A Citizen's Guide to Understanding and Monitoring Lakes and Streams

Chapter 4 - From the Field to the Lab

Measuring Chlorophyll a in Lakes

Field Sampling Considerations

Due to the fact that algae live primarily near the surface of a lake, chlorophyll a samples are typically collected just below the surface. Collecting a sample at one station near the midpoint of the lake is often adequate for a simple characterization of seasonal changes or possible trends in chlorophyll. If the monitoring objective is to compare portions of the lake, then chlorophyll samples should be collected accordingly. Because algae can be blown by the wind, samples collected near shore or at the downwind end of the lake may not be representative of average lake conditions.

Since algae can quickly reproduce or die, which will change the relationship between live cells and dead cells in your sample, samples need to be preserved in the field. A few drops of magnesium carbonate solution will adequately preserve a 200-mL sample. (NOTE: Be sure the lab doing the analysis is aware that you are interested in the chlorophyll a concentration, not just chlorophyll.)

The density of the algae population will determine how much sample is needed for the analysis. During the winter when populations are low, 1,000 mL (l liter) may need to be filtered to get an accurate reading. On the other hand, in summer months during an algae bloom, the filter may clog before 50 mL have been filtered through it. To ensure the lab has ample water, a 1,000-mL sample should be collected.

Measurement Methods

Chlorophyll a is measured by filtering a known amount of sample water through a glass fiber filter. The filter paper itself is used for the analysis. The filter is ground up in an acetone solution and either a fluorometer or spectrophotometer is used to read the light transmission at a given wavelength, which in turn is used to calculate the concentration of chlorophyll a. Because of the equipment requirements for this test, it is assumed that the filtering and analysis will be done by a professional lab.

QA/QC Considerations

Randomly select 10 percent of the samples for lab replicates. Field variability can be assessed by collecting field replicates for 5 to 10 percent of the samples.

The next section discusses measuring fecal coliform bacteria in lakes and streams.

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Last updated on April 01, 2008