Algal Blooms
What is an algal bloom and why do they occur?
An algal bloom is a rapid increase in the population of algae (microscopic photosynthetic organisms) in a water system. Algae and algal blooms are a natural part of water ecology.
During periods of warm, settled, dry weather, conditions are ideal for the growth of algae, leading to an algal bloom. In recent years, algal blooms have been seen more regularly due to climate change bringing excessively high summer water temperatures and low flowing water, as well as high nutrient levels such as those from agricultural pollution, which increase the chances of algae developing.
What do algal blooms look like?
During a bloom, the water becomes less clear and may look green, blue-green or green-brown, and appear a little like oil drops in the water. There may also be dense, colourful layers on the water's surface. When algal blooms start to breakdown, creamy-brown foam can appear on the water’s surface, and it may give off the smell of rotten eggs or vegetables – which is often mistaken for sewage.
Examples of free-floating microscopic alga (A; Dictyosphaerium), and filamentous algae (B) growing on the riverbed at Ross on Wye in 2022
Are algal blooms toxic and how do they affect aquatic life?
There are many types of algae not all of which are harmful. Some algae can produce toxins that pose a risk to aquatic life, animals and sometimes humans, through direct contact with them, although these are usually blue-green algae which are more commonly associated with still waters rather than watercourses like the River Wye.
Algae can change the water chemistry by fluctuating the levels of pH and dissolved oxygen. Large blooms may also block out sunlight from reaching plants in the water and use up oxygen in the water at night, which can stress or kill fish and contribute to ecological decline in the river, particularly among other aquatic plant communities.
Read our guidance here: Algal blooms: advice for the public and landowners - GOV.UK (www.gov.uk)(External link)
Algal blooms in the River Wye
Our algae surveys and sonde data have not shown any harmful blue-green algae (cyanobacteria) blooms in the Wye. In the Wye two types of bloom are known to occur:
- Free-floating (planktonic) blooms, which are caused by single-celled algae (can be colonial), usually microscopic algae and cyanobacteria.
- Filamentous algal blooms on the riverbed, which are formed by multi-cellular algae, which can smother stones and other submerged plants.
Filamentous algae start to grow in Spring and is most prevalent in the summer months. Most of the filamentous algae seen in the Wye are simply a form of harmless green algae (Chlorophyceae), or brown algae that grow on the riverbed. These types of algae can be unsightly and may interfere with recreational activities but pose no risk to humans. However, they can pose a danger to fish as they use up oxygen overnight which can lead to fish kills.
During the hot summer of 2022 it appeared that many fish kills in the Wye were related to high water temperatures rather than algal blooms. During this period some free-floating algae (phytoplankton) blooms were identified from samples collected, but these were green algae or diatoms. Whilst harmless to humans, some diatoms can cause irritation to fish’ gills.
To find out more about the types of monitoring we do, visit the ‘Science and Data’ section.
Read our blogs on algal blooms:
Telling the difference between an algal bloom and sewage - Creating a better place (blog.gov.uk)(External link)
Understanding algal blooms in the River Wye(External link)
Examples of free-floating microscopic alga (A; Dictyosphaerium), and filamentous algae (B) growing on the riverbed at Ross on Wye in 2022
What are Diatoms and how do they affect fish?
Diatoms are a type of single-celled algae that possess intricate silica shells called frustules. Some species are free-floating, but most freshwater species are attached to hard surfaces such as stones or plants. The colonial diatom Melosira forms slimy brown growths that can smother the riverbed and submerged plants, especially in the spring and during hot weather. It is unclear if Melosira benefits from increased nutrients, increased temperatures, or a combination of the two, and further research is needed. Some diatoms have spines or other protrusions on their frustules. When diatoms die and their frustules break down, the spines can be released into the water.
In certain cases, these spines can irritate the gills of fish and other aquatic organisms. The delicate tissues in the gills may be sensitive to physical irritation, and the spines can cause discomfort or damage to the gill structures. This can potentially interfere with the fish's respiratory process.
It is important to note that diatoms themselves are not toxic in the sense of producing harmful substances or toxins. The irritation caused by the spines is mechanical rather than chemical. The overall impact on fish health would depend on factors such as the concentration of diatoms, the species of diatoms present, and the tolerance of the fish species to this type of irritation.
Diatom Stephanodiscaceae under a microscope (1000x magnification)
Macroinvertebrates
Aquatic macroinvertebrates are small organisms without a backbone that are visible to the naked eye and live in freshwater, these include snails, worms, crayfish, and shrimp that spend part, or all their life in water. They play a large role in freshwater ecosystems by recycling nutrients, as well as being a food source for other wildlife. Understanding national trends of macroinvertebrates help us to evaluate the effects of environmental pressures such as climate change and pollution on the habitat.
Macroinvertebrates are a well-studied set of organisms with a known response to water quality issues. This is the main reason why we routinely monitor them in our rivers. Most macroinvertebrates do not tend to move far, and because of this they are good at showing the effects of water quality and habitat alteration over time. This means they are good indicator of overall river quality.
Our data shows that the Wye supports a highly significant aquatic macroinvertebrate community in terms of biodiversity and conservation value. Some of the many rare species found include Riolus nitens, Stenelmis canaliculata, Potamanthus luteus, Ephemera lineata, and Leptocercus interruptus. These species can only be found in a very small number of large rivers in Britain.
Macroinvertebrate species with high conservation value present in the Wye, a)Riolus nitens b)Stenelmis canaliculata c) Potamanthus luteus d) Ephemera lineata; e) Leptocerus interruptus (Not to scale with each other) To find out more about the types of monitoring we do in the Wye, visit our Citizen Science and Monitoring page.
Algal Blooms
What is an algal bloom and why do they occur?
An algal bloom is a rapid increase in the population of algae (microscopic photosynthetic organisms) in a water system. Algae and algal blooms are a natural part of water ecology.
During periods of warm, settled, dry weather, conditions are ideal for the growth of algae, leading to an algal bloom. In recent years, algal blooms have been seen more regularly due to climate change bringing excessively high summer water temperatures and low flowing water, as well as high nutrient levels such as those from agricultural pollution, which increase the chances of algae developing.
What do algal blooms look like?
During a bloom, the water becomes less clear and may look green, blue-green or green-brown, and appear a little like oil drops in the water. There may also be dense, colourful layers on the water's surface. When algal blooms start to breakdown, creamy-brown foam can appear on the water’s surface, and it may give off the smell of rotten eggs or vegetables – which is often mistaken for sewage.
Examples of free-floating microscopic alga (A; Dictyosphaerium), and filamentous algae (B) growing on the riverbed at Ross on Wye in 2022
Are algal blooms toxic and how do they affect aquatic life?
There are many types of algae not all of which are harmful. Some algae can produce toxins that pose a risk to aquatic life, animals and sometimes humans, through direct contact with them, although these are usually blue-green algae which are more commonly associated with still waters rather than watercourses like the River Wye.
Algae can change the water chemistry by fluctuating the levels of pH and dissolved oxygen. Large blooms may also block out sunlight from reaching plants in the water and use up oxygen in the water at night, which can stress or kill fish and contribute to ecological decline in the river, particularly among other aquatic plant communities.
Read our guidance here: Algal blooms: advice for the public and landowners - GOV.UK (www.gov.uk)(External link)
Algal blooms in the River Wye
Our algae surveys and sonde data have not shown any harmful blue-green algae (cyanobacteria) blooms in the Wye. In the Wye two types of bloom are known to occur:
- Free-floating (planktonic) blooms, which are caused by single-celled algae (can be colonial), usually microscopic algae and cyanobacteria.
- Filamentous algal blooms on the riverbed, which are formed by multi-cellular algae, which can smother stones and other submerged plants.
Filamentous algae start to grow in Spring and is most prevalent in the summer months. Most of the filamentous algae seen in the Wye are simply a form of harmless green algae (Chlorophyceae), or brown algae that grow on the riverbed. These types of algae can be unsightly and may interfere with recreational activities but pose no risk to humans. However, they can pose a danger to fish as they use up oxygen overnight which can lead to fish kills.
During the hot summer of 2022 it appeared that many fish kills in the Wye were related to high water temperatures rather than algal blooms. During this period some free-floating algae (phytoplankton) blooms were identified from samples collected, but these were green algae or diatoms. Whilst harmless to humans, some diatoms can cause irritation to fish’ gills.
To find out more about the types of monitoring we do, visit the ‘Science and Data’ section.
Read our blogs on algal blooms:
Telling the difference between an algal bloom and sewage - Creating a better place (blog.gov.uk)(External link)
Understanding algal blooms in the River Wye(External link)
Examples of free-floating microscopic alga (A; Dictyosphaerium), and filamentous algae (B) growing on the riverbed at Ross on Wye in 2022
What are Diatoms and how do they affect fish?
Diatoms are a type of single-celled algae that possess intricate silica shells called frustules. Some species are free-floating, but most freshwater species are attached to hard surfaces such as stones or plants. The colonial diatom Melosira forms slimy brown growths that can smother the riverbed and submerged plants, especially in the spring and during hot weather. It is unclear if Melosira benefits from increased nutrients, increased temperatures, or a combination of the two, and further research is needed. Some diatoms have spines or other protrusions on their frustules. When diatoms die and their frustules break down, the spines can be released into the water.
In certain cases, these spines can irritate the gills of fish and other aquatic organisms. The delicate tissues in the gills may be sensitive to physical irritation, and the spines can cause discomfort or damage to the gill structures. This can potentially interfere with the fish's respiratory process.
It is important to note that diatoms themselves are not toxic in the sense of producing harmful substances or toxins. The irritation caused by the spines is mechanical rather than chemical. The overall impact on fish health would depend on factors such as the concentration of diatoms, the species of diatoms present, and the tolerance of the fish species to this type of irritation.
Diatom Stephanodiscaceae under a microscope (1000x magnification)
Macroinvertebrates
Aquatic macroinvertebrates are small organisms without a backbone that are visible to the naked eye and live in freshwater, these include snails, worms, crayfish, and shrimp that spend part, or all their life in water. They play a large role in freshwater ecosystems by recycling nutrients, as well as being a food source for other wildlife. Understanding national trends of macroinvertebrates help us to evaluate the effects of environmental pressures such as climate change and pollution on the habitat.
Macroinvertebrates are a well-studied set of organisms with a known response to water quality issues. This is the main reason why we routinely monitor them in our rivers. Most macroinvertebrates do not tend to move far, and because of this they are good at showing the effects of water quality and habitat alteration over time. This means they are good indicator of overall river quality.
Our data shows that the Wye supports a highly significant aquatic macroinvertebrate community in terms of biodiversity and conservation value. Some of the many rare species found include Riolus nitens, Stenelmis canaliculata, Potamanthus luteus, Ephemera lineata, and Leptocercus interruptus. These species can only be found in a very small number of large rivers in Britain.
Macroinvertebrate species with high conservation value present in the Wye, a)Riolus nitens b)Stenelmis canaliculata c) Potamanthus luteus d) Ephemera lineata; e) Leptocerus interruptus (Not to scale with each other) To find out more about the types of monitoring we do in the Wye, visit our Citizen Science and Monitoring page.