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Monitoring Chlorophyll A

High levels of Chlorophyll often indicate poor water quality and low levels often suggest good conditions. Annual median Chlorophyll A concentrations in a waterway are an important indicator in the state of an environment. Elevated concentrations of Chlorophyll A can reflect an increase in nutrient loads and increasing trends can indicate eutrophication of aquatic ecosystems.

It is, nevertheless, natural for Chlorophyll A levels to fluctuate over time; they are often higher after rainfall, particularly if the rain has flushed nutrients into the water. Higher Chlorophyll A levels are also common during the summer months due to water temperatures and light levels.

What is Chlorophyll A?

chlorophyll aChlorophyll is any of several related green pigments found in the mesosomes of cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words χλωρός, khloros (“pale green”) and φύλλον, phyllon (“leaf”). Chlorophyll allows plants to absorb energy from light.

Two types of Chlorophyll exist in the photosystems of green plants: Chlorophyll A and B. Chlorophyll A is the most common form of Chlorophyll within photosynthetic organisms and, for the most part, gives plants their green color. Chlorophylls absorb light most strongly in the blue portion of the electromagnetic spectrum as well as the red portion. Conversely, it is a poor absorber of green and near-green portions of the spectrum. Hence Chlorophyll-containing tissues appear green because green light, diffusively reflected by structures like cell walls, is less absorbed.

Monitoring Chlorophyll A

As all phytoplankton have Chlorophyll A, a Chlorophyll sensor can be used to detect these organisms in-situ. In addition to providing immediate data, it can be also be used for continuous or long-term monitoring and recording. In-situ Chlorophyll measurements are recommended in standard methods for the examination of water and wastewater to estimate algal populations. Chlorophyll sensors are also an in-situ method for determining the trophic state (nutrient-rich, stable, or nutrient-poor) of an aquatic system. A high Chlorophyll measurement is an indicator of eutrophication.

“Chlorophyll sensors rely on fluorescence to estimate phytoplankton levels based on Chlorophyll concentrations in a sample of water. Fluorescence can be measured to determine how much Chlorophyll is in the water, which in turn estimates the phytoplankton concentration. These estimates are then used to develop parameter limits for bodies of water.”

Chlorophyll sensors rely on fluorescence to estimate phytoplankton levels based on Chlorophyll concentrations in a sample of water. Fluorescence means that when the Chlorophyll is exposed to a high-energy wavelength (approximately 470 nm), it emits a lower energy light (650-700 nm). This returned light can then be measured to determine how much Chlorophyll is in the water, which in turn estimates the phytoplankton concentration. These estimates are then used to develop parameter limits for bodies of water.

As an example, the New Hampshire Department of Environmental Services provides the following Chlorophyll guidelines for river quality: a Chlorophyll measurement below 7 µg/l is within a desirable range. 7-15 µg/l is less than desirable, while over 15 µg/l is considered problematic, potentially an indicator of an imminent algal bloom.

Algae and Chlorophyll A

Algae are aquatic organisms, encompassing a wide variety of structures, from single-celled phytoplankton floating in the water, to large seaweeds (macroalgae) attached to the ocean floor. Algae can be found residing in oceans, lakes, rivers, ponds and even in snow. Macroalgae are simpler and attach themselves to the seabed without a true root system. Aquatic plants, whether floating, submerged, or emergent (starting in the water and growing out) have specialized parts such as roots, stems and leaves; most plants also have vascular structures (xylem and phloem), which carry nutrients throughout the plant. While algae contain Chlorophyll (like plants), they do not have the specialized structures that higher plantforms have.

Algae can be single-celled (cell-like), filamentous (string-like) or large (plant-like) but are difficult to classify. Most organizations group algae by their primary color (green, red, or brown), but this is a simplification at best. The various species of algae are vastly different from each other, not only in pigmentation, but in cellular structure, complexity, and chosen environment. As such, algal taxonomy is still under debate, and to further complicate the nomenclature, single-celled algae often fall under the broad category of Phytoplankton.

What are Phytoplankton?

Phytoplankton are microorganisms that drift about in water. They are single-celled, but at times they can grow in colonies large enough to be seen by the human eye. Phytoplankton are photosynthetic and have the ability to use sunlight to convert carbon dioxide and water into energy. While they are plant-like in this ability, phytoplankton are not in fact plants!

Phytoplankton can be divided into two classes, algae and cyanobacteria. These two classes have the common ability of photosynthesis, but have different physical structures. Regardless of their taxonomy, all phytoplankton contain at least one form of Chlorophyll (Chlorophyll A) and thus can conduct photosynthesis for energy.

Phytoplankton, both algae and cyanobacteria, can be found in fresh or saltwater. As they need light to photosynthesize, phytoplankton in any environment will float near the top of the water which receives sunlight. Most freshwater phytoplankton are made up of green algae and cyanobacteria, also known as blue-green algae. Marine phytoplankton are mainly comprised of microalgae known as dinoflagellates and diatoms, though other algae and cyanobacteria can be present. Dinoflagellates have some autonomous movement due to their “tail” (flagella), whereas diatoms are current-propelled.

Nutrient influence

An increase in the nutrient concentration of a body of water is called eutrophication. Eutrophication is often an indicator of agricultural runoff, which can raise phosphorus and nitrogen concentrations to very high levels. If there are too many nutrients, the algae will form a bloom, which can be very detrimental to water quality and aquatic health.

“Annual median Chlorophyll A concentrations in a waterway are an important indicator in the state of an environment. Elevated concentrations of Chlorophyll A can reflect an increase in nutrient loads and increasing trends can indicate eutrophication of aquatic ecosystems.”

Eutrophication is caused by an increase in nutrient levels. This can lead to an algal bloom and can cause low levels of dissolved oxygen. While phytoplankton rely on photosynthesis to produce sugar for energy, they still need other nutrients to grow and reproduce. These nutrients are typically phosphorus, nitrogen and iron, though some species also require silicon, calcium and other trace metals. The more nutrients (particularly phosphorus) that are present in a body of water, the more algae and phytoplankton that will grow.

The lack of iron in the open ocean limits phytoplankton growth. Nitrogen and phosphorus are also scarce away from coastlines and can be limiting factors as well. However, ocean circulation can cause an upwelling, which moves deep, nutrient-rich water up into the photic (sunlight zone), replacing the nutrient-depleted surface water. Upwelling, seasonal ice melts and agricultural runoff can all increase nutrient levels, leading to an increase in phytoplankton populations.

Typical freshwater levels

In temperate fresh waters, growth is limited in winter because light and temperatures are low. A large increase in the spring normally occurs as light conditions improve and water begins to mix. In the summer, phytoplankton flourish until the nutrient supply begins to run low. In tropical lakes, the phytoplankton distribution is fairly constant throughout the year and seasonal population changes are often very small. In temperate and subpolar waters, the seasonal fluctuations are normally fairly large. Fluctuations in population also occur if agricultural runoff brings additional nutrients into a body of water.

Chlorophyll detection from Chelsea Technologies

UniLux Chlorophyll AChelsea’s highly sensitive UniLux Chlorophyll A fluorometer for the detection of Chlorophyll A. UniLux is a low cost, miniature fluorometer for monitoring Chlorophyll A. The highly sensitive sensor indicates small changes in Chlorophyll at very low concentrations. Robust ambient light and turbidity rejection ensure suitability in a wide variety of water environments.

The UniLux fluorometer is easy to integrate with monitoring platforms and systems, or combine with our Hawk Hand-held Display and Data Logger for on the spot readings and logging and offers highly sensitive readings to <0.01 μg/L.

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