		Ammonia - Ammonium Ammonium (NH₄⁺) — or its uncharged form ammonia (NH₃) — is a form of nitrogen which aquatic plants can absorb and incorporate into proteins, amino acids, and other molecules. High concentrations of ammonium can enhance the growth of algae and aquatic plants. Bacteria can also convert high ammonium to nitrate (NO₃^-) in the process of nitrification, which lowers dissolved oxygen.

		Blue-Green Algae Blue-green algae (BGA) is actually cyanobacteria, and can range in colors from blues, greens, reds, and black. BGA can reduce nitrogen and carbon in water, but can also deplete dissolved oxygen when overabundant. Monitoring BGA is important because they pose a serious threat to water quality, ecosystem stability, surface drinking water supplies, and public health through toxin production and the large biomass produced in algal blooms.

		Biochemical Oxygen Demand - BOD Biochemical oxygen demand, or BOD, measures the amount of oxygen consumed by microorganisms in decomposing organic matter. BOD also measures the chemical oxidation of inorganic matter (i.e., the extraction of oxygen from water via chemical reaction). A test is used to measure the amount of oxygen consumed by these organisms during a specified period of time (usually 5 days at 20 C). The rate of oxygen consumption is affected by a number of variables: temperature, pH, the presence of certain kinds of microorganisms, and the type of organic and inorganic material in the water.

		Chloride The chloride (Cl-) ion is a component of salt found natuarally in minerals and oceans. Chloride salts are typically soluble in water, and high concentrations of chloride in water bodies can negatively impact fish and aquatic communities.

		Chlorophyll Chlorophyll in various forms is bound within living cells of photosynthetic organisms, such as phytoplankton and cyanobacteria (blue-green algae). The amount of chlorophyll found in a water sample is used as a measure of the concentration of phytoplankton. These measurements contribute to the understanding of the general biological “health” of the system, such as its trophic status or primary production. Chlorophyll measurements can also identify algal bloom events and their effects on water quality and anticipate toxic algal blooms.

		Conductivity Electrical conductivity is an indicator of water quality. Conductivity data can determine concentration of solutions, detect contaminants and determine the purity of water. YSI conductivity sensor measures conductivity by AC voltage applied to nickel electrodes. These electrodes are placed in a water sample (or other liquid), where the current flows through the electrodes and the sample. Current level has a direct relationship with the conductivity of the solution.

		Depth Depth of water is measured by a non-vented strain gauge. A differential strain gauge transducer measures pressure with one side of the transducer exposed to the water and the other side exposed to a vacuum. Depth is calculated from the pressure exerted by the water column minus atmospheric pressure.

		Dissolved Oxygen "We have used YSI products for many years. We still use DO meters purchased in 1975." Every species on our planet depends on water and oxygen. For aquatic species, adequate dissolved oxygen is of prime importance to their continued survival. Since dissolved oxygen levels are directly related to good water quality, the two are highly interdependent. Many factors can affect DO levels, and an understanding of these levels in order to make informed decisions concerning wastewater treatment operations, hypoxic zones, aquaculture facilities or large-scale ecosystems is essential.

		Free Chlorine Free chlorine, or hypochlorous acid, is a strong oxidant and the active species in most drinking water disinfection processes. For quality assurance, the chlorine concentration of treated water is often continuously monitored within the plant and throughout distribution pipelines.

		Water Level Level of water is measured by a vented strain gauge. A differential strain gauge transducer measures pressure with one side of the transducer exposed to the water and the other side vented to the atmosphere. The transducer measures only the pressure exerted by the water column, and does not factor in atmospheric pressure.

		Nitrate Nitrate (NO3-) forms in water when bacteria use dissolved oxygen to oxidize ammonium. Nitrate is mobile and may seep into streams, lakes and estuaries from ground water enriched by animal or human wastes or commercial fertilizers. High concentrations of nitrate can enhance the growth of algae and aquatic plants.

		ORP or Redox ORP is typically measured to determine the oxidizing or reducing potential of a water sample. It indicates possible contamination, especially by industrial wastewater. ORP can be valuable if the user knows that one component of the sample is primarily responsible for the observed value. For example, excess chlorine in wastewater effluent will result in a large positive ORP value and the presence of hydrogen sulfide will result in a large negative ORP value.

		pH pH determines the acid and base characteristics of water. A pH of 7.0 is neutral; values below 7 are acidic and values above 7 are alkaline. Excessively high or low pH levels are often associated with nutrient deficiencies, metal toxicities, or other problems for aquatic life. High pH makes ammonia more toxic. During algal blooms, photosynthesis increases the water pH, especially in stagnant or slow-moving water.

		Photosynthetically Active Radiation - PAR PAR measures irradiance--or the amount of sunlight or ambient light that diffuses through water compared to surface light. PAR focuses on the dynamics of the photic zone, typically 1-5 meters below the surface, and leads to an understanding of photosynthesis, toxic algae blooms, and eutrophication (nutrient loading).

		Rhodamine Rhodamine is a red dye which measures the TOT (Time of Travel) for surface and ground water. It indicates how water moves, and is useful for tracing pollutants, studying aeration and dispersion, as well as waste buildup and flushing in estuaries and bays and waste water retention and flushing in wetlands.

		Temperature Temperature of water is one of its most basic properties, and many other parameters depend on temperature for accuracy. With temperature data, we can monitor thermal loading or discharge and determine changes in the thermocline, which affect the health of aquatic species and organisms. Many aquatic organisms are sensitive to high temperatures. The solubility of oxygen is lower in warmer water, thus limiting oxygen supply.

		Turbidity Turbidity is the measurement of water clarity. Suspended sediments, such as particles of clay, soil and silt, frequently enter the water from disturbed sites and affect water quality. Suspended sediments can contain pollutants such as phosphorus, pesticides, or heavy metals. Suspended particles cut down on the depth of light penetration through the water, hence they increase the turbidity -- or "murkiness" or "cloudiness" -- of the water. High turbidity affects the type of vegetation that grows in water.

		Photometer Parameters Photometers allow you to easily take readings directly in the field for 100+ parameters.