One of the advantages of using macroinvertebrates in assessing the health of aquatic ecosystems is their ease of collection. Benthic organisms are typically abundant in most streams and can be quickly identified by experienced biologists. Minimal equipment is necessary for collection, and the process can provide a cost effective approach to assessing habitat and water quality in New Hampshire streams and rivers.
An important consideration in sampling macroinvertebrate communities is the time of year that the collection work takes place. Mid summer to early fall is usually best for the program's purposes, since this represents a period when most invertebrates are in the later stages of development and are large enough to be collected and identified with the greatest confidence. Stream conditions are also favorable this time of year as flows are low and more wadeable, posing less risk and making sampling easier.
"Kick" sampling and artificial substrates are the two predominant methods used for sampling benthic macroinvertebrate communities from running waters. Both of these methods provide reliable and consistent data to biologists. Questions of whether the two methods are comparable to one another are debated in stream ecology. Can artificial substrate data be compared to kick net data across several locations? In order to remove this uncertainty, the biomonitoring program tries to use artificial substrates at all "routine" sites if feasible. We often are involved in special projects with different hydrologic characteristics, requiring kick netting. An obvious advantage of kick netting is the immediacy of obtaining the sample. There is no minimum 6-week waiting period for organism colonization. This also makes it the method of choice for special projects.
The Kick Sample
The technique is quite simple. With a net placed downstream, the substrate is disturbed by kicking, rubbing, and tumbling the material on the stream bed. This process dislodges the organisms that are residing amongst the various substrate materials and are caught in the net as they drift downstream. The standard nets used have an 18" rectangular frame with a mesh size of 600 microns. Larger nets can be used, provided enough people are available for securing the net to the bottom, and kicking. Also, if the analysis is to be quantitative, the disturbed area should be measured and constant. The procedure is as follows:
- Looking upstream, toss a one-fifth square meter quadrat into a riffle area that is representative of the dominant substrate material in the stream. Make certain that you are well away from the streambanks to avoid your samples being influenced by streambank habitat.
- Place the kick net on the bottom of the stream directly downstream of the quadrat. Disturb the area within the frame by rubbing stones and stirring up embedded sand and gravel with your hands for one minute.
- While remaining in the middle two-thirds portion of the stream bed, move upstream and toss the quadrat once again. Continue this procedure until a total of five one-minute collections have been completed, ultimately representing one sample collected over a selected one square meter area.
- Empty the contents of the net into a white enamel pan or "wash bucket" and add any organisms that remain attached to the net. Large substrate materials and detritus that have been collected should be cleaned of organisms and returned to the stream.
- The contents remaining in the pan or wash bucket are transferred into a one-quart wide-mouthed canning jar and preserved with 1/3 water and 2/3 ethanol for an approximate 70% mixture.
- Proceed upstream of the first sample collection point and return to Step 1, repeating the process for as many replicates as needed.
Artificial substrates are used when semi-quantitative sampling is desired and when comparisons need to be made among different sites to address water quality related issues. Artificial substrates are also used for sites that are too deep for wading, and in areas where bottom substrates are not suitable for kick netting (such as bedrock slabs).
The colonization of artificial substrates by macroinvertebrates tends to be more selective towards the scraper and collector-filterer communities, and may differ from the resident community. This may occur if the site habitat is varied from the artificial substrate (i.e. ledge river bottom). In these cases, artificial substrates are more representative of the potential for colonization than of the resident community. These factors should be considered in the survey planning stages.
There are numerous designs and materials in use for artificial substrates. The Biomonitoring Program uses rock baskets which are comprised of regionally indigenous bank run gravel ranging in size from 1.5 - 3.0 inches in diameter and are housed in a 6.5 inch diameter cylindrical plastic coated wire basket 11 inches in length. The mesh opening is 1 square inch. One end is hinged so that rocks can be added and removed. Once the basket is filled with rock, the hinged end is closed and fastened shut with a nylon tie. Baskets are placed in stream riffle habitats at depths that cover the artificial substrate by at least 5 inches. Each biomonitoring station uses three baskets that are anchored to the stream bed by driving ½ inch steel reinforcing rod into the stream bed and then attaching the baskets downstream in an array pattern with a loop of nylon coated steel cable.
Substrates are left undisturbed at the site for a period of six to preferably eight weeks in order for adequate colonization to take place. Subsequent retrieval and processing takes place within a couple days of the designated time frame.
Rock basket retrieval is done by approaching the substrate from a downstream direction and placing a sieve bucket (3 gallon bucket with 600 micron mesh covering the bottom) against the stream bottom just downstream of one basket. Debris/algae clinging to the rock basket should be gently removed and discarded and then the basket quickly lifted inside the sieve bucket. Once the rock basket is placed into the bucket and removed from the water both are transported to the stream edge. The protocol for rock baskets processing is as follows:
- Using the nylon-coated steel cable, raise the rock basket above the sieve bucket so the nylon ties can be clipped. This should release all the contents of the basket into the sieve bucket.
- Fill a 5-gallon compound bucket with stream water and submerge the empty rock basket, scrubbing with a soft-bristled brush. This will dislodge any organisms clinging to the wire. Once this is completed, set the basket aside and pour the water from the compound bucket over the rocks in the sieve bucket.
- Fill the compound bucket with fresh stream water. Slowly place the rock-filled sieve bucket in the compound bucket. This nested arrangement should submerge the rocks enough to allow scrubbing and rinsing each rock.
- Get comfortable and begin scrubbing each rock with the soft-bristled brush, covering the entire surface. If the rocks are to be retained, return them to the empty rock basket.
- Once all the rocks have been scrubbed, rinse the sieve bucket containing all the organisms in stream water by flushing water repeatedly up through the sieves. Form the contents into a "ball" on one edge.
- Grasp the balled organisms and debris and transfer them to a jar. Rinse the sieve bucket several times with stream water to get all the visible organisms. Preserve the sample with 70% ethanol and label appropriately.
Sampler substrate material should be thoroughly cleaned and allowed to dry for a sufficient length of time for complete desiccation before reuse.