Skip to main content
Creating a better place

Blue Green Algae in the Lake District

Posted by: , Posted on: - Categories: Uncategorized

Blue-green algae certainly made a name for itself in 2018, to the extent that it hit the headlines and tripped off the tongue of many locals and visitors to the Cumbrian Lake District throughout the summer and autumn of 2018…and is now making a nuisance of itself again this spring of 2019.

Blue Green algae in Derwentwater with shoreline in the foreground and a fence in the water in the background
Blue Green algae in Derwentwater

Naturally occurring in most water environments

To be technical, also known as Cyanobacteria, blue green algae are a naturally occurring photosynthetic organism that range from unicellular and filamentous forms to colony forming species. Some species have the ability to utilise nitrogen from air and water for photosynthesis (to create food), and can be found in most terrestrial and aquatic environments.

They can utilise low levels of both phosphorus and nitrogen meaning that they can out-compete other algae, however their growth rate is usually much slower than others.

Water bodies affected by blue green algae, or algal blooms may be green, blue-green or greenish brown and can produce musty, earthy or grassy odours. Blooms can also cause foaming on the shoreline, which can sometimes be confused with sewage pollution. During a bloom, the water also becomes less clear, blocking sunlight and can slow down plant growth in water.

Blue-green algal blooms are nothing new, as reports date back to the 12th Century. However, in the Lake District, lakes such as Derwentwater and Ullswater have not had reported blooms for several years, in fact, prior to 2018, a bloom hadn’t been confirmed on Ullswater since 1999!

The most notable bloom appeared on Derwentwater in early September 2018, with numerous reports and public concerns - especially as all blue-green algal blooms are potentially toxic to human and animal health. The bloom persisted through to November with confirmed reports in February and now again in May 2019.

Appearing in the sunshine

Their ability to control their own buoyancy means that during bright sunny weather, the algae migrate to the surface layer of lakes and stillwaters. During long periods of settled, sunny and warm weather with little or no wind, blue green algae can multiply to such an extent that ‘blooms’ can form.

In deeper water, under such conditions, the lake will form layers (stratify) with the warmer, denser water lying above the cooler water. Gentle breezes can then move the blue-green algae into bays downwind where the algal bloom is much more visible when it approaches the shoreline.

Once algal numbers are high, the bloom is likely to persist throughout the season, declining only on the onset of winter conditions.

Blooms can also move with the wind, so in a lake such as Derwentwater, a bloom can start in one location, but then move to a new location the next day. In soft breezes, the energy is too light to break the surface so the scum drifts downwind and is deposited in bays and on the shoreline. It does not survive this for long, cells become stressed releasing this contents and blue green pigment into the water and eventually die, leaving the unsavoury, un-aesthetic appearance on the shore. Then if these periods persist, it will take longer to wash away from the shore.

Affected by winter conditions

It is unusual for blue-green algal blooms to persist into the winter, especially in the Cumbrian lakes. The disturbance caused by winter storms, lower temperatures and faster flushing rates mean that this slow growing organism cannot grow to such densities until the conditions become more conducive for growth in late spring/early summer.

The winter of 2018/19 was not typical as far as Lake District winters go. Milder than average temperatures, the lack of significant storms or rainfall, and much lower lake levels following the hot dry summer, allowed the naturally occurring algae to persist in the deeper areas of Derwentwater.

As the lake refilled through autumn and early winter, the algae persisted without being flushed from the system, so that on bright, sunny, still days, reports were submitted to the Environment Agency which were then confirmed.

The onset of storms through March and the cold weather of early April - more akin to winter – would have flushed much of the algae from the system and caused the remaining algae to subside into deeper water.

The more settled and sunny weather in this area during May, has then caused the algae to migrate to the surface of the lake again and form a bloom.

St Herberts Island on Derwentwater
St Herberts Island on Derwentwater

Is it safe to swim or let my dog swim?

Bloom and scum forming blue-green algae can produce toxins. Toxin producing blooms are called Harmful Algal Blooms. These toxins can be harmful to wild animals, farm livestock and domestic pets. In humans, they have been known to cause rashes after skin contact and illnesses if swallowed. Not all blue-green algae blooms and scums are toxic, but you can’t tell just by looking at them, so it’s best to assume they are. People (and pets), therefore should enter the water at their own discretion.

What’s the Environment Agency’s role?

The Environment Agency collects samples when reports are submitted of the presence of blue-green algae on lakes and other waterbodies, and analyses them to confirm its presence.

Notifications of confirmed blooms are then sent to the landowners and other statutory bodies so that signs can be displayed to warn lake users of the presence of potentially toxic blue-green algae.

Once we have a confirmed bloom in a particular location, Environment Agency officers sample every week until we’ve had two clear samples in a row.

Can anything be done to prevent it?

As algal blooms are naturally occurring and require the right conditions to form, there are no quick and easy solutions for reducing their occurrence.

The Environment Agency issues and monitors discharge permits throughout the country which contain strict limits on phosphorus concentrations discharged to the environment. We also work with water companies to ensure that their activities are compliant with their discharge permits as well as advising them on any improvements that they may wish to make.

Over the last twenty years there’s been huge progress in enhancing the water environment thanks to tougher regulation and years of hard work, and there is ongoing work to maintain water quality and improve it even further.

Is there anything I can do?

There are many things which residents and users around the lakes can do to reduce the persistence of blooms in lakes, including:

  • Regular checking and emptying of holding tanks on boats to prevent pollution
  • Reducing detergent use (and using phosphate-free)
  • Reducing fertiliser on lawns, gardens and farmland that can ultimately end up in the lake
  • Checking the condition of septic tank systems to reduce discharge

We always encourage people to reduce phosphate use as this helps improve water quality. You can buy phosphate-free washing detergent and cleaning products from your local supermarket.

It’s important to note though, that although increased phosphorus and elevated nutrients can impact the amount of algae present, it’s not the whole picture. Algae isn’t completely reliant on phosphorous and can utilise low levels of both phosphorous and nitrogen. It’s adept at utilising it from dirty water such as from general land erosion.

As always, it’s important that people remember to report environmental incidents, such as potential blue green algae blooms or pollution, to the Environment Agency incident line on 0800 80 70 60 open 24/7, so we can investigate and take appropriate action to protect people and the environment.

When it comes to protecting our iconic Lake District, we also always encourage people to ‘Check, Clean, Dry’ every time we leave a lake, stream or pond, to help stop the spread of invasive plants and animals in our waters. Freshwater invasive non-native species are able to ‘hitchhike’ on our equipment, footwear, clothing and boats and if they become established in a new waterbody, can often have devastating effects.

This is especially important for those using equipment on our lakes, such as boats, kayaks/canoes or paddle boards, as well as anglers, however, it’s also important for those open water swimming – such as open water swims as part of the Keswick Mountain Festival or triathlons such as the Lakesman.

To protect the health of yourself and your pets from blue green algae, check for signs at all popular access points to your lake/waterbody, check the @EnvAgencyNW Twitter feed for confirmed reports and contact the Lake District National Park or National Trust.

Sharing and comments

Share this page


  1. Comment by Valerie Clark posted on

    Is there blue algee in Bassenthwaite?

    • Replies to Valerie Clark>

      Comment by eileenroffe posted on


      You would need to submit this as an information request by emailing our team at:

      If you notice any blue-green algae in rivers, canals, lakes and coastal waters, you can call our Incident Line on 0800 80 70 60 and we will take a report and investigate further.

      Regards Amy

  2. Comment by Janine Mansfield posted on

    Possible outbreak at Brown How Coniston?

    • Replies to Janine Mansfield>

      Comment by eileenroffe posted on

      Hi Janine, please report any sightings of blue-green algae to our 24hr Incident Hotline on 0800807060. They can ask more questions, create a report and assign an environment officer to investigate.

      Thanks, Joz.

      • Replies to eileenroffe>

        Comment by Janine Mansfield posted on

        A flier was posted up later in the day informing people of the B/G algae and its effects at Brown How.
        Although several people had already been in the water.

  3. Comment by A. posted on

    Is there an up to date site with information about where it is safe for dogs to swim in the Lake District?

    • Replies to A.>

      Comment by eileenroffe posted on

      Hello - We would always recommend that dogs are kept out of water but we appreciate that can be difficult: Here is the latest information we have for blue green algae:

      27-Jun-19 Coniston - Beck Leven SD 30963 95063 Sample clear - no further monitoring required N/A
      28-Jun-19 Scandal Beck, Ravenstonedale NY 72415 04021 Confirmed Bloom - weekly sampling Sampling has ceased as 2 consecutive clear samples
      01-Jul-19 Scandal Beck, Ravenstonedale Follow up sample to bloom of 28-Jun-19 - first clear sample - continue weekly monitoring
      10-Jul-19 Scandal Beck, Ravenstonedale follow up sample to bloom of 29-Jun-19 - second CLEAR sample - Monitoring can now STOP
      10-Jul-19 Windermere - Rayrigg Meadow SD 40177 98465 Sample clear - no further monitoring required N/A

      Best wishes - Eileen

  4. Comment by DM posted on

    We thought we saw blooms on the north of Bassenthwaite, near Armathwaite Hall on 28/07/19.

    Is there a section of your website for the most recent updates?

    • Replies to DM>

      Comment by eileenroffe posted on

      Good Morning

      Please report any blue green algae sightings to our National Incident Team on: 0800 80 70 60 - the latest information we have on our spteadsheet for blue green algae is:

      27-Jun-19 Coniston - Beck Leven SD 30963 95063 Sample clear - no further monitoring required
      28-Jun-19 Scandal Beck, Ravenstonedale NY 72415 04021 Confirmed Bloom - weekly sampling
      01-Jul-19 Scandal Beck, Ravenstonedale Follow up sample to bloom of 28-Jun-19 - first clear sample - continue weekly monitoring
      10-Jul-19 Scandal Beck, Ravenstonedale follow up sample to bloom of 29-Jun-19 - second CLEAR sample - Monitoring can now STOP
      10-Jul-19 Windermere - Rayrigg Meadow SD 40177 98465 Sample clear - no further monitoring required

      Best wished


  5. Comment by Steve Ward posted on

    Is there a website address for up to date information on locations where BGA is.

    • Replies to Steve Ward>

      Comment by eileenroffe posted on

      Hello Steve

      You would either need to submit this as an information request by emailing us at:

      or if its something you have reported to us by ringing our incident team on 0800 80 70 60 you can request feedback and speak with the allocated officer for an update on that particular incident.

      Regards Amy

  6. Comment by Carole Stainton posted on

    Suspected Blue Green Algae in Lily Tarn on Loughrig Fell seen on 10th Sept 2019. phone calls not easy due to poor signal here.

    • Replies to Carole Stainton>

      Comment by eileenroffe posted on

      Hello - please can your email it to us at: for the attention of ICS - please include as much information as possible such s dates first notice, extent - sorry to ask you to do this but the team would appreciate the information - thank you in advance - Eileen

  7. Comment by Nina posted on

    Do you have an update on the blue green algae testing situation at Derwent Water?

  8. Comment by Tony Hanson posted on

    Hi Dan, Could I use your image of blue green algae for an awareness article I am putting together for the golf industry?

  9. Comment by Peter Gosling posted on

    The environment agency must publish the latest blue green algae reports on the web. It is not acceptable to say the public can email or phone to ask. (That's not an efficient use of EA staff time either).

    There are still warning signs up at at Derwentwater on 24 November 2019. But relying on the signs is not safe because they don't cover all the routes to the lake. There were no signs posted on the Cumbria Way from Grange to Derwentwater.

  10. Comment by Stormy Winfrey posted on

    An excellent post, congratulations !!

  11. Comment by Paul Tresca posted on

    This post is really good. We've been struggling with HAB's in our lakes for the last 10 years. We hired several companies to help us get this behind us, but to no avail. Finally we just started working with Clean-Flo in West Chester and their plan seems really solid. Has anyone else worked with them? Here is their website: - If anyone has any info they'd be willing to share, I'd be most grateful!

  12. Comment by Claire posted on


    I have autism and can't phone the number to report algal blooms covering a public pond for a few years. The public pond contains protected newts. And often stinks really bad.

  13. Comment by Mark posted on

    is it feasible to reduce the phosphate content of the waters with soluble iron salts to reduce algal blooms?


Leave a comment

We only ask for your email address so we know you're a real person

By submitting a comment you understand it may be published on this public website. Please read our privacy notice to see how the GOV.UK blogging platform handles your information.