Editor’s note: David Deen is river steward for the Connecticut River Watershed Council.
The small voice just asked, “What are you doing, mister?” The surprise of the child who was not there a minute before is what caused me to drop my collection net into the stream. Sampling a stream for macroinvertebrate life requires focused attention and the child had walked unseen and unheard up to the sample site.
In June the accidental dumping of a chlorine solution into a small stream caused a major fish kill. The accident involved 500 gallons of chlorine solution dumped into the stream when a potable water feed line was cleaned out with a chlorine disinfectant solution. Instead of collecting the cleaning solution for proper disposal once it had passed through the pipe, the solution was discharged directly into the stream. Vermont Fish and Wildlife estimated that some 600 fish were killed.
The South Branch of the Williams River that is a small rocky mountain stream that heads up 10 miles west of the point where it joins the main stem of the Williams River. The stream is friendly and lent itself to being a secret place, being screened by the trees and low bushes from the homes on both banks. Children used the stream on a regular basis. Several stopped by over the sampling period and offered to help complete the work. In talking with them about where to fish it seems “big rock” is the place.
Getting on stream within a day of the incident offered an opportunity to assess the damage to the aquatic insect life at the site of the spill and develop some information on how much time it took the stream to recover from the discharge. Although more sampling visits were scheduled only four samplings were needed to determine how much time it would take the stream’s macro-invertebrate life to recover in richness and diversity to the control site levels.
Working definitions of richness and diversity will be helpful in understanding the results of the field work. Richness is a measure of the total numbers or weight of biological life found in a habitat area. Diversity is a measure of how many different species are found in the habitat area. Of the two, diversity is the most important in indicating the health of a habitat area. Having a lot of a limited number of species does not indicate a healthy habitat.
The samplings were done once each week on the same day at the same time of day. There were four sample sites established. Site A was 50 feet above the discharge point and served as the control site reflecting undisturbed macro-invertebrate life. Site B was 25 feet below the discharge reflecting the maximum effect of the chlorine. Two lower sites 100 and 250 feet below the discharge were sampled to ascertain the effects of the chlorine as distance increased downstream.
The basic in stream sampling tool is a Surber net. The net allows for a 1/10th of a square meter of the bottom to be disturbed with the life forms being swept by the current into a collection net. A measuring tape and meter stick for delineating sample sites, collection pan, test tubes and an 8x magnifying glass were necessary tools to count and identify the species.
The selected sample sites were riffle areas of shallow but swift-flowing water over rocky bottoms in that they support the highest concentrations of life forms. Collection areas were randomly selected on a paper diagram and the net was then walked into those positions. No collection site was used twice. The bottom was disturbed by hand digging. Any life forms adhering to larger rocks were scraped into the net and counted as part of the population in that sample area. Larval, nymphal and other life forms swept into the net were then deposited in the collection pan. A rough sorting by order or family was done streamside. Collected organisms were put into test tubes and later, final identification and sorting was done with the aid of the magnifying glass.
The results showed just how resilient nature proved she is. The chlorine discharged into the South Branch destroyed nearly all of the insect life in the 100 feet below the discharge point based on results of the first field samples. Fortunately the chlorine discharge was one time, short lived and being that chlorine is a volatile gas it did not adhere to the substrate on the bottom of the stream. The field study documented that the insect habitat below the dumping point was re-colonized through nymphs drifting down stream and adults flying upstream to deposit eggs to levels of richness and diversity found at the control site upstream of the dumping point within four weeks of the incident. Now that’s resilient.
