Ecosystem and Lake Productivity
Chlorophyll Analysis

Laboratory Testing Methods - Lake Productivity Analysis and Trophic Status Monitoring and Interpretation

After filtration - samples preserved and frozen

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The Online University provides educational, training, and analytical services related to estimating and monitoring the trophic status of a lake or primary productivity of an ecosystem. 

Chlorophyll a is the photosynthetic pigment that causes the green color in algae and plants.  The concentration of chlorophyll a present in the water is directly related to the amount of algae living in the water.  Excessive concentrations of algae give lakes an undesirable “pea soup” appearance.  The concentration of chlorophyll a can be used to estimate the trophic status of the lake.

Chlorophyll is a green pigment found in most plants, algae, and cyanobacteria. Its name is derived from ancient Greek: chloros = green and phyllon = leaf. Chlorophyll absorbs light most strongly in the blue and red but poorly in the green portions of the electromagnetic spectrum, hence the green color of chlorophyll-containing tissues like plant leaves.  The pigments are named after the wavelength (in nanometers) of their red-peak absorption maximum. The identity, function and spectral properties of the types of chlorophyll in each photosystem are distinct and determined by the protein structure.

Chlorophyll is vital for photosynthesis, which allows plants to obtain energy from light. Chlorophyll molecules are specifically arranged in and around pigment protein complexes called photosystems which are embedded in the chloroplasts. In these complexes, chlorophyll serves two primary functions. The function of the vast majority of chlorophyll (up to several hundred per photosystem) is to absorb light and transfer that light energy by resonance energy transfer to a specific chlorophyll pair in the reaction center of the photosystems. Because of chlorophyll’s selectivity regarding the wavelength of light it absorbs, areas of a leaf containing the molecule will appear green.


Spectral Analysis of Extract- Lake Monitoring Tools

To measure chlorophyll a concentration, a composite sample of the lake column within the photic zone is collected on a monthly basis during the growing season.  The water sample is “composited” because the purpose is to calculate an average chlorophyll concentration within the photic zone.  The photic zone is where plants (algae and other aquatic plants) have sufficient sunlight to permit photosynthesize. Below the photic zone, there is not enough sunlight for most plants to photosynthesize.  The depth of the photic zone can be estimated using the secchi disk depth.  The integrated sample allows us to examine the water column where phytoplankton live (i.e., the part of the water column with enough sunlight for photosynthesis to occur).

If the composite sample is to be filtered in the laboratory, the sample is placed in a dark bottle and wrapped with aluminum foil and then placed in a cooler.   In the laboratory, a given volume of the sample is filtered using a glass fiber filter.  All of the algae and other suspended particles in the water will collect on the filter paper.  The filter paper is then processed, ground, and leached to extract the chlorophyll.  Once extracted from the protein structure and dissolved into a solvent, such as: Acetone or Methanol, these chlorophyll pigments can be separated using a simple paper chromatography method or a spectral analysis method using a spectrophotometer.

Trophic State Index Equations
(Source: Carlson, R.E., 1997)

TSI = 60 - 14.41 (ln (Secchi Depth)  - Secchi Depth (meters)
TSI = 14.42 (ln (Total Phosphate)+ 4.15   - TP (ppb)
TSI = 9.81 (ln Chlorophyll a) + 30.6   - Chlorophyll a (ppb)


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