Chlorophyll a

Photosynthetic pigment found in all green plants, and the main
pigment in algae. The concentration of chlorophyll a is used to
estimate the amount of algae (all phytoplankton in a given area)
in surface water. Measured in micrograms per liter
(µg/L).
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While it is normal to find algae in streams and lakes, high
levels of algae can lead to low levels of dissolved oxygen in a
water body.