Macroinvertebrate collection

 

Benthic macroinvertebrates are animals without a backbone that can be seen with the naked eye, and have to the ability to cling to bottom surfaces such as rocks, leaves or roots.  They include crustaceans, mollusks and annelids but in many aquatic environments most of the macroinvertebrate community are the larvae of aquatic insects. These communities are important links in the food web between producers (leaves, algae) and higher consumers such as fish, and are key indicators of biological integrity in streams, rivers and wetlands.

Depending upon the stream environment a variety of methods and equipment are used to collect benthic macroinvertebrates from wadeable streams.  In rocky-bottom streams, WV Save Our Streams recommends using a two-pole screen-barrier net (commonly known as a two-pole kick-net) or a single pole rectangular style kick-net (sometimes called surber on a stick).  Both types should be equipped with 500-micron mesh netting.  Two-pole kick-nets require at least two people working together to collect the sample.  Groups may purchase other nets as needed but only the two-types discussed here are recommended for rocky-bottom collections.  D-nets are used for macroinvertebrate collections from low gradient and wetland habitats.  Low-gradient streams and wetlands require a much more intensive collection procedure.  Ultimately, the types of nets used depend upon the goals and objectives of the volunteer monitoring program and the kinds of streams they plan to monitor. 
 
Benthic macroinvertebrates live in a wide variety of aquatic environments.  In lakes, wetlands and large river systems they are common in shallow edge microhabitats along shorelines in tangles of vegetation, roots, and leafs, in gravel shoals or along rocky and undercut banks; some kinds burrow in mud and sand of in shallow flowing water.  In streams and swift-flowing rivers they are more common and diverse in rocky areas (riffles), but are also found in runs, which are sampled when riffles are not present.  The collection procedure described here is designed for rock-bottom streams from riffle habitats.  For more information about the muddy-bottom (low-gradient) collection procedure CLICK-HERE or visit WV Save Our Streams’ Volunteer Manual website.

The number of samples collected depends upon the type and size of the net.  Standard two-pole kick-nets are approximately 3-feet wide, so usually three-samples are adequate.  These types of nets can only be used in riffles and runs.  Other nets such as the rectangular kick-net or D-nets are much smaller and more versatile than the two-pole net.  However, smaller nets require additional samples.  For example, if you use the rectangular kick-net, six to eight samples should be collected instead of three.                                                     

 
Choose the best habitats: Your goal is to collect macroinvertebrates from three different riffle areas. (If the riffles are as wide as the stream then multiple samples could be collected within the same riffle.)  The riffles should have different characteristics (i.e. different composition but mostly cobble and gravel and different velocities).  Often different types of riffles hold different varieties of macroinvertebrates, so to properly assess the biological conditions you need to collect a representative sample.  Of course your choices of habitats will ultimately depend upon what your reach provides.  Once you’ve chosen your location always approach from the downstream end, sampling the site farthest downstream first.  This approach insures that the sample is representative of its location and reduces the chances of biasing your second and third sample.
 
Get into position (place the net): Select an area approximately the same width (or slightly less) than the width of your kick-net.  Most two-pole screen-barrier kick-nets have about 1-meter or 3-foot width. The width of the single-pole rectangular kick-net is about ¼ to ½ meter.  So if you are using a smaller diameter-net you will need to collect more samples (2-3 samples using the rectangular kick-net is equivalent to 1 sample with the screen-barrier kick-net). The net holder will place the net snugly against the bottom of the streambed; rocks can be removed if necessary to make sure you have a close-fit.  Once you are satisfied with the position, line the front of the net with rocks heavy enough to hold the net in-place.  However, be careful to choose rocks that are not too heavy or too wide or high.  Large rocks will damage the net and will influence how the macroinvertebrates flow into the net thus making capture less successful.  The net holder now tips the kick-net backward at about a 45-degree angle from the water’s surface.  This provides greater surface area and more even flow into the net.  If the net is held to high some of the macro-invertebrates will wash around the sides and not be captured in the net.  While holding the kick-net backwards the net-holder must make sure that water does not wash over the top of the net.  Have one 3 ½ to 5-gallon bucket ready before you begin collecting the sample.  The bucket will be needed during sample collection. 
 
Begin disturbing the streambed: The second person approaches the sample area from upstream and determines the approximate sample-size.  Once the area is delineated, the sampler begins disturbing the streambed directly in front of the net.  The process starts with rock rubbing.  First, pick-up all large rocks (cobble size and larger) and inspects them.  You are looking for snails, clams and caddisfly cases.  These animals often cling very tightly to the rocks and are not removed by just a simple rub with the hands or a small brush.  If the rocks have any of these animals, remove them from the rocks and place them inside the bucket.  Move the rocks you picked-up towards front and slightly into the net; brush all sides of the rocks with your hands or a small vegetable-brush to dislodge other clinging macroinvertebrates.  Continue the rock rubbing process until the larger sized stones have been thoroughly cleaned.  If the rocks cannot be lifted from the streambed, simply rub them where they lay.  As much as possible the rock rubbing should proceed from the upstream portion of the sample area towards the front of the net.  After the rocks are rubbed they should be placed aside (outside of the sample area) so they are not rubbed a second time. 
 
Some volunteer programs choose a timed approach to rock rubbing since this is a rather intensive and somewhat time consuming step.  The recommended time frame for adequate rock rubbing is four - six minutes (or less) depending upon the abundance of cobbles and boulders within your sample area.  If you choose a timed approach you should make a note on the survey data sheet and record the time frame you use.  The recommended kick-time is two minutes, so your entire sample should take about six - eight minutes to collect. 
 
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