Studying the Find
Directions - Studying the Find
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In your survey you are recording two important pieces of information: (1) the diversity of orders or families of macroinvertebrates and (2) the population within each of these groups. Diversity is a strong sign of health, especially if the orders are diverse in those families that are pollution-sensitive. Populations tell us something about the trend in stream health. Increases in numbers of sensitive species may indicate an improved food supply or better water quality. Decreases could be due to seasonal variations or lowering water quality. In tolerant species, a population increase could indicate poor water quality or a change in stream bottom conditions due to sediments. The chart below, "Assessing Impairment," clarifies these distinctions.
Streams can suffer from a variety of problems that may be discovered by consistent stream quality surveys through time. These usually fall into three categories:
- Physical problems. Physical problems may include excessive sedimentation from erosion, street runoff, or a discharge pipe. Sediment may create poor riffle characteristics, contribute to excessive flooding, reduce flow rates, change temperature of the water (which decreases oxygen levels), and smother aquatic life. The result is usually a reduction in the number of all animals in the study area.
Sometimes the physical problems are not in the stream itself but result from changes in the structure of the stream bank. Reduced shading from the riparian vegetation increases water temperature and lowers oxygen levels in the stream. This has an adverse effect on fish populations and sensitive macroinvertebrates. Any substantive change to the stream bank can have an effect on stream health.
- Organic pollution. Organic pollution normally comes from excessive human or livestock wastes or high algal populations due to an increased nutrient load in the stream. Sources of organic pollution include runoff from farms, treated sludge from sewage treatment plants, runoff from impervious surfaces like streets, parking lots, and roofs, leaking septic systems, and excessive fertilizer from lawns or golf courses.
The result of organic pollution is usually a reduction in the number of different kinds of macroinvertebrates in the stream. The organism populations most commonly reduced are shredders like stoneflies or some mayflies, leaving more collectors/scrapers such as netspinning caddis flies, scuds, or lunged snails.
- Toxicity. Toxicity includes chemical pollutants such as chlorine, acids, metals, pesticides, herbicides, and oil. One of the most serious examples is acid mine drainage from old coalmines. This condition leaves the stream clear, clean, and dead. A low level of toxicity generally lowers the variety and numbers of all macroinvertebrates.
| In the case of... | Look for... |
|---|---|
| Increases in diversity and or population of intolerant groups | Improved stream health |
| Decreases in diversity and/or population of intolerant groups |
|
| Little variety of insects but high numbers of each kind, normally of tolerant groups | Enrichment of water with organic material, commonly from effluent, manure spreading, or increased runoff |
| Only one or two kinds of tolerant groups in great abundance (commonly worms or blackflies) | Severe organic pollution (manure or other nutrient pollution) or severe sedimentation |
| A variety of macroinvertebrates but only a few of each kind | Toxic pollution at low levels |
| No macroinvertebrates, but a clean stream | Severe toxic pollution |
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