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Aquatic biologists use the amount of algae growing in a stream and the types of algae species present as indicators of water quality.

Heavy algal growth caused by nutrient enrichment Heavy algal growth caused by nutrient enrichment


Algae are plants found in all of Philadelphia's streams and rivers, where they are an important part of the aquatic food chain. In small streams, most algae grow on the surfaces of rocks in a slippery layer called periphyton. In large rivers such as the Schuylkill and Delaware, most algae are suspended in the water and called phytoplankton.

Water Quality Affects Algae

Most of Philadelphia's streams have too much algae, which can cause water quality problems. Algae become over-abundant when the water has too much of the nutrients that algae need for growth, a process called nutrient enrichment or eutrophication. Just as nutrient-rich fertilizers help plants grow in our farms and gardens, nutrients in the water cause algae to grow. Most natural unimpaired streams have a healthy balance between nutrients and algae. In developed areas, water pollution causes excessive concentrations of these nutrients (specifically phosphorus and nitrogen) in waterways. Nutrients can come from non-point sources, such as fertilizers, sediment, and natural organic matter in stormwater runoff, or from point sources such as wastewater treatment plant effluent.

Filamentous green algae growing on rock Filamentous green algae growing on rock

Algae Affects Water Quality

Everything that lives in the water, including plants, uses oxygen through the process of respiration, which takes oxygen out of water. In healthy natural streams, water contains plenty of oxygen that is frequently replenished. But when algae become too abundant and the stream biological community becomes unbalanced, oxygen levels can fluctuate. Low dissolved oxygen conditions can occur at night, causing fish and other sensitive organisms to die.

Plot of dissolved oxygen and stream discharge data from Cobbs Creek. Note fluctuations in DO due to heavy algal growth. Plot of dissolved oxygen and stream discharge data from Cobbs Creek. Note fluctuations in DO due to heavy algal growth.

Algae Monitoring

PWD aquatic biologists estimate the biomass of algae on stream surfaces by scraping algae off rocks and measuring the amount of chlorophyll (a green pigment needed for photosynthesis) in the layer of "scum" or biofilm. Since algae contain chlorophyll, while bacteria and other components of the biofilm do not, the amount of chlorophyll present in biofilm allows us to estimate the degree of algal growth. Streams with excessive algal growth are generally more likely to have strong fluctuations in dissolved oxygen and unbalanced aquatic life.

PWD Aquatic Biologist collecting algae from rock PWD aquatic biologist collecting algae from rock

Algae Sampling Methods

We collect and analyze algae using Rapid Bioassessment Protocols (RBPs) recommended by USEPA, described briefly within this section. For more information, refer to the USEPA guidance documents, linked below under Additional Resources

Field Methods

Periphyton is collected from natural substrate particles (rocks) in shallow (~20cm) run habitats. Substrate particles for periphyton analysis are collected by walking transects through the stream along a randomly selected angle until appropriate depth of flow is reached. Biologists then walk heel to toe and select the first rock encountered by reaching down at the very tip of the wading shoe. At each monitoring site, three replicate samples are taken. Each replicate consists of 1-3 rocks, the upper surfaces of which are scraped to dislodge the attached algae. Once the rock has been thoroughly sampled, the surface is carefully wrapped in aluminum foil to measure the surface area that was sampled.

Algae Laboratory Methods

Periphyton samples are brought to the PWD Bureau of Laboratory Services (BLS)and processed in the Wastewater Laboratory using a modified version of EPA Method 445.0. All the material that was scraped off rocks in each replicate sample is added to a laboratory blender and blended to break up large pieces of algae. Additional water is added, and then a small amount of this sample is passed through a very fine (45 micron) glass fiber filter. The filter traps algal cells, which contain chlorophyll.

Filters containing algae are placed in test tubes and ground up with a laboratory tissue grinder. Chlorophyll and other pigments are extracted with an acetone solution and analyzed with a fluorometer. Once the concentration of chlorophyll has been determined, the foil molds of each rock scraped for algae in each replicate sample are scanned to measure the total rock surface area. The final result is an estimate of the amount of chlorophyll per unit surface area in mg/square meter.

Algae Taxonomic ID

Just as ideal growing conditions for garden plants can vary, different types of algae grow better under different water quality conditions. Algae can thus serve as water quality indicators. However, identifying algae requires a high level of training, so these water quality indicators have not been used to the same extent as those using aquatic invertebrates. We have enlisted the help of the Phycology Section of the Academy of Natural Sciences of Philadelphia to incorporate species-level algal community data into our watershed assessments. Lab technicians prepare algae samples, and taxonomists methodically scan, identify, and count algal cells.

Additional Resources

EPA Rapid Bioassessment Protocols for Periphyton

EPA Analytical Chemistry Methods Manuals:Chemical Substances in Marine and Estuarine Environmental Samples

EPA Method 445.0 In Vitro Determination of Chlorophyll a and Phaeophytin a in Marine and Freshwater Phytoplankton by Fluorescence (PDF)

ANSP Algae Image Database