What Blue-Green Algae is and How to Test for it in Water

Blue green algae is a photosynthetic bacteria which grows in both fresh and marine water. Most typically, blue green algae grow in lakes, ponds and slow-moving streams where the water is warm and nutrient-rich. Most species are buoyant so float to the water’s surface, and form layers of scum known as algal blooms.

Toxic blue-green algae is able to multiply because it’s generally not eaten by any other species. Algal blooms can be very noticeable because of the smelly scum that can form on the surface of the water, however, there are not always visible signs of it in water bodies. This is where a blue green algae test comes in.

Aquaread Blue-Green Algae sensors

Aquaread’s blue-green algae sensor is available in two different forms: one to measure phycoerythrin (salt water-PE) and one to measure phycocyanin (fresh water-PC). The sensors are constructed using dense ABS/POM plastics and aluminium. They feature robust sapphire lenses for the highest optical performance and scratch resistance when testing for blue-green algae.

These sensors are fixed response fluorometers, meaning they excite the algae in the water at a fixed wavelength (BGA-PE at 520nm and BGA-PC at 590nm). They then measure the subsequent emitted fluorescence (BGA-PE at >575nm and BGA-PC at >655nm).

Blue-Green Algae

Key Features

  • Dense sapphire lenses are used on our BGA sensor for their high clarity and scratch resistance.
  • Measurement range BGA-PE: 0 – 200,000 cells/ml
  • Measurement range BGA-PC: 0 – 300,000 cells/ml
  • Resolution: 1 cell/ml.
  • Repeatability +/- 10% of reading
  • Simple to install on a wide range of Aquaprobes.
  • Easy 2-point calibration using rhodamine dye.
  • Site specific calibration with grab sample calibration option.

Our BGA sensors can be installed into many of our probes – below are some examples. To see all of the probes that can house the BGA sensor please visit the Products section.

Our BGA sensors can be fitted to many of our probes, below are some examples

To see all of the probes that can house the BGA sensors please visit the Products section.



Advanced portable multi-parameter Aquaprobe.



The Aquasonde-5000 has 4 spare ports. They can house any combination of ISE and Optical sensors.



Single parameter Aquaprobe, for use with any of our sensors.

Simple installation

Installing the BGA sensor into an Aquaprobes is a very simple process. Simply unscrew the blanking plug from an appropriate aux socket, apply some silicon grease to the thread of the sensor (grease provided), and screw in the sensor. After installation, full calibration is required.

Simple calibration

Part of our ethos at Aquaread is “ease of use”, so we have simplified and automated the calibration process as much as possible. The BGA sensor is calibrated at 2 points. A zero point and a 8µg/l rhodamine dye point for BGA-PE or a 100µg/l rhodamine dye point for BGA-PC. A stock solution of rhodamine dye is provided, and a small dilution of the stock is required to make the required concentration.

Once your solution is made, the calibration involves placing the probe into the solution and selecting the correct calibration option from the menu. A calibration report is saved when complete.

Grab sample factor

When taking measurements at a particular site there can sometimes be a difference between measured values with the sensor and laboratory data. These differences are caused by various things as discussed above. It is possible to apply a multiplier to the data obtained from our sensor in order to make it better correlate with lab data for that particular site. To do this we use a grab sample factor. This factor is calculated by dividing the average grab sample value (measured in the lab) by the average value measured by our BGA sensor.

This is a great addition to the software when trying to correlate new data with historic readings from the same site.


Trying to accurately determine the number of cells present using fluorometric sensors is an indirect method of counting, thus not as accurate as direct cell counts that can be performed in a laboratory setting. Various external factors can affect the readings, for example, other microbiological species and compounds that fluoresce at similar wavelengths, differences in the fluorescent response between various species of cyanobacteria, temperature, ambient light, and turbidity.

For these reasons BGA measurement taken directly in the field is intended for qualitative data collection over time, to monitor changes in trend - not quantitative analysis.