical source areas that account for the majority of nutrient losses to water and target appropriate strategies to mitigate loss,” explains McDowell.

“This targeted approach may speed up water quality improvements, but it’s wise to follow New Zealand’s example and mitigate the loss of both N and P.”

“As in New Zealand, if changing practices alone is unlikely to meet water quality goals, land use may have to change to something less leaky.”

How can global water quality be improved?

Our Land and Water also contributed knowledge to a second multi-country investigation of global freshwater management, published in August 2019 in the Journal of Environmental Quality.

Comparing the freshwater management strategies of New Zealand, the US and Western Europe, the research suggests New Zealand has found a comfortable middle ground with our approach.

“Europe relies on input rules, which are effective when targeted correctly but risk being circumvented and manipulated, and discourage innovation,” explains McDowell, the New Zealand co-author of the paper.

“The US relies more on voluntary actions and in some cases regulations for total maximum daily load restrictions. The voluntary approach encourages innovation, but without drivers to make change this innovation is slow, and can be ineffective if it’s not targeted.”

“New Zealand’s strategy sits halfway between the US and European strategies, emphasising clear objectives and a targeted approach.”

Our Land and Water research clearly shows that targeting mitigations to critical source areas is a cost-effective strategy – and research points the way to some critical next steps.

“Our objectives need to start speaking to a whole-catchment plan for improved water quality,” explains McDowell. “We need a compulsory, audited Farm Environment Plan process, backed by good decision-support tools.”

Participating in international research is important for Our Land and Water. New Zealand is seen as a world leader in agriculture and water quality management. Keeping open channels to researchers around the world, and taking a global approach, means that what Our Land and Water learns can be applied in catchments that are home to billions of people.


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Annabel McAleer

Communications Manager, Our Land and Water. Text in this article is licensed for re-use under Creative Commons Attribution 4.0 International (CC BY 4.0)

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