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DEP Home : Water : Biological Monitoring : Biological Monitoring: Benthics
Watch streamside, how benthics are captured and classified during monitoring. Double click the image above to launch a bigger view.
All images courtesy of EcoAnalysts (under contract to USEPA) and Wayne Davis, USEPA.
Benthic = bottom dwelling Macro = visible without a microscope Invertebrates = lack a backbone This class of organisms, which posess the qualities listed above, are found in and around waterbodies during some period of their lives. They are often referred to as aquatic insects. Some live in water for all of their larval or juvenile life stages. Others live on rocks, logs, debris, sediment, and/or aquatic plants during some period of their lives. Benthic macroinvertebrates are used as indicators of of stream health.
Benthic macroinvertebrates are good indicators of watershed health because they:
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Ordinary flatworms (Dugesia or Planaria) are 5-8 mm long and look like an elongated arrowhead. They live in the mud or under decomposing organic debris along stream banks. An abundance of flatworms might indicate organic enrichment.
Sewage worms (Tubificidae) are generally 20-40 mm in length, reddish in color, and resemble ordinary earthworms. Like flatworms, they reside in mud and organic debris, and are tolerant of organic pollution (especially the genus Tubifex). If they are very common, and other invertebrates are scarce, they might indicate pollution.
Leeches (Oligochaetes) are dark brown and leathery, with a grippy-hold at one end to attach to the skin and suck blood. Many are predators to other invertebrates. If abundant, leeches can also indicate organic pollution.
Snails (Physidae or Prosobranchia) build their shells in a spiral. Snails are "right-handed" or "left-handed." You can tell the difference by holding the shell so that its tip is upward and the opening toward you. If the opening is to the right of the axis of the shell, the snail is termed dextral, or right-handed. This type of snail has gills, needs plenty of oxygen to survive, and is very sensitive to pollution. If the opening is to the left of the axis of the shell, the snail is termed sinestral, or left-handed. These snails have lungs and are also known as pouch snails, or Physidae. When common, they indicate organic pollution, but they are sensitive to toxic water conditions. http://rock.geo.csuohio.edu/norp/bmi1.htm#SNAIL: This site requires a special plugin to work - is there another site we can use?
Fingernail Clams (Plececypoda) are cream to light brown and measure less than 0.5 inches. Fingernail clams filter detritus, algae, and bacteria, and are sensitive to pollution.
Isopods (Isopoda) are less than 0.7 inches in length, brown to light gray in color, are flattened and broad-bodied, and have seven pairs of legs. They often indicate poorer water quality.
Amphipods (Amphipoda) have extremely flattened sides and a hump back, and somewhat resemble large "fleas." They have several pairs of legs and can be white or brown but are usually gray. Most are very small, but some can reach 0.5 inches in length. Certain species are sensitive to pollution and others are not.
Fairy Shrimp (Anostraca), an indicator species of seasonal pools, have colorful bodies measuring 0.5 - 2 inches in length. They swim upside-down with their 11 pairs of legs as they filter microbes and detritus. See a video of Fairy Shrimp swimming.
Crayfish (Decapoda or Cambaridae) look like small lobsters. They are decapod crustaceans (ten legs) with a hinged abdomen that curls the tail under the body. A large population may indicate organic enrichment, but in some cases crayfish have no diagnostic value. Learn more about our crayfish monitoring.
Mayfly Nymphs (Ephemeroptera) usually have three tails, and always have a single claw per leg. Their tails may be feathered with hairs or may have short rough spines. Mayfly nymphs grow to be less than 15 mm long, and are sensitive to pollution. They are sensitive to low levels of oxygen in the water, and prefer cool water as it holds oxygen more easily than warm water. They are sensitive to increased chemical pollution and prefer shaded waters.
Stonefly Nymphs (Plecoptera) are characterized by two tails (instead of the Mayfly's three). Varying in length from 6 to 20 mm, stonefly nymphs have six legs that are usually fringed with hairs and have two claws. Their abdomen is about the same size as their thorax, and has gills near each end. Stonefly nymphs are very sensitive to pollution and to fine sediment (silt) in the water, and will become rare as nutrient enrichment increases. They also prefer cool, oxygen-rich water.
Caddisfly Larvae (Trichoptera) can grow up to 20 mm long, and are worm-like with three pairs of legs on the first three body segments and hooks on the last one. Caddisflies are related to butterflies and moths. Some Caddisfly larvae build cases using silk to bind pieces of substrate together, while others use their silk to spin nets to catch their food. The case-builders are slightly more pollution sensitive than the net-spinning caddisflies. They cannot tolerate low oxygen levels and those that break up leaf litter for food require vegetated streams, with trees that overhang the water.
Dragonfly Nymphs (Odonata) are the "teenage" form of a dragonfly, just before it becomes an adult. They tend to have a long abdomen and usually only a short spine for a tail. They are moderately sensitive to habitat disturbance, and can tolerate a moderate degree of organic enrichment.
Damselfly Nymphs (Odonata) are related to dragonflies, but have slender bodies with three long tail-like gills on their end. Damselfly nymphs are also moderately sensitive to habitat disturbance, and can tolerate a moderate degree of organic enrichment.
Dobsonfly Larvae or Hellgrammites (Megaloptera) usually have three pairs of legs, and also have several or many-paired appendages that resemble legs along their abdomen. They are a favorite delicacy of the Largemouth Bass. Hellgrammites are sensitive to pollution.
Water Pennies (Coleoptera) look like small, gray, oblong Frisbees with six tiny legs underneath. They are sensitive to pollution.
Riffle Beetles (Coleoptera) are brown or black, with six jointed legs, and have a hard covering over their bodies. They are moderately sensitive to pollution.
Midge Larvae (Diptera) are the larvae of tiny flies. They are very small, often C-shaped, and have a spastic, squirmy movement. Midge larvae are very tolerant of organic pollution, because they feed on organic particles. They also have a high tolerance for low levels of dissolved oxygen, as they are able to swim to the surface to breathe.
Black Flies (Diptera) are shaped like a little bowling pin, with tiny bristles on their black head used for filtering food. They are tolerant of pollution and low oxygen levels.
DEP collects a sample of benthic macroinvertebrates from the stream and returns to the laboratory for further sub-sampling. Individuals from the sub-sample are identified (usually to the genus level) and enumerated. From these data, metrics are calculated, scored, and then summed to obtain a final Benthic IBI score.
There are a total of eight metrics comprising measures of biological structure and function; each metric is scored either as a one, three, or five. The highest possible final score is 40 (5 x 8). To determine the overall stream condition, benthic IBIs are averaged with fish IBIs.
Taxa richness (Total number of taxa)
Biotic index
Ratio of scrapers (Scrapers divided by (scrapers + filter feeding collectors))
Proportion of Hydropsyche sp. & Cheumatopsyche sp.
Proportion of dominant taxa
Total number of EPT* taxa
Proportion of EPT* individuals
Proportion of shredders
* EPT = taxa that are either mayflies (Ephemoptera), stoneflies (Plecoptera), or caddisflies (Trichoptera); aquatic insects that spend all of their juvenile or larval life stages instream.
To determine a narrative based on a final metric score, the following breaks are used. These breaks are determined by plotting reference stream benthic conditions.
Montgomery County has tabular raw benthic data and benthic narrative summaries from 1994-present for most monitoring sites. Also available are GIS coverages (or maps) showing benthic conditions. Maps can be made to order depending on the request.
STATION Two letter stream code + two letter stream reach code + order (1-4) + reach number (01-99).
DATE The date the sample was taken.
TAXA Benthic macroinvertebrates are identified to family.
[#GRIDS] Number of grids sampled during subsampling routine to estimate benthic population sampled.
[#INDVS] Number of macroinvertebrate individuals sampled from subsampling routine.
EXCLUDE This is a taxa that is automatically excluded from further use in calculating metrics for IBI.
Replicate This is a repeat of another sample.
Station The station field is a nine character code that identifies the station name. The stations are a combination of the two letter code for the watershed+the two letter code for the subwatershed+ the single digit stream order code+ the sequential reach number.
Date The date the station was sampled.
SummaryScore The final IBI summary score (1-5).
Narrative Descriptive word to describe the condition of the stream in relation to reference streams. Narratives are either Excellent (>4.5), Good (3.3-4.5), Fair (2.2-3.2), or Poor (<2.2).
If you are interested in obtaining data or protocols, please contact DEP at askdep@montgomerycountymd.gov. In your request, please provide the following information:
Name, organization (if applicable), phone number, and/or email address Type of data requested Time frame requested Explanation for use of data (helps to personalize the data request) Preferred method of data retrieval (email, CD by mail, FTP, pick up CD or materials from DEP offices)