This diagram provides an accessible description of the key processes that guide the water quality within a lake.
Farming and forestry activity generally involve the application of nutrients, especially through fertiliser application, and can encourage their loss to water through some 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 livestock, to protect its capacity to reduce the amount of nutrients that enter the lake.
The edge of the lake at the land-water interface 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 livestock, to protect its capacity to reduce the amount of nutrients that enter the lake.
Nutrients enter the water column of the lake.
Nutrients can enter from groundwater into the water column of the lake. 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 limited.
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. 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 sufficient water clarity. 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 compromise clarity in a lake.
Clarity is important to maintain plants rooted on the lake bed. A marked decline in clarity is often associated with a reduction in the 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 the clarity present in a lake.
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.
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.