Nitrogen
Nitrogen is an essential protein component that is found in the cells of all living things.  Inorganic nitrogen can be found in three forms in a water body: nitrates, nitrites, and ammonia.  Significant sources of nitrates include fertilizer, septic tanks and cesspools, animal waste, and discharge from car exhaust.   Nitrites are easily converted to nitrates by bacteria and are not present long in the water body. A high concentration of nitrites however, can cause a serious illness called brown-blood disease to develop in fish.  Excess nitrates cause algae and other aquatic plants to increase in number.

Phosphorus
Phosphorus is an essential element for plant and animal growth.  Phosphates, an important form of phosphorus, stimulate plant and algal growth.  Phosphates come from fertilizers, pesticides, industry, cleaning compounds, phosphate-containing rocks, and wastes.

Sediments
The term sediments refers to soils and other particulate matter that comes from the surrounding land.  The sediments often contain pollutants and other materials that can affect the health of the water body.

Heavy Metals
Heavy metals are metals with a large atomic number. These metals include such elements as copper, cadmium, lead, selenium, arsenic, mercury, and chromium. Some sources of heavy metals are industry, municipal wastewater, atmospheric pollution, urban runoff, river dumping, and shore erosion.

Toxins
The term toxins refers to a wide range of molecules and compounds that detriment the health of a water body.   It includes oils, fats, and other hydrocarbons, pesticides, herbicides, and other compounds.  They therefore have a wide variety of sources as well.  They can be imbedded in sediments, dissolved in runoff or dissolved in groundwater groundwater.

Water Temperature
The water temperature of a given body can fluctuate with land use, shade, and temperature of feeding water bodies.  It is an important indicator because many aquatic organism can only stand an optimal range of temperatures.

Air Temperature
This indicator is data.

Runoff
Runoff refers to the mass of water that flows from the surface of the surrounding land to a water body.  This indicator is a facilitator for the entrance of other indicators into the system.

Precipitation
Precipitation is the amount of water that falls on the surrounding land area, not what falls onto the water body itself.

pH
pH refers to the number of free hydrogen ions in a liquid.  As organic material decay carbon dioxide forms which in turn lowers pH making the water body more acidic.  Aquatic life can only withstand a small pH range, anything below or above that range can cause failure for eggs to develop, insufficient growth, or death.  pH can also have a synergistic effect on other indicators.  This means that the effect of that indicator or pollutant is magnified by the change in pH.

D.O.
D.O. stands for dissolved oxygen.  This is the concentration of oxygen that is dissolved in the water.  This indicator is important because most aquatic animals are not able to breathe air, they must acquire their oxygen in liquid form.  Oxygen dissolved in the water comes from atmospheric oxygen and oxygen produced by aquatic plants.  Plants also use the DO when it is overcast or at night when they are not able to photosynthesize, leaving less DO for the aquatic animals to utilize.

B.O.D.
The letters B.O.D. stand for Biochemical Oxygen Demand.  This refers to the amount of oxygen needed by microbes to decompose the decaying organic matter in the water body.  This process increases with more organisms in the water body.  It is an aerobic process meaning it uses oxygen and will depleted the amount of DO present.

Saturated D.O.
Saturated D.O. is the maximum capacity of D.O. that the water can hold at that time.  This value is related to water temperature, the lower the temp, the more D.O. the water can hold.

From Hach.com Important Water Quality Factors