Blue Planet Prize Story

The phosphorus cycle is the movement of phosphorus through the global environment through its use in farmlands as fertilizers, flowing out to rivers with soil, or via the waste excreted by organisms, and so on. In addition, phosphorus also moves when people mine and use phosphorus from minerals. Eutrophication occurs when a body of water, such as lakes, becomes enriched with nutrients. While the accumulation of nutrients may seem positive at first, an excessive amount of nutrients causes phytoplankton blooms, leading to a sequence of wide-ranging problems, resulting in a rapid and complete change of ecosystems. This is called regime shift.

According to Professor Carpenter, chemical inputs into rivers, lakes, and oceans come from either point sources, such as factory effluent pipes, or nonpoint sources, like agriculture and urban activity. Eutrophication is mainly caused by domestic wastewater and agricultural activities that use large amounts of phosphorus in fertilizers and livestock feed. He suggests that eutrophication is largely linked to the public's lifestyles and also agricultural activity rather than specific sources of contamination. The phosphorus contained in fertilizers and feed accumulates in the soil and flows into rivers and lakes with rainwater, leading to pollution.

There are two types of regime shift: one is a long-term shift in broader areas over a few hundred years, and the other is a short-term shift in specific areas. A long-term regime shift occurs after the accumulation of nutrients, including phosphorus in coastal ecosystems, over a long period of time, while a short-term regime shift occurs with eutrophication in lakes and rivers.

The long-term regime shift starts from eutrophication in lakes and rivers. Nutrients including phosphorus flow into the ocean, accumulate at the bottom of coastal ecosystems, and promote the growth of living organisms. This leads to the depletion of dissolved oxygen in the water, which eventually results in the death of fish and other marine organisms. Meanwhile, when there is little oxygen in the water, the phosphorus accumulated in the sediment at the bottom of lakes and oceans starts to dissolve in water, accelerating eutrophication. Once such negative chain occurs in a long-term regime shift, it is impossible to restore the system with the current technology.

A short-term regime shift is caused by phosphorus being discharged from the surrounding farmlands, which causes algae blooms. This also causes a phenomenon similar to those seen in coastal ecosystems. However, because it occurs within a short period of time after phosphorus flows into the water, there is a possibility that we can manage the problem. Prof. Carpenter's method, which he developed from studies on trophic cascades, has been applied to Lake Mendota since 1987. That is, increasing the number of large fish and reducing the number of small fish while increasing small organisms (a type of Daphnia) that eat algae. This organism then becomes food for large fish. As a result, the water quality of Lake Mendota improved significantly around 1988.

As described above, phosphorus is being used in fertilizers to increase productivity in agriculture. However, phosphorus-based fertilizers stick to the soil, making it difficult for crops to absorb them, forcing farmers to use increasing amounts to be effective. Over time, a large amount of this phosphorus eventually flows into the water, and this has a real impact on the global environment. It is necessary to increase agricultural productivity to support the increasing population of the earth. However, because it is necessary to use more phosphorus fertilizers to do so, there is concern about further harmful effects on the global environment. Prof. Carpenter and other researchers have warned that we need to consider measures to maintain agricultural production while protecting the global environment.

4. Human imagination