DISSOLVED OXYGEN is oxygen in the water that fish and aquatic animals breathe. Levels of 40 to 50 percent are required for fish to survive.

NITROGEN is a nutrient that aquatic plants use for fertilizer. High nitrate levels can cause excessive plant growth and water problems. Levels of 4 ppm or less are considered unpolluted; levels above 40 ppm are considered unsafe for drinking.

pH measures the activity of hydrogen ions in the water. Levels below 7.0 are acidic and levels above 7.0 are basic. A level of 7.0 is considered neutral.

PHOSPHATES are also nutrient fetilizers for aquatic plants. Levels above 0.03 help to cause increased plant growth.

TEMPERATURE is more important than you might think. It affects the amount of dissolved oxygen, rate of photosynthesis, and how sensitive organisms are to pollutants.

TURBIDITY is a measurement of relative clarity in the water. It is caused by suspended matter such as clay, silt, and microscopic organisms. This should not be confused with dark color in the water.