• People and the River

Biodegradable Organic Matter, Oxygen & Temperature: what are they and where do they come from?

Barely a day has gone by since the COVID-19 pandemic without river pollution being a news headline. Most of the headlines focus on the issues of excess effluent entering rivers. While a serious issue, this is not the only source of river pollution, nor is there a lot of detail about what river pollutants actually are, what their effects are and what safe levels would be.

 

What are they and where do they come from?

Biodegradable organic matter (BOM) are particles or larger amounts of biological material suspended in river water. This could include matter like leaves, part digested material from sewage or waste material from food and drink manufacturing processes for example.

Oxygen is the 8th element (symbol: O). It is vital for life. Most organisms need to take in oxygen from either the air or water around them, the exception being anaerobic organisms, almost all of which are single celled. Oxygen makes up around 20% (1/5) of air. It gradually dissolves in water from both their air and aquatic plants and algae which release oxygen.

 

What is the problem?

In of itself, BOM is not very dangerous, excluding where it may be contaminated with harmful chemicals or bacteria. However, this matter gets broken down by bacteria which consume oxygen dissolved in the water. This uses up oxygen that is needed by other organisms, the fish and invertebrates of our rivers. This demand for oxygen from bacteria is called the biological oxygen demand (BOD).

 

White foamy river water visible from the bankside
A chemical spill in the Great Ouse near Brackley, Northamptonshire resulted in the river water turning thick and foamy and the deaths of around 2,000 fish in 2018

 

Oxygen concentration in water is also highly dependent on temperature. Hot water cannot hold as much oxygen as cooler water. This, combined with lower volumes of water being available in the summer, increasing the density of animals in rivers, can lead to very low oxygen levels in summer. One of the best ways to spot this is if ‘gulping’ fish are seen. These are fish that have pushed their head out of the water to breath air. Fish can take in oxygen from air, though not nearly as efficiently as in water. They only do this is the oxygen they can get from air is more than that which is available in the water. If you should every see this phenomenon, call the EA’s 24 hour hotline on 0800 80 70 60. Water temperatures are raised by warm outfalls, a lack of shade, warm air temperatures and long days.

 

Planting bank-side trees is a really effective way of reducing temperature by providing shade. This in turn improves oxygen conditions. They also help hold back nitrogen and phosphorus that would come off fields. Tree planting is one of the most vital and cost-effective methods of improving river conditions. Composting biodegradable waste rather than sending it down the sink reduces the BOD of rivers.

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Pressures

The topics below represent the pressures that many waterbodies in the Combined Essex catchment face. They have been divided into six main categories, but it is quite often that these categories can overlap as pressures relate to each other.

Diffuse Pollution

Diffuse pollution occurs as water moves across the land or through the ground and picks pollutants. These pollutants can come from a variety of places, including urban and field run off. The pollutants that enter the river can range from sediment to toxins to excess nutrients, meaning that diffuse pollution can cause a whole range of different issues. The variety in these pressure groups means that it is something that can be quite difficult to tackle. It requires groups of people, business and stakeholders to work together in order to solve this problem.

Fish Passage

Fish should be able to travel up and down a river freely, allowing them to move and breed in the most suitable habitats for them. It is important that fish populations do not become isolated, as this makes them more susceptible to disease and puts pressure on their survival. Unfortunately, there are often many barriers along rivers that prevent fish from being able to migrate up and down stream. Where barriers have been identified, they will be seen as a ‘pressure’ on a waterbody. Thankfully, there are many solutions now that can be put in place to aid fish passage, even over large barriers.

Flow

The flow in a river can vary greatly throughout the year as rainfall and run off can have an effect. This is a natural process. It is when flow is impacted by non-natural processes that it can cause problems. Sometimes, water can be intercepted or removed from a system; this will reduce the flow, therefore changing the habitat conditions. Some species are happy in high energy rivers. This means that when flow is reduced, these species will no longer survive. The opposite of this can occur when excess water is entering a river, for example through increased runoff. Low energy systems then become high energy and displace the species that live there.

Invasive Species

A species that is not meant to be found in a particular area is known as an invasive species. Invasive species can be from a different habitat or a different country altogether. Most of the invasive species that we find on our rivers have come from other countries – plants that people have imported for their gardens or animals that have been released for food or by animal rights activists. Control of invasive species requires a lot of time and effort. We are fortunate that we do have a range of methods to manage most of the invasive species that cause havoc on our rivers, but there are still some which we are still struggling to control.

Physical Modification

Many of our rivers have been heavily modified over the years as rivers have been used for a wide range of purposes. Physical modification is one of the biggest factors that causes our rivers to be unhealthy. The issues that it can cause range from reducing habitat, preventing migration of mobile species, and even have an effect on the water quality. Where structures and modifications are no longer in use or necessary, they should be removed to allow the river to regain its natural state. Unfortunately, this action is not always taken which means that many of our rivers are over straightened and contain redundant structures. It is possible to return a lot of our rivers to their natural state, through one off projects, but in other cases it is not possible as the river has been changed to protect assets or manage flooding. It is recognised that some modifications cannot be removed without having severe negative impacts both socially and economically.

Point Source

This is pollution that comes from a single identifiable source. The pollution entering the river could include a whole range of pollutants. Some point source pollution is known about and licences, for example sewage treatment works. Other sources are not licensed, and therefore work needs to be done with landowners to fix the problems that are allowing the pollution to enter the river. Point source pollution is more easily controlled than diffuse pollution as it often only takes one management approach to solve the issue.