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

Chapter 2 - Lakes

Chlorophyll a in Lakes

Chlorophyll is the green pigment in plants that allows them to create energy from light – to photosynthesize. By measuring chlorophyll, you are indirectly measuring the amount of photosynthesizing plants found in a sample. In a lake water sample, these plants would be algae or phytoplankton. Chlorophyll is a measure of all green pigments whether they are active (alive) or inactive (dead). Chlorophyll a is a measure of the portion of the pigment that is still active; that is, the portion that was still actively respiring and photosynthesizing at the time of sampling.

As described in the previous discussions on DO, pH, nutrients, and Secchi disk depth, the amount of algae found in a lake greatly affects the lake’s physical, chemical, and biological makeup. Algae produce oxygen during daylight hours but use up oxygen during the night and again when they die and decay. Decomposition of algae also causes the release of nutrients to the lake, which may allow more algae to grow. Their processes of photosynthesis and respiration cause changes in lake pH, and the presence of algae in the water column is the main factor affecting Secchi disk readings. Algae, of course, also can cause aesthetic problems in a lake; a green "scum," and rotting scent are common problems associated with high algae concentrations.

Reasons for Natural Variation

Sunlight, temperature, nutrients, and wind all affect algae numbers and, therefore chlorophyll a concentration. During the spring when water begins to warm, the days are sunnier, and nutrients are still plentiful, the first outbreak or "bloom" of algae may occur. As the days become increasingly warmer and sunnier, algae will continue to grow; however, they may soon outgrow the available supply of nutrients. Consequently, the total amount of algae growth may be limited.

Wind also can impact algae populations. A good strong wind may mix the lake, causing an immediate decrease in algae concentrations as they become mixed throughout the water column. On the other hand, the wind also may cause a release of nutrients into the lake system by stirring up nutrients-laden bottom sediments. Then, after the wind dies down, the number of algae and the chlorophyll concentration may increase.

As summer turns to fall and temperature and sunlight decrease, algae concentrations will decrease as well. Often, in deeper lakes where temperature stratification has occurred (see discussion on temperature), there will be a fall algae bloom when the lakes mixes again and nutrients are released to the entire water column.

Algae populations, and therefore chlorophyll a concentrations, vary greatly with lake depth. Algae must stay within the top portion of the lake where there is sunlight to be able to photosynthesize and stay alive. As they sink below the sunlit portion of the lake, they die. Therefore, few live algae (as measured by chlorophyll a) are found at greater depths. Some algae, notably blue-greens, have internal "flotation devices" that allow them to regulate their depth and so remain within the top portion of the lake to photosynthesize and reproduce.

Algae Toxicity

Some algae produce a poisonous toxin. Typically, the amount of toxin produced is too small to have a serious impact. However, if populations of these algae get very dense, the concentration of the toxin can become seriously high. Dogs and farm animals have been known to die from drinking water that contained too many of these algae and their toxin. The algae of concern in this case is a group called the "blue-greens" – named after their particular pigment color. Sometimes, you can identify a "bloom" of blue-greens in your lake or pond by the oily, bluish-green sheen they produce in the water.

Expected Impact of Pollution

As previously described, the most common concern associated with pollution or development of a lake’s watershed is the increase in nutrients to the lake. Since the lack of nutrients is often what limits the number of algae that can grow in lake, the increase in nutrients caused by pollution usually results in more algae. The populations will continue to increase, causing the aesthetic problems described above.

Chlorophyll a is reported in ug/L. There is no Washington State water quality standard for chlorophyll a. Chlorophyll a concentrations for three Western Washington lakes are shown below to provide examples of the range you may expect to measure.

Chlorophyll a concentrations can be used to determine a lake’s trophic status. Though trophic status is not related to any water quality standard, it is a mechanism for "rating" a lake’s productive state. Information on calculating trophic status is included in the interpretation section at the end of this chapter.

The final section of Chapter Two - Lakes discusses fecal coliform bacteria in lakes.

Chapter Four provides information about how to measure chlorophyll a in lakes.

Temperature | Oxygen | pH | Secchi | Nutrients | Turbidity | Chlorophyll | Fecal Coliforms

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Last updated on April 01, 2008