The river appears to have taken a sudden turn for the worse with slimy green stuff floating on the surface. What is happening to the river and is the floating algae toxic ?
Over recent months, the river has developed a green tinge caused by thread-like green algae growing abundantly while attached to any stationery object in the river – stones, plants, roots. It is particularly in evidence in still waterways like the Beckenham Ponds. Over the recent hotter period, rafts of slimy-looking olive-green algae have formed on the river surface, sometimes trapped by weed beneath, sometimes floating freely. Why has this algae suddenly appeared? Does it denote a problem with the river and is it a toxic form of algae?
Algae in the Beckenham Ponds. This is not a cyanobacterial bloom at present although conditions here appear suitable. This algae is growing because of nutrients in the spring that rises at this point. Farming is almost certainly the root source of most of the nutrients.
What is the green algae? The thread-like green masses are mostly likely Spirogyra (Zygnemataceae) which is a very common algae that grows in relatively clear water that contains excess nutrients where it develops slimy green filaments.
Typically, this green algae grows underwater, but when there is enough sunlight and warmth it produces large amounts of oxygen which can collect as bubbles between the tangled filaments. The tangled masses, buoyed by the bubbles, come to the surface and become visible as slimy green floating mats often involving strands of the pond weed that grows in the river. Exposed to the air and sun, the uppermost algae can die and in hot weather, can start to smell rather unpleasantly as it decomposes.
Why has is suddenly appeared ? Green algae is always in the river but its growth is generally kept in check by the turbidity (rate of flow) of the river. The recent spell of fine weather has reduced inflows and warmed the water.
What causes the excess nutrients in the water? Excess nutrient loading can occur naturally, often through the entry of sediment into the water from erosion. We have had a fairly wet winter with several rain events that brought a lot of sediment out of the Cashmere Valley into the river. However, excess nutrient is also caused by a variety of pollutants found in sewage, stormwater, and fertilizer runoff.
We can look to the leaching of fertiliser and urea into the groundwater from the intensive farming of cows on the Canterbury plains as a source – the river is fed by springs drawing from the shallower aquifers which are most prone to this pollution. In addition, we should also look to the runoff from roads and properties within the city since all of this stormwater ends up in the river and brings with it increased nutrient loadings.
Is this green algae dangerous ? No…you and your pets could eat it if you had a mind to do so and in some countries, it is harvested as part of the diet. However, there is matter likely to be in the water which can be fatal to pets and humans if consumed. Strictly speaking, this dangerous matter is not algae but bacteria – cyanobacteria – but they also live in aquatic environments, get their energy from the sun and produce oxygen just like algae. They even look like algae and until recently were called algae.
What are cyanobacteria and what do they look like ? Cyanobacteria (often called blue-green algae) are microscopic organisms that are naturally occurring and live in a range of waterways, from near-pristine to those more impacted by land use. Cyanobacteria have existed since the very first forms of life on Earth and played a significant role in the development of life as we know it.
In rivers, this cyanobacteria (called benthic cyanobacteria) grows on the bottom of river beds. It appears as thick dark brown or black mats that have a slimy or velvety texture and musty smell. However, in slower-moving waterways, planktonic cyanobacteria are often suspended in the water.
Pond weed reaching the surface of the river is often mistaken for some sort of algae bloom. There is usually green algae attached to the weed and this can form rafts of algae with the weed on the water surface. However, this is generally not toxic although it may smell offensive on warm days.
Most of the year cyanobacteria are present at low levels, don’t impact ecosystem health and are not much of a danger to people and animals. However, they can become a problem when they increase to high concentrations, forming ‘blooms’. Blooms are more common during the summer months, when low rainfall, warm temperatures, the right level of nutrients and more sunlight create an environment where they can thrive. When it blooms, it can make water look cloudy or discoloured – sometimes strangely green or blue or yellow or even pink.
Why are cyanobacteria dangerous when blooming ? Some cyanobacteria are known to produce toxins. Blooms of these particular cyanobacteria therefore create high concentrations of toxins. These natural toxins can be a threat to humans and animals when licked, or eaten, or when water containing the toxins is swallowed, inhaled or comes into contact with skin.
Unfortunately, the level of toxins in a waterway can only be determined by laboratory testing and actual toxin levels in the water are highly variable even over short periods. Environment Canterbury, which is responsible for testing the region’s waterways, takes a conservative risk-based approach that assumes that toxins are present at all times in all blooms of cyanobacteria species that are known toxin producers.
Are there toxic blooms in the Ōpāwaho/Heathcote River ? The Ōpāwaho/Heathcote Riverr is not currently listed by Environment Canterbury as being subject to toxic blooms, but the conditions currently appear to be favourable for these to occur. Environment Canterbury can’t monitor the entire river and blooms can occur quite rapidly. If you see what you believe to be an algae bloom, you can report it (24/7) to Environment Canterbury by phone at 0800 765 588 or alternatively use the Snap Send Solve app.
What is the advice about dogs in the river ? Given the attraction of dogs to blooms of cyanobacteria due to its musty smell, and given that a mere tablespoon of toxic algae if swallowed can be fatal to dogs, let alone children, it would be prudent to keep your pet on a lead and both it and your children out of the river during the warm summer months. Given the high E. coli levels present in the river, particularly after rainfall, the river is currently not recommended for swimming. These levels of contamination are from large numbers of waterfowl but contamination increases following rain events due to the impact of human (sewage overflows) and dog sources.
Can the nutrients be reduced to get rid of the algae ? Short answer: yes. Long answer: It will take commitment by the agricultural industry to change land use practices on the Canterbury plans to vastly and quickly reduce nutrient leaching. Even with fast action, there will be a lengthy delay for resulting improvement. As far as the city is concerned, reducing nutrient inflows will have to focus on reduced transport outputs, improved pre-treatment of stormwater (by means of swales and similar) and reduced use of chemicals and fertilisers on residential land. In other words, what can you do to help? The solution involves all of us.
Check out this useful video about cyanobacteria. It was created by Environment Canterbury.