Algae are a natural part of freshwater ecosystems and you don’t always need to worry about taking a dip. All over the world, and in no small part thanks to determined toddlers, I have swum in the pea-soupiest of waters, from Seljavallalaug pool in Iceland to the Serpentine and Hampstead Ponds.
Misinformation – in the form of perennial signs about blue green algae – abounds, so here are some basic tips to help swimmers.
Algae is a natural part of a healthy aquatic ecosystem.
Algae grows most during summer, when the water is warmer and there is more sunlight. Sometimes, a combination of factors such as a period of still sunny weather and a high level of nutrients in the water (from surrounding agriculture or towns and villages) can trigger an algae ‘bloom’. A bloom is a rapid increase in algae which can have a negative impact on plants, insects, fish, wildlife and swimmers, by deoxygenating the water and releasing biotoxins. One year I cycled to my go-to reservoir, where the water is always pretty green and algae-rich in summer, to find a large fish gasping in the sunlit shallows and whelks floating on the surface. It looked like deoxygenation without knowing anything about it – the water seemed unhealthy and off-putting. A lot of swimmers feel able to make visual risk assessments before they swim because of warning signs like this: blooms will tend to let you know they are there in some way, water may be scummy or look or smell bad.
Like other algae blue green algae occurs in healthy water bodies around the world, and health effects are not likely from water that is not noticeably suffering a bloom.
However, if there is a high concentration of the algae during a bloom and it is touched, swallowed or inhaled in high levels then it can be toxic, with side effects including diarrhoea, nausea or vomiting; irritation of the skin, eyes and throat; and allergic reactions or breathing difficulties. Water with concentrations at this level will be discoloured, there may be surface scum, and a horrible taste and smell.
Blooms can occur in rivers but tend to be found around the edge of lakes where the water is warmest. They are less likely to occur in mountainous lakes, and more likely in water bodies surrounded by intensively managed farmland or towns.
Blue green algae is not actually an algae, but a type of bacteria, called cyanobacteria.
Blue Green Algae, Mihály Köles
Pond Weed, Ted Balmer
Pond Weed, Ries Bosch
Not all green water is algae in bloom: some might be duckweed, which on closer inspection, looks like small green lentils or tiny green quinoa. Shoreline surface scum may also be tree pollen, which on closer inspection, resembles yellow dust. If you put a long stick into filamentous algae it will drape over it, whereas blue green algae will drip off it like paint.
One for the swim nerds: if you’re not sure what’s in the water, you can do a KILNER JAR SAMPLE TEST, which involves collecting a water sample in a large Mason or Kilner jar (a pint or more) and leaving it to settle in the fridge overnight. Using gloves, collect a sample just below the surface of the water (avoid the surface scum). Fill the jar three quarters full: algae gives off gases that may cause pressure build up that could lead the jar to explode. Wipe the outside of the jar and leave undisturbed overnight. If the algae settles on the bottom, it is likely to be normal algae. If it settles on the top, it may be blue green algae. But 2% of blue green algae don’t float, so there is a margin for error.
Blooms can also clump together to look like mats of green algae. A STICK TEST is useful. Find a long stick which you can put in the water without getting algae on your hands. Slowly lift part of the surface mat out of the water. If the stick comes out draped in stringy silky strands, the mats are probably filamentous green algae, which are not a health hazard. If it comes out looking like it’s dipped in paint, then it could be blue green algae.
There is a type of blue green algae that creates brown mats and foaming edges on the shore, which make water look stagnant and can even stain the water purple and brown. This type would hold together on the stick test, but while I have swum in almost everything, I don’t know anyone who would swim in that!
Photos often show huge blooms, but it’s much more likely that you’ll find small patches of blue green algae along the shore. Some swimmers just move elsewhere in the same lake, where these patches have not formed.
Algae comes and goes so any permanent signage sign warning of blue green algae will not reflect current information. Landowners are expected to assess and warn visitors about blue-green algae, but some take an overcautious approach. They might warn about a whole lake when there are only issues in one area, or not remove signs after the problem has gone. By its nature, algae can fluctuate in a very short time with temperature, wind conditions and other factors. And unfortunately we know that in some places, such as reservoirs or other water bodies where swimming is banned, signs have been put up or left up deliberately to put people off going in.
As an example of what happens in practise, in the Lake District in England if the Environment Agency gets a call to say blue green algae has been spotted, then they will go along and look for the algae if they see a clump along the shoreline then they will test that. Some lakes are big and most of the lake could be clear of blooms: it is the shoreline where algae may have formed or been blown that is tested. If it looks dodgy, then swimmers won’t go in, but if it looks clear and there are no major surface blooms, then locals may. Two days later the bloom may have gone: but often the warning signs are still on display and generally don’t get taken down for a month or so. Historically the EA has taken this approach to warn farmers for cattle and then dog walkers to stop their dogs from lapping up a belly full of green scum. Now the policy is having more effect on swimmers, perhaps it will change – but the EA does not have the resources to constantly test the water in different areas all the time, so signage may be historic and over-cautious.
To pose a health risk to people, there needs to be more than 8ppb (parts per billion), 10 times the concentration of algae that poses an issue to dogs (0.89ppb).
If you are worried about algae while swimming, get out and wash yourself and your kit with warm water and soap as soon as possible. If you swim and develop skin rashes, eye irritation, vomiting, diarrhoea, fever or cold-like symptoms, and muscle and joint pain, go to A&E immediately and mention your fears.
This feature is based on information from The Outdoor Swimmers’ Handbook by OSS Founder Kate Rew. The Handbook is widely available in bookstores. Signed copies are also available from the OSS Shop.