Following the Fertilizer Trail: Study Explains How Nutrient Run-off Leads to Far Off Impacts

“People aren’t aware of how connected things can be, and how local actions can have big implications far away.”

This cautionary warning is being offered by Dr. Kevin McCann, a professor of theoretical ecology in the Department of Integrative Biology, following a new study from his lab that shows human changes to the environment are allowing nutrient run-off to cause major disruptions in far off habitats.  

It’s a disturbing twist on a decades-old problem linked to the modern use of agricultural fertilizers. Frequently, due to rain or other weather events, the nutrients applied to a crop are not absorbed by the plants, but end up washed away with the rain or snow melt and into waterways.

These excess nutrients can upset the balance between vegetation (say, duckweed) and consumers (say, minnows) in a given ecosystem (say, a pond), causing an overgrowth of the duckweed. If the duckweed grows faster than it can be eaten by the minnows, the duckweed can cover the entire surface of the pond. This can limit the amount of oxygen present in the water, killing the minnows. If the minnows are gone, so too will be the animals that eat the minnows, and the animals that eat the minnow-eaters, and so forth. In some cases, the excess nutrients result in the overgrowth of a less edible plant (like some forms of algae), which could end up crowding out the duckweed, intensifying the problem.

This is what ecologists call the “paradox of enrichment”, where increasing the productivity of the ecosystem (by adding extra nutrients) destabilizes the entire system.     

But nutrients aren’t contained to one area. Ecosystems are often interconnected, linked by networks of streams, rivers, lakes and oceans. This means that the harmful ecological impacts of fertilizers can be felt far away from where they were applied. 

How far and how fast nutrients move along waterways from one area to the next can depend on how much humans have modified the landscape.

“Wetlands are critical for slowing the flow of nutrients. If you slow the flow, you give the nutrients a chance to be absorbed, rather than allowing them to flow downstream,” McCann explains.

In contrast, agricultural lands often have drainage and irrigation systems in place to keep their fields hydrated while preventing them from getting waterlogged. While this is good for the crops, it hastens the flow of nutrients downstream.

And once the nutrients reach lakes and oceans, they can accumulate thousands of kilometres away from where they started. “Oceans act as conveyor belts, moving massive amounts of nutrients around,” says McCann.

Armed with theories explaining the effects of nutrients and how they move between locations, McCann and colleagues created a model of a simplified aquatic ecosystem to compare what happens when nutrients flow through agricultural lands, wetlands, or a mix of the two.  

Unsurprisingly, nutrients had the least impact when they flowed through less developed areas, such as wetlands. But when nutrients flowed through agricultural areas only, the results were less predictable.

“We found that nutrients have almost no impact until they slow down, which was surprising,” says McCann.  Nutrients flowing through agricultural areas move quickly and have little time to be absorbed along the way, amassing in their final destinations and spurring an overgrowth of inedible algae. In the model, this resulted in the local extinction of edible plants at distant locations. In the real world, this would also be followed by the local extinction of the animals that eat those plants.

The findings explain how landscape modifications are amplifying the ecological effects of nutrient run-off over large distances by connecting ecosystems that are geographically separated. 

But research and policy changes can help mitigate the problem, says McCann.

“If we can understand the river network, we can slow and assimilate the nutrients with green space. Emerging evidence shows that it’s possible.”

Read the full article in Ecology Letters.

Read about other CBS Research Highlights.