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.
Division Chrysophyta: This division contains 6650 unicellular species, including golden algae, yellow-green algae, and diatoms.
Class Chrysophyceae (golden algae): Most members are flagellated and are found in both fresh and salt water. The class contains 500 species
Class Xanthophyceae (yellow-green algae): Most are non-motile and are found in fresh, brackish, and salt water. The class contains 550 species.
Class Bacillariophyceae (diatoms): Members do not have a flagella and many are non-motile. Most occur as plankton and are found in both fresh and salt water. The diatoms have a characteristic thin siliceous shell (cell wall). The shells contain minute depressions which form intricate patterns that are used to identify different species. The class contains 5600 living species (Raven et al., 1986).
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)
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.
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.
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.
Class Charophyceae: Algae of this class are either unicellular, few- celled, or filamentous in nature. Spirogyra, a slimy filamentous algae, is a well-known resident of fresh water impoundments (Raven et al., 1986).
Class Ulvophyceae: This flagellated-cell algae occurs only in marine environments.
Class Chlorophyceae: Most green algae belong to this class. Except for a few planktonic marine groups, Chlorophyceae primarily occurs in fresh water (Raven et al., 1986).
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.
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 Recreation, Aesthetics < .025 mg/lLake 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.
Nutrient Nonpoint Sources:
Agriculture:Primary agricultural sources of nitrogen and phosphorus include livestock waste (from barnyards, pastures, rangelands, feedlots, manure land application practices, and uncontrolled manure storage areas); land application of sewage sludge; nitrogenous fertilizers; irrigation return flows; and decomposing plant material (Straub, 1989).
Residential and Urban:Primary residential sources of nutrients include fertilizers applied to lawns and gardens, leaky on-site wastewater disposal systems, and domestic pet excreta.
Other:The combustion of fossil fuels, and industrial and agricultural discharges of N-containing gases, aerosols, and air-borne particles contribute to the atmospheric nitrogen load. Evidence suggests that the atmospheric deposition of nitrogen in water bodies (directly and via rainfall) constitutes a large portion of total nitrogenous inputs to estuarine and marine systems and a somewhat lesser portion of total N inputs to freshwater systems (Paerl, 1993). Additional nitrate and phosphorus sources include excreta from wild animals in surrounding watersheds and excreta from wildfowl that congregate on the water body.
Nutrient Point Sources:
Industries that use nitrogen and phosphorus in manufacturing may release nutrients in effluent water. Nitrates are used in the following processes: meat curing; production of fertilizers, explosives, glass, heat-transfer fluid and heat-storage medium for solar heating applications (Kubek and Robillard, 1990). Phosphorus is used in the following industrial products: toothpaste, detergents, pharmaceuticals, and food-treating compounds. Sewage treatment systems may contribute high levels of nitrogen and phosphorus to a water body, especially during high flow periods.