Cyanobacterial blooms and chlorophyll-a - how to monitor them
In summer, when it turns warm and windless over a lake, a question returns that every bathing-site keeper and every water researcher knows: is this a bloom yet? People who watch the water regularly - monitoring staff, lifeguards, naturalists, lakeside communities - learn to read the early signs: streaks on the surface, a greenish “scum”, a sudden drop in transparency. Their vigilance really matters, because cyanobacteria can produce toxins, and the safety of bathers depends on a sound assessment.
It is a rewarding subject of observation, because it combines simple looking with a concrete measurement - and because it so clearly depends on the weather. Cyanobacteria like warm, calm, sunlit water rich in nutrients. Anyone who monitors them systematically, setting observations against atmospheric conditions, begins to predict the risk instead of merely reacting after the fact. It is very satisfying, useful work.
What and how we observe
Monitoring combines several simple paths:
- Visual observation - streaks, scum, water colour, smell; noted descriptively and with a photo.
- Transparency (the Secchi disk) - a sudden drop can be an early signal.
- Chlorophyll-a - an indicator of algal biomass; measured with a fluorometer in the field or determined in a laboratory (after extraction). Dedicated sensors also detect phycocyanin - a pigment characteristic of cyanobacteria.
What drives a bloom - i.e. why the weather
Cyanobacteria are favoured by: high water temperature, plenty of nutrients (phosphorus, nitrogen), calm and sunny weather, and stratification (no mixing). Wind, in turn, can “push” the scum into one corner of the lake. That is why the measurement gains a lot when you set it against temperature, sunshine and wind - here the weather is a co-author of the result.
How to take the measurement - step by step
- Set up fixed sites (including typical scum-accumulation spots) and a fixed time.
- Note the visual observation (description + photo) and the transparency.
- Measure chlorophyll-a / phycocyanin (sensor) or take a sample for laboratory determination.
- Record the conditions: water temperature, wind, cloud cover - they explain the picture.
- Remember that a bloom can be patchy - wind moves it within hours.
What the result means
Rising chlorophyll-a, falling transparency and intensifying visual symptoms are the picture of a developing bloom. The greatest value lies in the seasonal trend set against the weather: that is when you can see how a run of warm, windless days translates into the water - and you can react in advance.
The most common mistakes
- A single point and time - a bloom is patchy and mobile; measure at several places.
- Skipping the weather conditions - without them the dynamics are hard to understand.
- Visual assessment only - it is worth supporting it with a measurement (chlorophyll/transparency).
How often
In the bathing season and during favourable weather - more often (even several times a week in risk periods), otherwise in a steady rhythm. What counts is the repeatability of sites and time.
Sources and further reading
Monitoring of bathing waters and lake-status assessment is run by public services (e.g. GIOŚ and sanitary-epidemiological stations in Poland); it is also worth reaching for guidelines on cyanobacteria and methods for determining chlorophyll-a.
How to make it easier
Bloom monitoring is by nature data from many sources at once: observation, transparency, chlorophyll - plus the weather, without which the picture is incomplete. Combining it all manually is a tedious puzzle.
LimnoLog assembles it into one picture - and here connectors help:
- you record observations and measurements (also your own indicators: visual assessment, chlorophyll, phycocyanin) right in the field, also offline;
- a connector automatically pulls weather data from public APIs (e.g. IMGW-PIB) and saves it alongside your measurements - temperature, wind, sunshine on the same timeline;
- so you see the bloom and the weather together - a run of warm, windless days and the lake’s response form a clear pattern that helps predict the risk;
- you can share the results via a public link (useful for bathing sites and communities) or export them.
The same system covers, alongside cyanobacteria, transparency, oxygen, temperature and any other indicator - a full picture of the lake in one place.
← All methodsEarly users get free access for now and keep their features for good. If you watch over your lake - take a look and try it.