A model of a Northland lake

This diagram describes the key processes that influence the water quality within a Northland lake.

This diagram describes the key processes that influence the water quality within a Northland 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 of 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 of livestock, to protect its capacity to reduce the amount of nutrients that travel further into 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 the water column from these sediments. On the other hand, these nutrients can be absorbed by the sediments. If this reservoir contains a high amount of nutrients, then these can ensure that the nutrient concentration of the lake water stays enriched, even if inputs from land and groundwater are greatly reduced.
Higher levels of sediment and decomposing material (e.g. dead algae) on the lake bed are available to be stirred back up into the water column by strong wave action caused by the wind. 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 the water is clear enough for them to be able to access sufficient sunlight. 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 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. On the other hand, an increase in clarity will typically bolster plant presence. So, the relationship between clarity and plants is a positive one, denoted by a green arrow.
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 the presence of what is known as a reinforcing loop: the fact that high clarity reinforces high clarity, while low clarity reinforces low clarity. 

View the model in Insight Maker