Chlorophyll Sensors: Water Quality Monitoring

Why would I want to measure Chlorophyll?

The measurement and distribution of living microscopic plant matter, commonly referred to as phytoplankton or algae, has been of interest to scientists, researchers, and aquatic resource managers for decades. An understanding of phytoplankton populations and their distribution enables researchers to draw conclusions about an aquatic system’s health, composition, and ecological status.

Chlorophyll is the pigment that makes photosynthetic plants look green.  The amount of chlorophyll present in a water sample indicates the level of photosynthesis.  And the more photosynthesis, the more primary food production.  Without this primary production, there would be little or no aquatic life higher in the food chain.  Thus, chlorophyll measurements are a good indication of the amount of solar energy that has been converted into sources of food for everything from zooplankton to fish and other aquatic animals.

While modest chlorophyll levels indicate good prospects for higher life forms, excessive chlorophyll levels are often the definition or indicator that a water body has become eutrophic.  Eutrophic waters are aesthetically unappealing, produce taste and odor problems in drinking water treatments, and can create fish kills when the photosynthetic organisms die and decay in large numbers.

Changes in long-term chlorophyll trends can signal the need for more detailed chemical study of the water and its contamination sources.

How is Chlorophyll Measured?

Solinst Eureka’s chlorophyll 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 chlorophyll, the chlorophyll absorbs some of the blue light and then emits red light.  More molecules of chlorophyll produce more red light, so the magnitude of the return light is relatable to the amount of chlorophyll present.

Chlorophylls are often divided into chlorophyll a and chlorophyll b, based on differences in their functions.  Chlorophyll a is the “important” chlorophyll.  The chlorophylls fluoresce slightly differently, so Solinst Eureka’s chlorophyll sensor is tuned to chlorophyll a.  However, the success of this tuning varies with the specific speciation in specific cases.

There are three ways to calibrate chlorophyll sensors.  First, you can calibrate to a known concentration of chlorophyll; the sample can be prepared gravimetrically, or can be purchased.  Second, you can calibrate with a solid standard or “cal cube”, 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 Chlorophyll measurement in the field?

Fouling can pose a 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 chlorophyll.  However, fouling should be a problem only in long-term deployments, not in daily surveys. For continuous deployment, anti-fouling accessories may be added, such as Eureka’s universal wiper system, and copper mesh sensor guard.