Small, shallow lakes: warmer, and worth checking for algae

Small lakes and ponds are often the first places that feel swimmable in spring, and in a warm spell they can feel almost like an outdoor pool. Small, shallow water does warm faster. Keep an eye on these lakes for something that can come with the warmth: blue-green algae.

Depth, size and water quality

Check the depth and size of every lake. For big, deep lakes the worry is cold. For small, shallow ones it's water quality.

Ask about algae directly too: whether the lake has ever had blue-green algae, and whether it has any now.

Why they warm faster

The difference comes down to layers. In a deep lake, summer sets up a warm top layer sitting on cold water below, and the two don't mix until autumn. That's covered in why big, deep lakes stay cold below the surface.

A shallow lake doesn't have the depth for that. In their review of lake stratification, Boehrer and Schultze describe lakes that aren't deep enough to form a cold bottom layer. The whole lake behaves like the warm top layer, mixed by strong winds from time to time, or even daily as the water warms by day and cools by night.

Comparing lakes in the same climate, Boehrer and Schultze found that smaller lakes tend to gain heat faster in spring, and that in hot spells they peak a degree or two higher than bigger lakes.

The Outdoor Swimming Society (OSS) puts warm, shallow lakes at the less risky end of outdoor swimming, because they behave much like outdoor pools. Warmer isn't the same as warm, though. RoSPA describes cold water shock as a response to entering water at 15°C or lower, so get in slowly early and late in the season, whatever the size of the lake. There's more in cold water shock.

Warm water and algae

The agencies' descriptions of when blooms happen help explain why small, shallow lakes are linked with blue-green algae.

Natural Resources Wales says blooms form where there are high levels of phosphorus and the other conditions for growth are met, naming light, mixing, flow and temperature. DAERA in Northern Ireland says blooms are favoured by plenty of sunlight, still or slow-flowing water and nutrients, and are most common from May through to September, though they can happen at any time of year. The Environment Agency says scums can form in calm weather.

A large study of 494 lakes in central and northern Europe, led by Richardson and published in Global Change Biology, found that cyanobacteria increased with temperature and with how long water stays in the lake in five of the eight lake types it looked at. The authors also found that lakes responded differently depending on their type, and concluded that a one-size-fits-all approach to managing the risk isn't appropriate.

Warmth is one ingredient, but not the only one. A small, shallow lake in a hot, calm spell gives algae some of what they need. Whether a bloom follows depends on nutrients and on the lake itself, so check every time rather than assuming.

What to look for

The Environment Agency describes affected water as less clear, and green, blue-green or greenish-brown. Scums can look like paint, mousse or small clumps. Natural Resources Wales adds that blooms can cause foam on the shoreline that's sometimes mistaken for sewage, and that some smell musty, earthy or grassy.

You can't tell whether a bloom is toxic by looking at it, so the Environment Agency's advice is to assume it is. Keep pets and children away, and avoid skin contact with the water or the algae.

There's much more on symptoms, dogs and reporting in blue-green algae.

Reeds and weed

Reeds and water weed are part of a small lake's character. They're also something to swim around rather than through.

The OSS advice on weed: if there are weeds, avoid them. If you do get caught, stay calm and slow down. Use a gentle, modified doggy paddle and ease yourself free without kicking or thrashing. Avoid breaststroke, which the OSS says will get you more tangled. If you feel the panic rising, the breathing in panic in the water helps.

The OSS's Outdoor Swimmer's Code also asks swimmers not to pick, uproot or trample plants, including waterweed, which is important for invertebrates. Take care getting in and out so you don't damage the bank, and check, clean and dry your kit before you swim somewhere else.

Before you go

Wild's spot pages show an estimated water temperature band for lakes, worked out from published models, alongside today's weather and recent reports from swimmers who've been in. If a recent report mentions scum or discoloured water, take it seriously. Where a lake is a designated bathing water, the official rating is on the spot page too.

For everything else to check before a lake swim, from getting in and out to permission, see safe lake swimming: the complete guide.

Sources

1. Reviews of Geophysics (Boehrer and Schultze, 2008), Stratification of lakes
2. Outdoor Swimming Society, Is It Safe?
3. Outdoor Swimming Society, The Outdoor Swimmer's Code
4. RoSPA, Open water swimming
5. Natural Resources Wales, Blue-green algae
6. Department of Agriculture, Environment and Rural Affairs (Northern Ireland), Blue-Green Algae
7. Environment Agency, Algal blooms: advice for the public and landowners
8. Global Change Biology (Richardson et al., 2018), Effects of multiple stressors on cyanobacteria abundance vary with lake type

This guide is general information, not a substitute for local knowledge or training. Conditions change: check them on the day, and if in doubt, don't go in. Images on the blog are illustrations, not pictures of specific spots.