Mississippi

Alkalinity measures the water's ability to neutralize acids, acting as a natural buffer that helps keep pH stable and protects aquatic life from sudden chemical shifts. Waters with low alkalinity offer little protection against acidification from acid rain or mining runoff, leaving sensitive species like fish eggs and invertebrates vulnerable even when other conditions seem fine. The U.S. Environmental Protection Agency sets a minimum criterion of 20 mg/L to protect aquatic life, though where natural alkalinity is already below that level the natural concentration becomes the standard, and where it exceeds 20 mg/L human activities should not cause it to drop below 25% of the natural level or 20 mg/L, whichever is higher

Chloride is a naturally occurring ion found in most water bodies, but concentrations have been rising in many freshwater systems due to road salt, agricultural runoff, urban drainage, and wastewater discharge. At elevated levels, it disrupts the ability of freshwater organisms to regulate water and salts across their cells, with invertebrates generally more sensitive than fish. The U.S. Environmental Protection Agency (EPA) sets the long-term freshwater criterion at 230 mg/L and the short-term criterion at 860 mg/L.

Chlorine is widely used to disinfect drinking water and swimming pools, but even small amounts can harm fish and invertebrates when it enters rivers and lakes, with chronic effects observed at very low concentrations. At elevated levels it damages gill tissue and disrupts the physiology of sensitive species, with invertebrates like water fleas among the most vulnerable. The U.S. Environmental Protection Agency (EPA) sets the long-term freshwater criterion for total residual chlorine at 11 µg/L and the short-term criterion at 19 µg/L.

Chlorine entering marine waters reacts with bromide ions to form chlorine-produced oxidants, which are harmful to saltwater fish and invertebrates across a similar range of concentrations as freshwater species. Sensitive species like eastern oysters can be affected at very low levels, and the effects can ripple through the food web. The EPA sets the long-term marine criterion for chlorine-produced oxidants at 7.5 µg/L and the short-term criterion at 13 µg/L.

E. coli is a bacterium found in human and animal waste, and its presence in recreational water signals fecal contamination and potential risk to swimmers. When levels are elevated, swimmers risk gastrointestinal illness and other infections. The U.S. EPA's 2012 Recreational Water Quality Criteria (RWQC) recommend a geometric mean of 126 CFU/100 mL and a statistical threshold value (STV) of 410 CFU/100 mL for freshwater, or a more protective option of 100 CFU/100 mL and 320 CFU/100 mL. The Beach Action Value of 235 CFU/100 mL or more protective 190 CFU/100mL is recommended.

Dissolved oxygen is the most fundamental parameter for aquatic life, and when levels drop too low fish and invertebrates experience reduced growth, reproductive failure, and death, with eggs and young organisms especially vulnerable. The U.S. EPA's 1986 Water Quality Criteria set coldwater standards at a 7-day mean of 9.5 mg/L for early life stages and 6.5 mg/L for other life stages, and warmwater standards at 6.0 mg/L and 5.5 mg/L respectively.

pH measures how acidic or alkaline water is, and it affects the toxicity of many substances in the water as well as the ability of fish and invertebrates to breathe, reproduce, and survive. Outside the acceptable range, fish experience increasing physiological stress, and sensitive invertebrates like mayflies can be affected at levels that seem only mildly acidic. The U.S. EPA's 1986 Water Quality Criteria recommend a freshwater pH range of 6.5 to 9.0 to protect aquatic life.

pH in marine waters is naturally more stable than in freshwater due to seawater's stronger buffering capacity, but plankton, benthic invertebrates, and oyster larvae are sensitive to changes beyond natural variation. In productive coastal and estuarine areas, even small shifts can stress communities already living near their tolerance limits. The U.S. EPA recommends that marine pH not deviate more than 0.2 units from natural variation, and in no case fall outside 6.5 to 8.5 for open ocean waters.