Water Quality Parameters

Dissolved oxygen: The concentration of dissolved oxygen (DO) in the water column provides a direct indication of the river's ability to support aquatic life such as fish and macroinvertebrates. Aquatic plants and bacteria in the sediments remove dissolved oxygen from the water when they respire. Because plants respire mainly at night, the lowest dissolved oxygen concentrations of the day occur in the early morning. During the day, plants add oxygen to the water column through photosynthesis. Both low morning DO concentrations and large changes in DO concentrations over the day (diurnal variation) are damaging to habitat.

Phosphorus: Plants need a balance of three major nutrients to grow: phosphorus (P), nitrogen (N), and carbon (C). Phosphorus is measured as total phosphorus (TP) and ortho-phosphate (ortho-P; soluble inorganic phosphate, the form required by plants). In most fresh waters, the concentration of phosphorus available to plants is low enough that they cannot grow at their maximum rate. But in water bodies where human activities add phosphorus, much greater growth of aquatic plants can occur. Large blooms of duckweed and algae are the result of an excess of nutrients.

Nitrogen: The other major nutrient supporting plant growth, nitrogen, is measured in its various forms as nitrate (NO3) and ammonia (NH3). Nitrate and ammonia are readily absorbed by plants and incorporated into proteins, amino acids, nucleic acids, and other molecules. Although most aquatic plant growth in rivers is limited by the availability of phosphorus, increased nitrogen availability can also lead to algal blooms. While ammonia can be readily utilized by plants, high concentrations of ammonia are toxic to fish and other aquatic life. A second effect of increased ammonia occurs when bacteria oxidize the NH3 to NO3, a process called nitrification, consuming four atoms of oxygen for every atom of nitrogen converted. This process can dramatically lower dissolved oxygen in the water.

Suspended solids: Excess nutrients in the Assabet come not only from wastewater treatment plant effluent, but also from non-point sources such as fertilizer runoff from lawns and golf courses and from sediments eroded at disturbed sites. Total suspended solids (TSS; the amount of silt, clay, organic material and algae in the water) gives us a measure of the inputs from erosion and the solids in effluent.

Conductivity: Conductivity (the ability of the water to conduct a charge) increases with increasing concentrations of charged ions in the water and is a rough indicator of pollutants, such as untreated waste, entering the stream.

pH: pH test measures the H+ ion concentration in water on a scale of 0 to 14, with pH 7.0 being neutral. The addition of nitrogen oxides or sulfur dioxide (combustion by-products) can lower the pH. Algal blooms can significantly raise the pH (the balance between alkalinity and acidity) of the water. Extreme pHs are harmful to all aquatic life; brook trout are particularly sensitive to changes in pH. pH also affects how other pollutants such as heavy metals behave in the environment.

Temperature: Temperature affects the ecosystem in a number of ways: many organisms, especially cool water fish, are sensitive to high temperatures; the solubility of oxygen is lower in warmer water, decreasing the supply of dissolved oxygen; algae, weeds, and pathogenic microorganisms can all grow faster in warmer water.