Lakes, Agriculture, and Thresholds
From RAWorkbook
Phosphorus is a common element that is necessary for plant growth and is often added to agricultural fields in order to increase the yield of crops. During and after rainfall, some of that phosphorus is carried in runoff to surrounding areas including wetlands and lakes. The added phosphorus then nourishes plant and algae growth in the lake. Over time with continuous phosphorus inputs from field runoff, the phosphorus accumulates in mud sediments at the bottom of lakes.
The amount (concentration) of phosphorus in the lake-bottom sediments is a key factor in determining whether the lake tends to be clear with green plants on the bottom (one state) or murky with algae blooms (alternate state).
Both images above are Lake Mendota, Wisconsin. In the image on the left the water is murky with algal blooms, in the image on the right the lake water is clear.
The dynamics of the phosphorus nutrient cycle are complex, as are the factors that trigger a transition between clear water and cloudy water (algal) states. Scientists have found that the amount of phosphorus in the sediments is the key factor that defines the threshold between the clear and murky water states. The speed at which sediment phosphorus concentration changes is slower than the rate of phosphorus input, or the speed at which algal blooms occur, and as such is called a slow variable. In this case, as in other examples where resilience is lost, the threshold is associated with components that change more slowly than other parts of the system.
A century ago, human sewage was usually funneled into lakes. This also increased the levels of nutrients in the sediments of these water bodies. Eventually, the process of eutrophication led to declines in water quality and shifts in lake state. Even with the construction of water treatment plants, which were designed to remove phosphorus from sewage, many lakes have not returned to their original clear-water state. In some systems, once thresholds have been crossed, it is difficult (if not impossible) to return to the previous state.
