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2021. március 15.

Balaton: A case of Regime Shift

Critical transitions: the Balaton case and a point made for resilience management.

Suddenly and somewhat surprisingly, the famous lake Balaton tourist destination became increasingly greenish and opaque by the end of the 1970s. As a consequence of deteriorating water quality, the change occurred rapidly, resulting in enormous ecological consequences: the death of many animals and plants. The main livelihood of the local people, tourism, also suffered a serious blow. What has happened? The management of the lake did not change much in the past decade. What happened, then?

One of the most important concepts in resilience theory is regime shifts, or in other words, critical transitions. These are large scale changes in social-ecological systems that happen relatively fast and are very hard to reverse. These changes occur suddenly often after a longer period of invisible preparation.

The eutrophication of Balaton was also a regime shift that was in preparation for decades. It started in the ’50s with the massive intensification of agriculture within the watershed, resulting in significant phosphorus pollution — first in the soil, then in the water. The lake sediment had a significant capacity to accumulate and to bind the pollutant, so it could not cause much damage for at least two decades, but after that period of time, this capacity was overwhelmed by the continuing pollution, and phosphorous concentration started to increase in the waterbody as well. As phosphorus is used as a fertilizer on land, it has a similar effect in the water, too: algae started to be produced en masse, making the water murky and greenish. This change had many other consequences: as the water turned murky, no light could reach the submerged plants in the water — this killed these plants in a short time. Losing them also removed their roots, which stabilized the sediment — this change leading to even more phosphorus to be released, making the problem worse. The changes in vegetation also reduced the oxygen concentration in the water, another factor that facilitated the release of phosphorus from the chemical bonds it stored for a long time.

As the shift happened, all the past pollutants that were safely stored in the sediment became available, reinforcing and stabilizing this new state of the lake. The sediment that was once a sink for the pollutant now became a source of it.

These are the self-reinforcing processes that are stabilizing a new stable state. Before the regime shift happened, a relatively moderate effort could have prevented it; after the transition, reversing the transformation needs a heroic intervention — if possible at all.

In Balaton, past pollutants that were once safely stored in the sediment became available, reinforcing and stabilizing the new state of the lake: the sediment that was once a sink for the pollutant now became a source of it.

Regime shifts are often depicted with a landscape with two valleys where a ball can be found in one of them. In normal circumstances, the ball is wobbling around randomly, following smaller disturbances and events, but it is always able to roll back to the bottom of the valley — this is the first stable state. If some external force changes the depth of the valley, or an unusually strong disturbance pushes it over the hill between the two valleys, it is possible that the ball ends up on the other side. These different disturbances are present, and a differently shaped valley will keep the ball from leaving its new stable state.

This transition is a regime shift. The shape of the valley defines the resilience of the stable state — the wider and deeper it is, the larger the force is necessary to push the ball out of the stability domain. This picture also explains why it is much easier to prevent a regime shift than to reverse it. The shape of the valley is determined by the value of the slow variables in the system — managing slow variables is the single most important intervention point for resilience management.


Further reading and source of images: Scheffer, Marten. 2001. ‘Alternative Attractors of Shallow Lakes’. The Scientific World Journal 1: 254–63. https://doi.org/10/dx3p5b.