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Chlorophyll a is the green pigment in plants that helps them change light energy into chemical energy through photosynthesis. Chlorophyll a is measured as the number of micrograms per one liter (μg Chl a/L) of water.
Chlorophyll is an important pigment that gives these aquatic plants their green color.
Measuring the concentration of chlorophyll a in water provides an estimate of algal biomass (i.e., amount of phytoplankton), which is an indicator of the productivity of a body of water. Too much or too little overall algal growth, or too much of a certain type of algae (such as cyanobacteria, often called blue green algae), can cause problems in an aquatic ecosystem. If levels of chlorophyll a in a body of water are insufficient, there might not be enough phytoplankton available to sustain the food web. On the other hand, too much chlorophyll a could signal a rapid growth of algae and indicate an algal bloom. One reason an algal bloom presents a problem for aquatic ecosystems is because when algae die they can sink to the bottom and the bacterial decomposition of algae, uses up the dissolved oxygen in water that aquatic organisms need to live.
The many types of microalgae also known as phytoplankton can be viewed under a microscope.
| Aquatic System Trophic Status | Mean Chlorophyll a Concentration (µg/L) |
| Oligotrophic | 0.3–3 |
| Mesotrophic | 2–15 |
| Eutrophic | > 10 |
An oligotrophic water body typically has low productivity and low nutrient input. Under mesotrophic conditions, a water body has moderate nutrient input and productivity. When a water body reaches eutrophic conditions, nutrient input and productivity are high. An excess of algae is often present, which can lead to reduced water clarity, low dissolved oxygen, and, possibly, harmful algal blooms.
The amount of chlorophyll a found in a body of water depends on the water’s temperature and nutrient content, as well as sunlight and wind. Sewage inputs and fertilizer runoff can pollute waterbodies and increase the amount of nutrients, such as nitrogen and phosphorus, present in rivers and lakes. The additional load of these nutrients spurs the growth of algae beyond what is healthy for aquatic organisms. Fish can die due to low dissolved oxygen levels in bottom waters that develop when algae sink to the bottom of a water body and decompose, and the number of aquatic plants can decrease as a result of shading by the algae. What’s more, some blue-green algae can produce toxins that can be harmful to living things.
Nutrient rich run-off from urban and agricultural sources can create an environment for algae to grow in excess.
Phytoplankton play an important role at the base of the great blue heron’s food chain.
Algal blooms along the shoreline can discolor the water.
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