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Macroinvertebrates

Aquatic macroinvertebrates are those that are large enough to be seen by the naked eye and commonly includes insect larvae, various arthropods, snails, and clams. In streams, the species composition and abundance of macroinvertebrates has long been used as indicators of pollution status.

There are a number of ways to gather macroinvertebrates. One is to use a sediment sampler (Ekman dredge or strong dip net for instance) to collect your sample. In EPA work, the primary sampling equipment used for the collection of benthic macroinvertebrates is the modified Hester-Dendy multiple-plate sampler. The sampler is constructed of 1/8 inch tempered hardboard cut into three inch square plates and one inch square spacers. A total of eight plates and twelve spacers are used for each sampler. The plates and spacers are placed on an eyebolt so that there are three single spaces, three double spaces, and one triple space between the plates. The total surface area of the sampler, excluding the eyebolt, is 145.6 in2.

Samplers placed in streams are tied to a concrete construction block which anchors them in place and prevents the multiple-plates from coming into contact with the natural substrates. In water deeper than 4 ft, a float is attached to the sampler to keep them within four feet of the surface. Samplers are exposed for a 4-6 week period before collection.

Sample collection – establishing transects

    For both macroinvertebrate and macrophyte sampling (next section) we will establish transects along the long axis of each pond. This will allow us to space our samples and to revisit sample locations if necessary.

Procedure

1. We will use a technique that is combination of qualitative and quantitative. Each group should have at least one dip net, several buckets, and an Ekman dredge.

2. Ideally, macroinvertebrates are sampled in the sublittoral region of the lake, that transitional area between submerged macrophytes and the profundal sediments. However, in small, shallow ponds, sufficient light is often available to support macrophyte growth across the entire bottom, so a classic sublittoral region does not always exist.

3. Qualitative : Use the dip net to sample at least 3 locations around the periphery of the pond as demonstrated by your instructor. Try to avoid locations that are easily accessible by the public since these tend to be highly disturbed areas. Qualitative sampling with a dip net can be completed from shore.

4. Quantitative : Use the Ekman dredge to collect a series of quantitative samples from along the transect. The number of samples to be collected depends on the size of the pond. For our farm ponds, collecting samples every 20 m is usually sufficient.

    a. When you have established your first sampling location along the transect, prepare the sampler by hooking one cable loop to one Twin-Pin pin. Repeat for the second cable loop. CAUTION : At this point, the dredge is considered armed and dangerous. Keep fingers away from the open ends of the scoop because the spring release mechanism is very powerful.

    b. Slowly lower the sampler so that it will descend vertically.

    c. On arrival at the bottom, allow the dredge to settle. Release the messenger, holding the line with just enough tension to keep it straight.

    d. Bring the dredge to the surface with moderate steady speed. Empty the contents into a bucket.

5. Back in the lab, process your sample by pouring it through a series of sieves and rinsing thoroughly with tap water. Once cleaned up, pour some of your sample into an enamel pan and sort the macroinvertebrates into major groups such as dragonfly larvae, beetle larvae, adult beetles, amphipods, isopods, etc. Try to differentiate into species, and we will attempt to identify as many as possible. Live samples can be returned to the pond of origin, preserved samples can be stored in 70% alcohol.