Eutrophication, explained
We know how vital nutrients are for all ecosystems, both terrestrial and aquatic – even if we don’t know exactly the reason behind it. Nutrients are chemical elements found in every living organism and are essential for breaking down food and providing energy for processes such as breathing, growth and repairing. However, when the concentration of nutrients – primarily nitrogen and phosphorus – increases in aquatic environments, a harmful process called eutrophication starts.
The excessive amount of phosphorus and nitrogen in the water leads to algal blooms as they feed on these, growing, and spreading quickly, forming a green blanket on the surface that can have a foul smell. This blanket blocks sunlight and prevents other aquatic plants from carrying out photosynthesis. When the algae and plants die, they’re decomposed by bacteria in a process that uses up oxygen dissolved in the water, leading to a drop in oxygen levels. As oxygen is vital for almost all aquatic species, if the water becomes hypoxic, meaning there isn’t enough oxygen for organisms to breathe, it creates a dead zone.
Despite being a natural event, human activities contribute to making eutrophication more frequent. Sewage discharges and agricultural and road runoff are commonly associated with eutrophication events in freshwater as they increase the level of phosphorus in the water through detergents, fertilisers, animal manure, and other chemical pollution. Deforestation can also play a role in eutrophication as it makes the soil erode more easily, increasing soil deposits, which can be rich in phosphorus, in the water.
According to an Environment Agency report from 2021, phosphorus is the most common cause of water quality failures under the Water Framework Directive (WFD) in England as it’s the main reason for water bodies not achieving good ecological status.
Besides leading to the death of whole freshwater ecosystems due to the lack of oxygen in the water, eutrophication is also harmful to animals that rely on them for food, such as mammals and birds, and to us. Algal blooms can produce toxins that are damaging to animals, moving up in the food chain when small ones are consumed by larger species.
Drinking and swimming in affected water can cause rashes, respiratory problems and stomach or liver illnesses in humans. Also, popular filter-feeder shellfish such as mussels and oysters can absorb microbes associated with algal blooms which are toxic to people, causing food poisoning when consumed.
Creating and enforcing environmental policies to protect our waterways from pollution are the main solutions to avoid human-induced eutrophication in our rivers and streams, and ultimately, the ocean. However, local improvements, such as constructing wetlands, can also help tackle its effects. Wetlands effectively help clean the water by filtering pollutants. At Thames21 we constantly use nature-based solutions in our projects, be it to mitigate flood risks, or to improve biodiversity and river health.
One of Thames21’s recent projects – Rewilding the Rom – saw the development of a constructed urban wetland connected to the River Rom in the London Borough of Barking and Dagenham. This wetland project aims to reduce flood risk in the local area and diversify the river habitat. To support this, Thames21 is now developing the wetland habitat through a series of planting events, including the creation of a wet woodland through the Trees for Climate grant and direct wetland planting to improve pollution filtration.
Another successful example of constructed wetlands used to improve water quality is Broomfield Park in Enfield. The Pymmes Brook, the local waterway, suffers from multiple pollution issues. In partnership with Enfield Council and with Replenish, Coca-Cola’s water stewardship strategy, a wetland was built in 2019 to reduce flooding in downstream properties, increase biodiversity and reduce pollutant levels. There was a reduction of 72% in ammonia (one of the primary forms of nitrogen in natural waters) and 69% in phosphate (the most common form of phosphorus in natural waters) levels between September 2019 and August 2020. This led to the reclassification of both nutrients in the Pymmes Brook from ‘poor’ to ‘moderate’ according to the water quality standards of the WFD.
Combining enforcement of environmental policies and nature-based solutions can help us secure a healthy future for our waterways. This is a multi-stakeholder action that requires all of us to get involved and change our habits.