Blue Green Algae Sensors: Water Quality Monitoring

Why would I want to measure Blue-Green Algae?

Blue-green algae is not algae, it is bacteria called cyanobacteria (phycobiliproteins).  It is called algae because it is, like algae, photosynthetic.  In fact, blue-green algae is responsible for over 20% of the earth’s total photosynthesis.  And, like algae, cyanobacteria can be found as individual cells in filamentous colonies.  Cyanobacteria have been found to be a numerically abundant faction of the phytoplankton community. Their roles in primary production, community structure, and spatial and temporal distribution are of interest for numerous scientific studies as well as natural water monitoring. Since chlorophyll fluorescence cannot be used to accurately determine cyanobacterial presence, analyzing phycobilin concentrations is essential for detecting, quantifying, and monitoring cyanobacterial levels.

Despite cyanobacteria’s beneficial role in earth’s oxygen cycle, they often multiply to the point that they exhaust their nutrient source and die in large numbers.  Their subsequent decay can so lower the oxygen level in the water that a fish-kill results. Dying cyanobacteria can emit toxins harmful to humans and other animals.

Changes in long-term blue-green algae trends can signal the need for more detailed chemical study of water bodies and their contamination sources.

How is Blue-Green Algae Measured?

Solinst Eureka’s blue-green algae sensor is a fluorometric sensor.  Fluorescence occurs when a molecule absorbs light energy at one wavelength and then emits that energy at a different wavelength.  Fluorometric sensors emit light at a certain wavelength, and look for a very specific, different wavelength in return.  If you shine a blue light at blue-green algae, the blue-green algae absorb some of the blue light and then emits red light.  More molecules of blue-green algae produce more red light, so the magnitude of the return light is relatable to the amount of blue-green algae present.

There are three ways to calibrate blue-green algae sensors.  First, you can calibrate to a known concentration of blue-green algae; the sample can be prepared gravimetrically or can be purchased.  Second, you can calibrate with a “cal cube” or solid secondary standard, which is an optical device available from Eureka that provides a consistent output for a given type of fluorescence.

Third, you can calibrate with a transfer standard, such as rhodamine.  The latter two methods are indirect, but fast, inexpensive, and practical.

What should I know about Blue-Green Algae measurement in the field?

Fouling is the biggest field problem for fluorescence sensors.  Any foreign material that accumulates on the active surface of the sensor will either reduce the amount of emitted light or received light, or both.  And sometimes the foreign material might provide a false signal by fluorescing at the same wavelengths as blue-green algae. Fouling typically does not pose a problem when profiling or spot checking for daily surveys; only when deployed for extended periods.  For continuous deployment, anti-fouling accessories may be added, such as Eureka’s universal wiper system, and copper mesh sensor guard.