Last lesson you used Daisyworld to see how negative feedback holds a system steady, how positive feedback pushes it away, and what a tipping point is. This lesson takes tipping points into real lakes, forests and farmland, and asks why some systems resist them better than others. By the end you should be able to explain a tipping point using eutrophication, give examples of emergent properties, define resilience, explain how diversity and the size of storages affect it, and explain how deforestation reduces it.
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1.2.12 Tipping points can exist within a system where a small alteration in one component can produce large overall changes, resulting in a shift in equilibrium.
Tipping points result in regime shifts between alternative stable states.
Use examples to support the explanation. For example, a change of nitrate/phosphate concentrations leading to eutrophication.
Identify other examples of such tipping points throughout the course.
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The guide's example of a tipping point is eutrophication: a lake receives more nitrate or phosphate than it used to, and at some point the whole lake changes. The video explains how it happens.
[14 May 2020]
![[Source]](attachment:743b933d-8f0c-408f-8e50-17ff15f61b94:Eutrophication.png)
[Source]
Satellite image of green algal bloom swirling across the western end of Lake Erie
Western Lake Erie on 5 October 2011, photographed by Landsat 5. The green swirls are that year's bloom, the largest in the lake's recorded history. Image: NASA Earth Observatory, using data from the US Geological Survey.
Lake Erie lies on the border between the United States and Canada. It is the shallowest and warmest of the five Great Lakes, with an average depth of 19 m.
Into the bloom state. In the 1950s and 1960s, phosphorus from farms, sewage and industry fed huge algal blooms in the lake's shallow western end year after year.
Back out. Under the Great Lakes Water Quality Agreement, first signed by Canada and the United States in April 1972, the phosphorus reaching Lake Erie was limited to 11,000 tonnes a year. The limit was met by the mid-1980s, and the lake recovered so well that it became known as the "Walleye Capital of the World" (walleye are a large predatory fish).
Back in again. From the mid-1990s the algae began to increase again, and blooms have grown rapidly since 2002. More of the phosphorus now arrives dissolved, washed off farmland by spring rain, and dissolved phosphorus is the form the bloom-forming cyanobacteria use most easily. Between 1995 and 2011 the dissolved phosphorus carried into the western lake by the Maumee River rose by 218%, while the amount of water it flowed in rose by only 42%. The rise matches changes in farming: more fertiliser spread in autumn, and more of it spread on the surface rather than worked into the soil.
In spring 2011 heavy rain washed the largest load of dissolved phosphorus into the western lake since detailed monitoring began in 1975. That summer's bloom was more than three times as intense as any bloom seen before. On 2 August 2014 a toxin made by the bloom, microcystin, got into the treated water of Toledo, Ohio, and half a million water customers were told not to drink from the tap for three days.
Sources: Michalak and others, PNAS, 2013; International Joint Commission; NASA Earth Observatory; Michigan Sea Grant; The Toledo drinking water advisory, Sustainability, 2015.
[29 April 2019]
Use the video and the case study above.
1. Identify the main source of the extra phosphorus reaching western Lake Erie today. [1]
2. Explain why the blooms came back in the 1990s even though the phosphorus limit had been met in the 1980s. [3]
3. Outline one way the blooms have affected people who depend on the lake. [2]
Diagram made for this lesson, after Scheffer and others (2001)
Lake Erie shows a lake switching between two very different conditions and staying in each for years. When a system can settle in either of two states like this, they are called alternative stable states, and a switch from one to the other is a regime shift.
The model above explains how it works. Think of the state of a lake as a ball in a landscape. Each valley is a stable state: nudge the ball and it rolls back, in the same way that Daisyworld's temperature came back after you killed half the daisies. The hill between the valleys is the tipping point.
The best-studied examples are shallow lakes. In the clear state, rooted water plants hold the mud down and take up nutrients, and small grazing animals called Daphnia shelter among them and eat the algae. In the turbid state, algae shade out the plants, and fish stir up the mud as they search the bottom for food and eat the Daphnia, which no longer have plants to hide in. Each state is held in place by its own feedback loops.
Adding nutrients does not push the ball at first. It reshapes the landscape: the clear-water valley becomes shallower until an ordinary storm or a hot summer is enough to tip the lake over the hill. Once it has tipped, cutting nutrients back to the level at which it tipped is usually not enough to bring it back, because the turbid state's own feedback loops now hold it there. Nutrients usually have to be cut much further. This is why regime shifts are so hard to reverse.
Source: Scheffer and others, Catastrophic shifts in ecosystems, Nature, 2001.
