What is the Right Chlorophyll a Level for Water?

Chlorophyll a is a key indicator of algal biomass and water eutrophication. While there is no universal standard, different countries and organizations have established reference values based on water use.

In China, the surface water quality standard (GB 3838-2002) classifies lakes and reservoirs as follows: oligotrophic (below 2 μg/L), mesotrophic (2–4 μg/L), lightly eutrophic (4–8 μg/L), moderately eutrophic (8–15 μg/L), and severely eutrophic (above 15 μg/L). For drinking water source protection zones, stricter limits apply: primary areas ≤1 μg/L, secondary areas ≤2 μg/L.

The World Health Organization (WHO) does not set a direct chlorophyll a limit for drinking water, but when cyanobacteria dominate, a provisional guideline of 50 μg/L is used to manage toxin risks.

In the United States, standards vary by state and ecoregion. For example, Florida sets a monthly geometric mean of 20 μg/L for freshwaters, while Colorado requires 5 μg/L for reservoirs used as drinking water sources. The US EPA provides ecoregional targets, generally ranging from 12 to 14 μg/L for lakes in the southern and central plains.

According to the OECD, general trophic thresholds for mean chlorophyll a concentrations are: ultra-oligotrophic (<1 μg/L), oligotrophic (<2.5 μg/L), mesotrophic (2.5–8 μg/L), eutrophic (8–25 μg/L), and hypereutrophic (>25 μg/L).

In practice, for clean drinking water sources, chlorophyll a levels below 5 μg/L are desirable to prevent taste, odor, and algal toxins. Levels between 10 and 25 μg/L indicate moderate eutrophication, while values above 25–40 μg/L signal high risk of harmful algal blooms and require management action.