This diagram provides an accessible description of the key processes that influence the water quality within a lake.
In this diagram it is useful to note that green arrows signify a positive relationship, while red arrows signify a negative relationship.
Farming and forestry activity generally involve the application of nutrients (especially fertiliser), some of which will be lost to water. These losses can be reduced by appropriate farming/forestry practices.
The area of land at the land-water interface is known as the riparian zone. It is important to safeguard the riparian zone from the entrance livestock, to protect its capacity to reduce the amount of nutrients that enter the lake.
The edge of the lake is known as the littoral zone. It is commonly home to large plants that are rooted in the lake bed and have leaves above the water surface. It is important to safeguard the littoral zone from the entrance livestock, to protect its capacity to reduce the amount of nutrients that enter the lake.
Nutrients can enter into the water column of the lake from the littoral/riparian areas or from groundwater. These nutrients may arise from outside of the area from which surface water collects and drains into a specific lake.
Sediments and organic material on the bed of the lake act as a reservoir of nutrients. Nutrients can enter/exit these sources from/to the water column. If this reservoir contains a high load of nutrients, then these can ensure that the nutrient concentration of the lake water stay enriched, even if inputs from land and groundwater are reduced.
Higher levels of sediment and decomposing material (e.g. dead algae) on the lake bed will also increase the amount of sediment available for suspension, I.e. there is more enriched material on the bottom of the lake to be stirred back up into the water column by strong winds. This relationship is not represented in the diagram, to sharpen the focus on the key relationship between plants and the amount of suspended sediment.
Algae take up nutrients from the water column.
Algae die and enter the pool of decaying material on the lake bed.
Plants grow on the lake bed, provided there is the water is clear enough. These plants take up nutrients from the sediment, but also enter the pool of decaying material on the lake bed following their death.
Higher algal populations cause water to become more turbid, I.e. less clear.
Clarity is important to maintain plant communities on the lake bed, a decline in clarity will probably be associated with a reduction in the lake's plant population.
Plants help to reduce the amount of suspended sediment present in a lake through keeping lake sediments anchored, especially when wave action threatens to stir up sediment in shallow lakes.
Higher levels of suspended sediment reduce water clarity.
If nutrients in the lake water are low, then algal populations will be low and the clarity will be high. This will help to increase the plant population, which will reduce the suspended-sediment load and thus further reinforce high clarity.
In contrast, if nutrients in the lake water are high, then algal populations will be high and the clarity will be relatively low. This will work to decrease the plant population, which will increase the suspended-sediment load and thus further reinforce poor clarity. This loop is a key relationship within a lake system. The plus sign signifies a reinforcing loop, the fact that high clarity reinforces high clarity while low clarity reinforces low clarity.