STREAM OF CONSCIOUSNESS: Assessing Water Quality, Part II | stream of consciousness


Editor’s Note: This is a region watershed column by Blyden Potts and guest columnists to spread awareness of the region’s tributaries and the efforts of region volunteers to keep them clean.

Just as there are physical assessments for physical threats to water quality and chemical and/or electronic tests to measure chemical threats, there are also biological assessments, but biological assessments are not just for detecting biological threats such as invasive species, toxic algae, or disease.

Each threat or disturbance has both a cause and one or more potential adverse effects on a waterway. Causes can be physical, chemical or biological, but the impacts are generally related to the ecological health of the waterway. In other words, an impact on the organisms and the relationships between organisms that make up the ecosystem.

Ecological assessments consider biological threats, but more importantly, the impact of any adverse impacts.

A pollution event may last only a few hours, making it difficult to detect without continuous monitoring, but when it harms aquatic organisms, there can be evidence of harm to the life of the organism. If it kills fish or other aquatic species, these losses may persist until migration, or growth and reproduction of surviving organisms replace those lost. When a species becomes extinct, the loss can be permanent. A life cycle assessment “snapshot” is more than a snapshot. There is a sense of impairments from the previous months or years.

Scientists interested in fish diversity, abundance, or growth rates sample fish, often by electrofishing.

Those interested in the overall health of an aquatic ecosystem usually sample aquatic macroinvertebrates.

Macroinvertebrates are at the core of aquatic food webs. Effects on macroinvertebrates indirectly affect fish, birds and species further up the food chain. This makes them a good indicator of impacts on other species.

The sample design and analysis takes into account that different species of macroinvertebrates live in different places in a stream, some on or under rocks, others among plants, in foliage or other debris. Tightly woven, flat edge-kick-nets are placed directly downstream of the site to be sampled and the macroinvertebrates are removed from the site to float with the water into the net, or a net is moved countercurrent through the sample area, scraping through the creek bed, plants and debris to remove and capture as it passes.

Organisms are removed from the mesh into containers for classification and enumeration. Larger species such as mussels and crabs can be identified and counted by eye. To identify and count small species or to distinguish similar species of the same genus, e.g. B. various mayflies or stoneflies, requires a magnifying glass or microscope.

Having an identification guide or reference book is invaluable.

Biologists know the acidity or alkalinity, mineral content, temperature ranges, and pollution levels that each species will tolerate and thrive in. You know the seasonality of macroinvertebrate life cycles. They usually have previous data from the stream that they can use to assess trends in population growth or decline. All this information and detail means that expert analysis can get both very specific and complex, but the core ideas are simple enough for elementary school children to learn. For citizen scientists who want a quick overview of aquatic health, rubrics are available that assign point values ​​for the relative prevalence of different macroinvertebrates based on their sensitivities.

The casual researcher sums the scores as an index measure of stream health. A simple example: see

As a rule of thumb, the greater the diversity of abundant macroinvertebrates, the better the water quality and the healthier the stream.


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