Concept Use in Management - Lake Eutrophication

Studying the eutrophication of lakes provides a real-world example of a system’s resilience being overcome and subsequently shifting to an alternative state (Scheffer et al. 2001; Wilkinson et al. 2018). Eutrophication occurs when increasing nutrient concentrations in a body of water cause a transition from a clear-water state to a turbid-water state with reduced water quality. Many shallow lakes have clear water and diverse underwater vegetation. Nutrients such as nitrogen and phosphorus enter the lake from various sources and atler stabilizing feedbacks in the clear-water lake (Wilkinson et al. 2018). If too many nutrients enter the lake, the resilience of the established regime can be overcome and the system suddenly flips from the clear, vegetated state to a cloudy, turbid state which kills off much of the underwater vegetation (Scheffer et al. 2001). In some cases, it can be very difficult to return to the previous clear state once eutrophication has occurred (Figure 3).

Figure 3. An example of two side-by-side lakes where one has transitioned to a turbid, cyanobacteria-dominated state (left) while the other remains in a clear-water state (right). Photo Credit: Dr. Stephen R. Carpenter (used with permission)

We can use the ball-in-cup model to help conceptualize this example. However, there is an alternative way to use this model (Figure 4). Up until now, we considered an example in which a disturbance moves the ball from one basin to another. This is easy to think about since we often view systems as static, unchanging, and at equilibrium until a disturbance shocks the system. But this is not the only way a system transitions to an alternative state; systems are dynamic and changing. Instead of disturbances that move the ball from one state to another, disturbances that alter the stabilizing feedbacks of an alternative state can change the shape of the cup itself (Beisner et al. 2003).

Figure 4. An alternative view of the ball-and-cup model in which changing system structures affects the movement of the ball. Adapted from Beisner et al. 2003.

Now let’s reconsider the example of lake eutrophication (Figure 5). The lake starts in the first cup of the model, before nutrients have been added to the system and its water is clear. Over time, as more and more nutrients enter the system, the first cup starts to become shallower as the nutrients increase. Eventually, the cup becomes so shallow that the ball can easily be pushed to the other cup representing the turbid state; the resilience of the lake’s clear-water state has been overcome and a transition to a turbid state becomes imminent. Although the switch may happen suddenly, it took a long time to reach that threshold in the first place. The lake’s resilience allowed it to absorb the disturbance for quite a while until a threshold was crossed. After that, resilience was overcome and the lake quickly shifted to an alternative state.

Figure 5. An example of a system’s structures changing as more nutrients are added into the lake. Nutrients fill the basin incrementally until filling the cup. Then the system’s resilience is overcome and it falls into a new state.

There are several methods to manage eutrophication and attempt to return a lake to a clear state. It is important to reduce the amount of nutrients entering the lake from outside the system (Sondergaard et al. 2001). Additionally, managers can add chemical compounds such as iron and alum to increase the lake’s capacity to absorb nutrients, dredge the sediment to remove absorbed nutrients, or add algaecides to remove excess algae (Sondergaard et al. 2001; Moss 1990; Wilkinson et al. 2018). However, the eutrophic regime itself can have high resilience. Although some eutrophic lakes can be reversed to their previous state by a proportional reduction of nutrient content, other lakes exhibit hysteretic behavior, in which extreme reduction of nutrient levels must occur in order to return to the previous state (Carpenter et al. 1999, Scheffer et al. 2001). In these cases, a manager may need to reduce the nutrient load to a level far below what even the original clear lake had when the transition occurred (Figure 6). The concept of hysteresis is important to understand for those managing complex systems and will be covered in a subsequent module.

Figure 6. An example of a hysteretic curve. As phosphorus increases, vegetation decreases (dashed line). As phosphorus is removed from the lake, vegetation levels do not increase proportionally (solid line). Much more phosphorus must be removed in order to return to previous levels of vegetation. Adapted from Scheffer et al. 2001.