This water quality snapshot focused on the four most commonly evaluated aspects of watershed viability: plant nutrients; heavy metals; physical conditions; benthic macroinvertebrate diversity. Phytoplankton at the bottom of the aquatic food chain (as well as stream macrophytes, for that matter) require appropriate levels of nitrogen and phosphorus for growth, although P is usually the most limiting nutrient for phytoplankton. Excessive amounts of these nutrients, from lawn and farm fertilizer run-off, as well as from industrial detergents and fecal matter (often from water fowl, produce algal blooms. After quickly exhausting other resources, the algae undergo a massive die-off and decomposition that robs water bodies of much or all of their dissolved oxygen. Abundant nitrate in drinking water is also harmful to babies. Before the age of six months they have a suite of intestinal bacteria different than that of adults. Some of these bacteria convert nitrate to nitrite, which inhibits oxygen uptake by red blood cells ("blue baby" syndrome).

Outflow from R.L. Martin Lake, showing bacteria growth. – Sample 10

Most aquatic organisms live best within a narrow range of physical conditions and can become functionally or even fatally stressed by out-of-range parameters. Clear waters enhance primary productivity by transmitting the maximum intensity of light at all wavelengths; excessively turbid waters (not caused by the organisms’ bodies themselves) absorb light that would otherwise benefit photosynthesis and raise water temperature as the absorbed light is converted to heat energy. Despite the high specific heat capacity of water, wide temperature swings injurious to aquatic life can be induced by unshaded, shallow depths or thermal pollution from human activities. In addition, there is a natural day-night cycle in the acidity and alkalinity of water bodies. Carbon dioxide (CO2) uptake during diurnal photosysthesis far exceeds CO2 production from respiration and drives the aquatic carbon dioxide-bicarbonate-carbonate equation toward higher water pH as hydrogen ions combine with carbonate and bicarbonate ions to restore the chemical equilibrium. Nocturnal respiration increases the concentration of CO2, which is not taken up immediately by plants. Thus, pH is lowered as hydrogen ions are produced by dissociation of the additional H2CO3 formed when CO2 dissolves in H2O. When there are ample carbonates in the soils surrounding a watercourse, additions of acid or base are well buffered. However, wide swings in pH harmful to aquatic organisms can arise when a lake or stream is poorly buffered and acid soils predominate in the watershed.

It has been recognized in recent years that water quality can be quickly assessed indirectly through the diversity and abundance of benthic macroinvertebrates living in a given stream. Using a sampling protocol that varies from method to method, and identification of taxa at varying levels, a water quality index is produced by assigning high index weight to very pollution-sensitive creatures, medium weight to those that tend to be somewhat pollution-sensitive and low weight to the ones that tend to be rather pollution-tolerant. After calibrating the suites of macroinvertebrates against abiotic water quality indicators for various kinds of streams, an excellent-good-fair-poor type of pollution assessment can be developed, usable by professionals and specially-trained amateur stream-watchers alike.