Phosphorus and nitrogen are already at the Earth's limits. We humans must act as members of the ecosystem
-- Interview with 2022 Prize Winner Professor Stephen Carpenter: The Future of Phosphorus and Nitrogen Cycles --

November 24, 2022

Professor Stephen Carpenter (USA), one of the two 2022 Blue Planet Prize winners, has been researching the problems of phosphorus and nitrogen cycles for more than 40 years, using lakes throughout Wisconsin for fieldwork. He points out that phosphorus and nitrogen cycles have already exceeded the Earth's limit and are broken, adversely affecting water resources around the world. We spoke with Professor Carpenter, who was in Japan for the Blue Planet Prize award ceremony, about the problems of phosphorus and nitrogen cycles, the regime shift caused by excess nutrients in lakes, and the solutions that came to light as a result of his research.

A wish to restore the beautiful appearance of the nearby lake. A limnologist who has continued to study the problems of phosphorus and nitrogen cycles

Professor Stephen Carpenter
2022 Winner Professor Stephen Carpenter at Lake Mendota, somewhat like a long-time companion.

Professor Stephen Carpenter (U.S.) began researching phosphorus and nitrogen cycles when he first visited Lake Mendota, which he says is now like a long-time friend. In 1974, Professor Carpenter entered the doctoral program at the University of Wisconsin-Madison to study ecosystems. It was there that he first saw Lake Mendota, located in front of the university. At that time, the lake was murky and dark green due to an algae bloom.

Professor Carpenter wanted to clean up the lake, and that is what triggered his studies. Lakes are very interesting subject matter when studying ecosystems because they are a small and closed ecological system. For more than 40 years, Professor Carpenter has been going back and forth between using Lake Mendota for fieldwork and expanding the knowledge he has gained there to a global scale.

Home to the University of Wisconsin-Madison, the city of Madison is also known as the "City of Four Lakes". This is due to its location on the Yahara River, which connects Lake Mendota, Lake Monona, Lake Waubesa, and Lake Kegonsa. Dairy farming, which makes use of this abundant water resource and crop production is a major industry in this region. However, research found that these agricultural and livestock industries were the major cause of algae blooms in the lake.

The total area of Lake Mendota is about 40 km2, Professor Carpenter explained. In contrast, the surrounding farmland area is about 15 times the size of the lake, or about 600 km2. All of the fertilizer used there and the manure of the livestock raised in the area was flowing into Lake Mendota. Phosphorus and nitrogen in the fertilizer and manure were flowing into the lake, causing eutrophication in the lake and causing algae blooms, he said.

From 1940 to 1960, the modernization of agriculture was promoted around the world to overcome a food crisis brought about by population growth. One feature of this change was the use of chemical fertilizers.

Chemical fertilizers refer to fertilizers chemically synthesized from ammonia and mineral resources, with the three major elements that are essential for the growth of crops: nitrogen, phosphorus, and potassium. From the 1950s, the use of chemical fertilizers increased dramatically due to the expansion of phosphate rock mining and the nitrogen fixation technology developed for ammunition production. By the 1960s, there was an excess supply of phosphorus and nitrogen, and it began to adversely affect the soil and the water environment.

Professor Carpenter noted that algae blooms, or outbreaks of harmful algae, have become a global problem. The situation is dire in the U.S., Western Europe, Japan, Africa, and China, he said. This is a problem that is still ongoing.

Excess phosphorus and nitrogen cause water pollution. We must keep their use to a minimum and circulate it above ground.

Planetary boundaries. Created by the Stockholm Resilience Centre
Planetary boundaries. Created bythe Stockholm Resilience Centre

In 2011 Professor Carpenter and colleagues estimated the planetary boundary for phosphorus. In later papers the phosphorus boundary was combined with studies of the nitrogen boundary to refine the estimate of the freshwater boundary.

The concept of planetary boundaries is based on the essential biophysical processes that maintain the stability of the Earth system and scientifically defines the range in which humanity can safely operate on the Earth. Professor Carpenter says phosphorus and nitrogen pollution have already reached levels that are beyond what the Earth can support.

The biggest problem with phosphorus and nitrogen is that Cyanobacteria that grow from excess inflows raise the toxicity of the water, Professor Carpenter explained. This causes toxic algae to grow in the water and poison animals that drink the water, including humans.

The professor emphasizes that phosphorus in particular is a finite element that cannot be wasted. The Earth's phosphorus resources are said to become depleted in 100 to 300 years at the earliest. Professor Carpenter gave an example of how phosphorus, which is important for both plant growth and human life, has the power to influence world affairs. Around 2007, when phosphate ore-producing countries halted exports one after another, the price of phosphorus skyrocketed. This caused a shortage of chemical fertilizers in agricultural countries and led to a fall in wheat yields, causing famine in the Middle East and North Africa, as well as political instability.

Professor Carpenter says that fundamentally, phosphorus should be used wisely and efficiently. In other words, it is important not to waste phosphorus and to keep it in the soil so that it does not flow into the water. Research has shown that planting a cover crop such as wheat after a harvest can decrease the runoff of phosphorus into streams and lakes. "Then you have two crops trading off and there is always something stabilizing the soil," Professor Carpenter said. This mimics a natural ecosystem where there is something growing all year round. It is also effective to maintain a buffer strip of native, deep-rooted plants for about 30 m or more along the edge of a river, he said.

From an economic point of view, a feasible political approach is to impose taxes on phosphorus and nitrogen so that costs of fertilizers account for costs of the environmental damage. The funds raised by the taxes should be used to reverse the damages of excessive fertilizer use by improving health of soil and water resources.

In Japan, it is also becoming more and more common to see organic vegetables and farm produce that do not use chemical fertilizers. They have become easier to purchase through online shopping and home delivery services. Could there be a positive impact on phosphorus and nitrogen cycles from the purchase of such organic vegetables?

"Organic farming is better for the environment because it does not use commercial fertilizer. It is creating a closed cycle of phosphorus and other chemicals that we need. Secondly, it does not use biocides. Organic agriculture avoids that," Professor Carpenter explained. Additionally, he said consumers could reconsider their consumption of beef and pork.

In general, large amounts of chemical fertilizers are used to grow grains for livestock feed, Professor Carpenter said. "I'm not saying zero meat. I eat meat, some of the deer that I shoot on my land. But we could eat far, far less meat than we do and still have strong muscles and a healthy immune system."

Two regime shifts in lakes. Exploring approaches at Lake Mendota

 Simon Levin and Steve Carpenter (right)
Simon Levin and Steve Carpenter (right) share a conversation on the boat to Asko Island

Professor Carpenter is also known for his research on "regime shifts" or large changes in ecosystems. A regime is the state in which an ecosystem tends to remain. While resilience is the ability to remain in a favorable regime, a regime shift may be a transition to an unfavorable, unexpected situation. In some over-enriched lakes the shift to the state dominated by toxic Cyanobacteria may not be reversible because large amounts of phosphorus are stored in the lakes sediments.

There are two kinds of regime shifts that take place in lakes: short-term changes and changes that take place in large cycles of hundreds of years, Professor Carpenter said.

Short-term regime shifts take place every summer. During the initial stage when phosphorus and nitrogen from the surrounding farmland flow into the lake, algae grow in large numbers using this as a source of nutrients. Soon, the level of algae grows beyond what algae eaters, known as grazers (*), can eat, and the lake becomes covered with algae.

As a countermeasure against this short-term regime shift, lake managers in Wisconsin have been using a water quality control method called biomanipulation in Lake Mendota since 1987. Large fish are released into the lake which reduces the number of small fish. This helps to increase the number of grazers that were previously eaten by the small fish, which then reduces the algae that have grown in large numbers. As a result, the water quality of Lake Mendota improved significantly around 1988.

On the other hand, regime shifts in long-term cycles are caused by phosphorus and nitrogen sediments at the bottom of the lake. The algae that grow in large numbers on the surface eventually sink to the lake bottom, and the sunken algae and chemical substances that flow in and accumulate at the bottom become decomposed with the oxygen in the water. When there is too little oxygen at the bottom of the lake, nitrogen and phosphorus dissolve out from the bottom sediment and are recycled from the deep water to the surface water. This in turn causes eutrophication again in the surface water. Once such a regime shift occurs, it is difficult to reverse.

At Lake Mendota, the county government is taking steps to deal with sedimentation based on scientific advice. In addition to scooping up the sediment and returning it to the land to use as fertilizer, efforts are being made to prevent phosphorus and nitrogen from flowing out in the first place. Devices have also been installed to decompose livestock manure and use it as compost. The biogas generated during decomposition is then used as energy. Such examples are also seen in livestock regions in Japan and western Europe and are expected to become the global standard in the future.

*Grazers are small animals that scrape off algae on rocks and eat them. Grazers in Japanese rivers include aquatic insects such as pond snails and stoneflies.

A scenario approach that involved a diverse group of people and created common understanding. Teamwork solves environmental problems

Prof. Carpenter(Jeff Miller / University of Wisconsin-Madison)
Prof. Carpenter(Jeff Miller / University of Wisconsin-Madison)

Lake Mendota has improved its water quality after being treated with various measures. However, Professor Carpenters says there is now a new problem. A ship from the Caspian Sea has released non-native fish, and the fish are eating algae predators, causing massive algae blooms to occur once again.

In order to prevent such a situation, each of us must be aware that we are part of the ecosystem and remember that we are influencing the natural environment around us, Professor Carpenter noted. Even if you are in a city surrounded by buildings, you are completely dependent on nature for your survival. Nature determines water, food, air quality, etc. We cannot live without nature, he said.

Professor Carpenter says that in addition to political and economic approaches, an approach to change people's awareness is also an important factor in solving environmental problems. In 2010, he and his colleagues started a scenario project titled "Yahara 2070", thinking about how to make people aware of the uncertainty of the future and to gain a sense of the environmental problems as realistically and accurately as possible.

The project involved a diverse range of stakeholders living in Madison including politicians, scientists, government officials, engineers, company managers, those working in non-profit organizations and industry groups, reporters, farmers, students, and artists. After lectures on the history of the region and environmental issues, each person thought of a scenario and presented it. The scenario team then rounded them up into four scenarios based on common themes and fleshed out the scenarios based on scientific data. For example, if there was a flood in the scenario, they added calculations on the risk and scale of the flood. The completed scenarios were later presented in various forms by illustrators, writers, and filmmakers. Plays were also performed by children at schools and churches, which people enjoyed seeing.

"Those stories embrace uncertainty. We use stories to tell people about the full range of uncertainties without having to teach mathematics. Humans think in stories. We like stories and communicate in stories. Scenarios are a way of organizing our collective ideas about the future and having an open conversation that everyone can participate in. That is what we want a democratic society to do. The scenario approaches can help us move into the future in a more coherent way, acknowledging the uncertainty, but not fearing the uncertainty and knowing that we have no choice but to go into the future."

Reflecting on the scenario project and his life as a researcher, Professor Carpenter believes that trust and teamwork are essential to solving environmental problems. He wants the young scientists who are responsible for the future to value solidarity that transcends specializations and communities, and to have respect for other fields.

Professor Carpenter said that he would like to convey to young people that they should study team science. By respecting other languages and fields and learning from each other, we will be able to solve environmental problems. He has a long list of what he would like to research going forward and this is the result of teamwork. He believes the dynamism of the team can be a disruptive factor, become a source of ideas, and lead to solutions to environmental problems.


Professor Stephen Carpenter
Emeritus Director of the Center for Limnology,
Stephen Alfred Forbes Professor Emeritus of Integrative Biology
University of Wisconsin-Madison

Through his research on lake eutrophication, from nutrients such as phosphorus and nitrogen, he studied the resilience of lakes using mathematical models, providing a new perspective on social-ecological systems. He also worked on the environmental pollution from phosphorus and nitrogen through land use, showing the critical state of the global phosphorus cycle and the need to review human activity from a broad geochemical viewpoint. He was awarded the 2022 Blue Planet Prize.

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