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Aquatic Invertebrates

Scientists collect information about invertebrates living on the stream bottom because they can serve as indicators of whether the stream is healthy or not.

PWD biologist holding a crayfish collected from Tacony Creek


Aquatic invertebrates are small animals, such as insects, crustaceans, mollusks, and worms that live in water. Most invertebrates are found living in the stream bottom among the rocks and gravel. Many types of insects, such as mosquitoes, dragonflies, and mayflies, begin their life cycle in water, but they are perhaps more familiar and more frequently encountered as adults that fly. The stream bottom may appear to be just a bunch of rocks, but in a healthy stream there may be thousands of individual animals and 100 different species living in a single square meter of stream bottom!

Aquatic Invertebrates as Water Quality Indicators

There are many reasons why aquatic invertebrates are useful as water quality indicators:

  • Aquatic invertebrates are relatively easy to collect and identify without a lot of specialized equipment.
  • Like fish, most aquatic invertebrates living in streams get oxygen directly from the water itself, not from the air. When animals that require a good supply of dissolved oxygen are not found, it may indicate types of pollution that take oxygen out of the water.
  • Aquatic invertebrates generally cannot move around very much. Unlike fish, these tiny bugs cannot swim away from pollution.
  • Based on years of study, scientists know which species of invertebrates are sensitive to pollution.

We have been collecting aquatic invertebrates for more than 10 years to evaluate the health of Philadelphia’s streams and rivers. We use techniques recommended by EPA and PADEP to sample wadeable streams (streams that are safe for trained aquatic biologists to walk in). Invertebrates are always sampled in late winter/early spring. We also sample invertebrates from nearby “reference” streams such as French Creek that are known to be healthy, with natural invertebrate communities.

Sampling Methods

We collect benthic macroinvertebrates using the same methods as PADEP for Instream Comprehensive Evaluations (ICE), described briefly within this section. For more information, refer to the PADEP ICE Protocols, linked below under Additional Resources

Field Methods

Macroinvertebrate samples are collected by holding a handheld D-frame net (500-µm mesh) against the stream bottom within a riffle. The stream substrate directly upstream of the D-frame net is then kicked and disturbed for approximately one minute to a depth of approximately 10 cm to dislodge the invertebrates living in and amongst the substrate. The invertebrates and associated debris are washed into the net. This procedure is then repeated at five other riffle locations within the sampling reach. The sample from each station is a composite of six riffle samples preserved in alcohol, which is then returned to the laboratory for subsampling and identification.

Laboratory Methods

Each composite sample contains a large amount of debris and thousands of macroinvertebrates, making it infeasible to pick out and count each one, so a subsampling procedure is used to speed up the process. The entire sample is placed into an 18 x 12 x 3.5 inch pan marked with 28 numbered four-square-inch grids. Four numbered grids are randomly selected from the pan, and the contents are extracted using a four-square-inch circular "cookie cutter," and placed into another identical empty pan. From this second pan, organisms are picked from randomly selected grids or “plugs” until a count of 200 (+/- 20%) individuals are subsampled.

Invertebrates are examined under a dissecting microscope and identified using a dichotomous key. The key is organized by pairs of contrasting statements describing the structures of the invertebrate. For example, The invertebrate has six legs or The invertebrate does not have six legs. The biologist steps through the key, narrowing down the invertebrate's identification. In most cases, the biologist identifies invertebrates to the genus level. Small midge flies and some types of worms are not identified as precisely and are limited to the family or higher taxonomic levels.

Data Analysis

Most of the ways we interpret biological data are based on the idea that healthy streams contain diverse communities with many different types of animals serving different roles within the ecosystem. One of the simplest ways to assess whether a particular stream is healthy is to compare the total number of different types of invertebrates found at a site to another site which is known to be healthy. If the stream being studied has fewer types of invertebrates than the healthy "reference" site, it may indicate that some sensitive species cannot survive at the site or some ecological roles are not being filled. This total number of different types of invertebrates, or taxa richness is one way to analyze the data. Aquatic biologists use a variety of different metrics to assess the biological health of streams.

Additional Resources

PADEP Instream Comprehensive Evaluation Protocols PWD generally performs Benthic Macroinvertebrate assessments in accordance with these guidelines.

EPA Macroinvertebrate Rapid Assessment Protocols

Family-Level Macroinvertebrate Images in the Stroud Water Research Center Schuylkill Project Slideshow