General Information: The term "algae" refers to a wide variety of photosynthetic organisms. Algae range in size from minute phytoplankton to giant marine kelp that may grow to 60 meters long (Raven et al., 1986).

Phytoplankton biomass is usually measured by the amount of chlorophyll-a in the water. Chlorophyll-a is a photosynthetic pigment that serves as a measurable parameter for all phytoplanktonic production. On average, 1.5% of algal organic matter is chlorophyll-a (Raschke, 1993). Thus, if chlorophyll-a levels are known, a manager can estimate the phytoplankton biomass in the water body. High biomass will discolor the water body. The following table illustrates the amount of discoloration that may be expected given a certain level of chlorophyll-a (Raschke, 1993).

Chlorophyll-a level                 
(micrograms per liter)        Degree of water discoloration
       > 10                   Water discoloration
      10 - 15                 Some discoloration; some 
                                 development of algal scums
     20 - 30                  Deep discoloration; frequent 
                                 algal scum formation
       > 30                   Very deep discoloration; 
                                 intense matting of algal scum
Six taxonomic divisions delineate the different types of algae: Divisions
Chrysophyta, Pyrrophyta, Phaeophyta, Rhodophyta, Chlorophyta and Euglenophyta.
 Many divisions are further broken down into classes. 
  1. Division Chrysophyta: This division contains 6650 unicellular species, including golden algae, yellow-green algae, and diatoms.

  2. Division Pyrrophyta (dinoflagellates): Most members of this division are unicellular biflagellates. Most occur as plankton and are found in both fresh and salt water. The dinoflagellates have thick cellulose plates that look like armor. This division has 1000 species.

    Blooms of dinoflagellates may pose health and environmental risks. These blooms are known as "red tides" because they color the sea red or brown. Dinoflagellates secrete a poison that can be toxic when there are many organisms in a small area, such as occurs during blooms (Raven et al., 1986). Many aquatic organisms, especially those in the benthos, may perish (Kennish, 1992)


  3. Division Phaeophyta (brown algae): Brown algae is a multicellular algae that grows primarily in salt water. Sizes range from microscopic to the largest existing seaweed (kelp). The division contains 1500 species.

  4. Division Rhodophyta (red algae): Red algae is a multicellular algae of which 3900 species occur in salt water and 100 species occur in fresh water. Red algae is usually found attached to a substrate.

  5. Division Chlorophyta (green algae): The green algae are very diverse. Most green algae occur in fresh water, although a few groups are marine. Some green algae is found in snow, soil, on tree trunks and in symbiosis with other organisms. This division contains 7000 known species that are either unicellular or multicellular (Raven et al., 1986). Division Chlorophyta is separated into three major classes: Charophyceae, Ulvophyceae, and Chlorophyceae.

  6. Division Euglenophyta (euglenoids): All euglenoids but one, Colacium, are unicellular. Most euglenoids are freshwater organisms. Sizes range from less than 10 micrometers in length to more than 500 micrometers (0.5 millimeters). Division Euglenophyta contains more than 800 species (Raven et al., 1986).
*Note: Blue-green algae is often mistakenly classified as algae because of the chloroplasts contained within the cells. These organisms are actually photosynthetic bacteria. The proper term for this group is Cyanobacteria.
National standards are not currently established for algae populations. States and other levels of government may use narrative language addressing eutrophication or excessive plant growth within their water quality criteria/standards for phosphorus and may use site-specific numerical criteria as well for chlorophyll a, as noted below. Algal blooms should be controlled by decreasing nutrient concentrations in the water body.


Algal production is correlated to the levels of nitrogen (N) and phosphorus (P) in the water. If the N:P ratio in a freshwater system falls below 10:1 by weight, algal growth will usually not occur if it is P-limited. Above a 10:1 N:P ratio in a freshwater system and above a 15:1 - 16:1 N:P ratio in an estuarine system or coastal area, the system will likely experience an algal bloom, the severity of which will be in relation to the excess phosphorus available (Schindler, 1978; Jaworski, 1981). Freshwater systems tend to be phosphorus limited. In those estuarine systems which are nitrogen limited, if the N:P ratio falls below 10:1, the system will likely experience an algal bloom, the severity of which will be in relation to the excess nitrogen available.

Generally, a phosphate concentration of 0.01 mg/l will support plankton, while concentrations of 0.03 to 0.1 mg/l phosphate or higher will likely trigger blooms (USEPA, 1986; Dunne and Leopold, 1978). A high availability of P does not always indicate continued production because the system may become nitrogen limited. Estuarine systems tend to be nitrogen limited.

Numerical Categories:

States, water districts, and EPA regions have established guidelines for the mean growing season levels of chlorophyll-a. Generally, for a water supply impoundment or reservoir (human consumption), a mean growing season limit of less than 15 micrograms per liter is recommended before treatment. For all other water uses, the mean growing season limit should not exceed 25 micrograms per liter (Raschke, 1993).

   Water Supply      < .015 mg/l chlorophyll-a
   Aesthetics        < .025 mg/l
Lake Champlain, Vermont has a chlorophyll-a target of .003 mg/l (3 micrograms per liter)(State of Vermont, 1991). However, in North Carolina, for all water supply impoundments, chlorophyll-a levels may not exceed 40 micrograms per liter at any time. For waters not serving as a water supply, chlorophyll-a may periodically exceed 40 micrograms per liter during the growing season. The following table illustrates how often exceedences should be expected (Raschke, 1993).
 Mean growing season                Percent of samples that 
    chlorophyll-a                      would be expected 
   concentrations                   to exceed 40 micrograms 
(micrograms per liter)                     per liter         
        15                            3.5%  (0.25 day/week)
        20                            9.5%  (0.7 day per week)
        25                           15.3%  (1.1 days/week)
        30                           21.2%  (1.5 days/week)  
Health Effects: Although algal blooms usually pose no direct health risk, certain species produce endo- or exotoxins that may accumulate in edible shellfish and also can have direct health effects. Some dinoflagellates, such as Gonyaulax catenella and Gonyaulax tamarensis, produce paralytic poisons. As a shellfish ingests the dinoflagellates, the poison accumulates in its tissue. Humans ingesting the tainted edible shellfish may contract paralytic shellfish poisoning (PSP), an acute illness characterized by numbness of the lips, fingertips, and tongue. In severe cases, PSP may be fatal (Carmichael, 1981; Kennish, 1992).

Environmental Effects: Although algal blooms usually pose no direct health effects, certain species produce endo- or exotoxins that may be harmful to aquatic life. The euryhaline chrysophyte Prymnesium parvum produces exotoxins (secreted into the water) that may cause mass fish mortality (Carmichael, 1981). Some dinoflagellates, such as Gonyaulax catenella and Gonyaulax tamarensis, produce paralytic poisons that may kill fish if ingested.

An abundance of algae will shade the water below, preventing photosynthetic activity. The worldwide decline of submersed aquatic vegetation is considered a result of reduced light levels attributable to high algal productivity (Dennison et al., 1993). Submersed aquatic vegetation provides food for waterfowl and aquatic life and essential habitat for finfish, shellfish, and other aquatic life in estuaries and along the coast. The decline of this essential habitat impacts the entire ecosystem.

Sources: Algal blooms are usually attributable to nutrient loading of nitrogen and phosphorus to the water system.

  1. Nutrient Nonpoint Sources: 

Mode of Transport: Because most phytoplankton are non-motile cells or are only capable of weak self-propulsion, most transportation occurs via currents within the water body. Transportation from one water body to another may occur if flooding occurs and the cells are carried in overland flow.
Analytical Techniques: (Determination of algal biomass)
  1. Chlorophyll-a Method: Dry-weight organic matter is 1.5% chlorophyll a. To estimate algal biomass, multiply chlorophyll-a content by a factor of 67.
  2. Biovolume Method: For each species, multiply the average algal cell volume (in cubic micrometers) by the number of algal cells of that species per millimeter. For total wet algal volume, add results of all species.
  3. Cell Surface Area Method: For each species, compute average surface area in square micrometers and multiply by the number of algal cells of that species per millimeter.
  4. Displacement Volume Method: An algae sample displaces a known volume of liquid.
  5. Adenosine Triphosphate (ATP) Method: Measures the living plankton biomass. Calculate the concentration of ATP and divide by 24,000 to determine total living plankton biomass as dry weight organic matter.
(APHA, 1992)