1 Water quality: an introduction
Nitrogen and phosphorus are two important nutrients essential for plant growth. They are often applied to soil, in a variety of forms, to boost yields.
Rainfall can wash them into watercourses, where they can be transported downstream, either in solution or attached to sediment particles suspended in the water.
During a flood event, sediment is deposited across the floodplain because floodwaters slow down, once they leave the river channel, such that their ability to transport sediment falls. The nutrients attached to sediment particles increase the fertility of the floodplain soil. While this is a natural process, the quantity of nutrients in the system are now much higher than they were historically, due to an increased discharge of wastewater from a growing population and an intensification of agricultural practices.
Water with high nutrient levels causes problems for biodiversity, stimulating blue-green cyanobacteria, whose toxins make water unsafe to drink or swim in or triggering algal blooms, which can suffocate aquatic organisms. However, nutrients are beneficial in moderate quantities; they make soil fertile and allow crops to grow. Keeping nutrient levels are at an appropriate level is a matter of balance.
The rich biodiversity found in floodplain meadows has developed as a result of the delicate balance created by the annual cycling of nutrients between flooding and haymaking.