Why Hasn’t All the Wildlife Died?

Understanding the Effects of Blue-Green Algae, E. coli and Ammonia in Walkford Brook

Following the recent death of a dog after swimming in Walkford Brook, many local residents have asked an understandable question: “If the brook is so polluted, why hasn’t all the wildlife died?"

The answer is more complicated than many people expect. People often imagine pollution acting like a pesticide or bleach: if a watercourse becomes contaminated, every living thing should die immediately. Rivers rarely behave like that.

Instead, pollution usually alters the balance of the ecosystem. Sensitive species disappear first, while more tolerant species survive and sometimes even thrive.

A polluted river can still appear to support plenty of life while, beneath the surface, the ecosystem has already changed dramatically.

We examine the effects of three different but potentially related hazards:

• Blue-green algae (cyanobacteria)

• E. coli and sewage contamination

• Ammonia

Each affects animals and aquatic ecosystems differently.

Blue-green algae (cyanobacteria)

Despite the name, blue-green algae are not true algae but are photosynthetic bacteria called cyanobacteria.  Some species produce powerful toxins known as cyanotoxins.

These toxins can affect the liver, the nervous system, the digestive system and the skin.

The most important cyanotoxins include Microcystins (liver toxins) Anatoxins (nerve toxins) Cylindrospermopsins (cell toxins).

Animals such as dogs may be exposed by drinking contaminated water, swallowing water while swimming, eating algal mats, eating contaminated fish or invertebrates, or grooming contaminated fur.

Although not every cyanobacterial bloom produces toxins it’s unfortunately impossible to determine whether a bloom is toxic simply by looking at it.

Dogs are particularly vulnerable and are among the animals most frequently affected by cyanobacterial poisoning. They swim, drink from rivers and ponds and lick their fur after leaving the water. Sometimes eat algae growing on the water’s edge.

Symptoms may develop within minutes or hours and can include vomiting, diarrhoea, excessive salvation, weakness, and muscle tremor, seizures and collapse.  Some toxins act so quickly that death can occur within a few hours. Published reviews have suggested that more than half of reported canine poisonings are fatal, and there is no specific antidote so early veterinary treatment is vital.

Horses and livestock

Cattle, sheep and horses are also susceptible to cyanobacterial poisonings and are usually exposed while drinking from ponds, lakes and streams.

Symptoms include weakness, diarrhoea, loss of coordination, liver damage and even sudden death. Cattle may be even more sensitive than dogs because they consume large quantities of water and graze along the water’s edge. Sheep too in other parts of the country.

Birds and fish

Cyanobacterial toxins can also affect fish, waterfowl and wild birds. However, wildlife deaths in these animals are not always obvious.

Fish can often move away from contaminated areas; birds may only be exposed occasionally and dead animals may also be removed rapidly by scavengers. Consequently, the absence of obvious wildlife deaths does not necessarily indicate that the water is safe!

Many people associate blue-green algae with floating green surface scums but these cyanobacteria do not always appear as green scum and that is particularly important when considering Walkford Brook. Toxic cyanobacteria can also form benthic mats.  These mats grow attached to stones, gravel and sediments in stream beds. Several fatal dog poisonings have been linked to benthic cyanobacteria. A river may therefore appear relatively clear while still containing dangerous concentrations of cyanotoxins.

What about E. coli?

Escherichia coli (E. coli) is a bacterium that normally lives in the intestines of humans and animals and most strains are harmless. However, high concentrations in rivers indicate faecal contamination. So for environmental scientists, E. coli is usually considered an indicator organism, acting as a warning sign that sewage or animal waste has entered the water.

High E. coli counts such as those we have seen locally in the Lymington River, Dan stream and tributaries entering Avon Water suggest that the water may also contain other pathogenic (harmful) bacteria, viruses, protozoa, sewage derived organic matter and ammonia

Therefore, E. coli should be viewed primarily as evidence of contamination rather than as the direct cause of ecological damage.

How does E. coli affect dogs?

Dogs exposed to heavily contaminated water may develop: vomiting, diarrhoea, abdominal pain, loss of appetite and lethargy. Most dogs recover. However, some strains of E. coli produce toxins that can cause much more severe disease.

Dogs become exposed by exactly the same means as described above in relation to Cyanobacteria. The presence of extremely high E. coli counts should always be treated as a public health warning.

Does E. coli kill wildlife?

Surprisingly, there is very little evidence that E. coli itself directly kills aquatic wildlife and it is a good reason why people become confused.

The ecological damage associated with sewage pollution usually results from other components of sewage described above, including organic matter, nutrients, and ammonia. Bacteria decompose sewage and these bacteria consume oxygen. As dissolved oxygen concentrations fall, the river becomes less suitable for sensitive aquatic organisms. This is why we measure dissolved oxygen content of a water body when we suspect serious contamination.

We also measure ammonia and this may be the most important pollutant in Walkford Brook. scientific literature suggests that ammonia may be the most significant ecological threat. Ammonia occurs naturally as part of the nitrogen cycle. However, elevated concentrations are often associated with sewage, septic tanks (such as we see giving problems in Sway, Tiptoe and Norleywood), agricultural run-off, and waste water discharges.

Ammonia can exist in two forms in water. For the enthusiasts, these are ionised ammonia (NH₄⁺) and un-ionised ammonia (NH₃). One form, un-ionised ammonia is particularly toxic, and it toxicity increases as pH and temperature rise. Warm, alkaline water can therefore become much more dangerous.

How ammonia affects fish

Ammonia can damage gills, disrupt ion regulation, interfere with respiration, impaired their growth and reduce their reproductive success. Fish kills we have seen locally have been associated with ammonia. Young fish are particularly vulnerable. However, fish can sometimes avoid polluted areas by moving elsewhere. This almost certainly happened in the Danestream.

How ammonia affects freshwater invertebrates

Recent reviews in scientific literature have concluded that aquatic invertebrates may be among the organisms most sensitive to ammonia. High concentrations have been shown to reduce feeding, growth and reproductive success, damage tissues, and disrupt metabolism.

Long-term exposure can reduce biodiversity and changes the structure of aquatic communities. Invertebrates are often used as indicators of river health precisely because they respond so quickly to pollution.

Not all invertebrates respond in the same way and this is the key to understanding why a polluted stream may still appear to support wildlife. Pollution-sensitive species disappear first. These include mayflies, stoneflies and some caddisflies.

When these disappear, pollution-tolerant organisms often remain. These include bloodworms, sludge worms, midge larvae and water hog lice. The result is an ecological simplification: the river still contains life but the community has changed. Species diversity declines as sensitive species disappear and pollution-tolerant species dominate.

The river may still look healthy to casual observers even though its ecological integrity has already been severely damaged.

Blue green algae (Cyanobacteria) also affect invertebrates

To ind produced by these organisms don’t just affect mammals. There are effects on zooplankton, crustaceans and molluscs.  Effects include reduced growth and reproduction, some tissue damage, and behavioural change.

Some cyanotoxins can accumulate within invertebrates and move through the food web into fish, birds, and mammals in a process known as bioaccumulation.

So why hasn’t all the wildlife died?

Rivers do not respond to pollution in a simple, all-or-nothing way. Several factors influence what we observe. Firstly, not every bloom is toxic; many cyanobacteria do not produce toxins. Secondly, exposure.differs between species.  A dog may drink a large quantity of contaminated water but a kingfisher may have almost no exposure. Thirdly, animals can escape!  Fish can swim away, birds, dragonflies and damselflies can fly so simply move away from polluted water. Another reason is that species differ in their tolerance with some species being naturally more resistant than others.

Ecological damage is often gradual with pollution removing the most sensitive species first. This means that the river remain remains biologically active but is progressively less diverse.

Conclusion

The recent concerns surrounding Walkford Brook should not be dismissed simply because wildlife is still present.

The available scientific evidence suggests that blue-green algae can poison dogs, livestock, birds, fish and invertebrates.

E. coli is an important indicator of sewage contamination and a significant risk to dogs and people.

Ammonia may be the most important ecological pollutant because it directly affects fish and aquatic invertebrates.

The presence of wildlife does not necessarily indicate a healthy ecosystem. A stream can still support life while undergoing profound ecological change. In many cases, pollution does not create a dead river. Instead, it creates a simpler river dominated by species that can tolerate contamination. That distinction is important when interpreting what is happening in Walkford Brook.