Water Quality Indicators and Aquatic Ecosystem Health

Unique Properties of Water

Water is a molecule that consists of two hydrogen atoms and one oxygen atom. Water is special because its highest density is in liquid form rather than solid form, which is the case for most substances. It is also called the “universal solvent” because of its ability to dissolve more substances than any other liquid found on Earth, due to its unique chemical composition and physical attributes.

Cohesion and adhesion are two very important water properties.

• Cohesion is the property that allows water molecules to stick to one another (e.g., a drop of water holding its shape).
• Adhesion is the property that allows water molecules to stick to another substance instead of slipping off.
• Capillary action is the movement of water up a thin tube against or without the force of gravity, propelled by both adhesion and cohesion.

Wastewater Treatment Processes

Wastewater treatment is a process used to convert wastewater into an effluent that can be returned to the natural water cycle without a significant impact on the environment. Sometimes, the water is also directly reused in a process called water reclamation. In the US, treatment costs $12 billion a year and is expected to double in 10 years.

Septic Tank Disposal Systems

This is a conventional method for treatment. A sewer line from the house leads to an underground septic tank in the yard. This tank is designed to separate solids from liquid, digest and store organic matter, and allow the treated sewage to seep into the surrounding soil. As the wastewater moves through the soil, it is further treated by the natural processes of oxidation and filtering.

Municipal Treatment Stages

• Primary Treatment: Involves screening, grit removal, and sedimentation/clarification.
• Secondary Treatment: Involves aeration, sedimentation/clarification, and disinfection.
• Tertiary/Advanced Treatment: Involves nutrient removal.

Potable Water Treatment

Potable water treatment is the process by which lake or river water is made drinkable for humans. These processes are both physical and chemical in order to remove as many waterborne germs as possible. There are six main steps:

1. Coagulation
2. Flocculation
3. Sedimentation
4. Filtration
5. Disinfection
6. Distribution

Ecological Species Interactions

Competitive Interactions

These interactions have a negative effect on both organisms competing for resources because resources are always limited. Types include:

• Competitive Exclusion
• Intraspecific Competition
• Resource Partitioning
• Scramble Competition
• Contest Competition

Positive Interactions

• Mutualism
• Commensalism
• Neutralism

Negative Interactions

• Amensalism: One species is harmed, while the other is unaffected.
• Competition: Both species are harmed.
• Antagonism: Interference with the action of one substance or organism by another.
• Antibiosis: An interaction between two organisms that is detrimental to at least one of them.
• Allelopathy: The chemical inhibition of one plant (or organism) by another.

Exploitative Interactions

• Herbivory: A form of predation where plants are the prey.
• Parasitism: Beneficial to one species and negative to the other (the host is not killed).
• Predation: Beneficial to one species and negative to the other (one species is killed).
• Altruism: Behavior that benefits another organism at one's own expense. For example, the Sacculina carcini parasite tricks crabs into caring for the parasite's eggs as if they were its own, taking up the crab's nutrients.

Key Water Quality Indicators

Salinity

Salinity is a property of water that describes its salt concentration. It is measured by dissolved salts in parts per thousand (ppt) or grams of salt per kilogram of water; these two quantities are equivalent. The category of salts includes compounds such as sodium chloride, magnesium sulfate, potassium nitrate, and sodium bicarbonate, all of which dissolve into ions. Salinity governs the physical properties of water, such as density, heat capacity, and electrical conductivity. As such, salinity is often measured by water density or, more commonly, conductivity.

Water with constant salinity is called homoiohaline. Environments in which salinity varies over time are called poikilohaline. Poikilohaline salinities may range from 0.5 to greater than 300 ppt.

Turbidity

Turbidity is a measure of the cloudiness or haziness in a fluid caused by large numbers of individual particles. It can be measured in several units:

• Formazin Turbidity Units (FTU) or Formazin Nephelometric Units (FNU): Measured using the ISO 7027 method, which determines the concentration of suspended particles by measuring incident light scattered at right angles from the sample.
• Jackson Turbidity Units (JTU): Measured using the Jackson Candle Method, where a candle is shined through a column of water, and the length of water needed to completely obscure the light source is measured. The longer the column, the lower the turbidity.
• Nephelometric Turbidity Units (NTU): A measure of the tendency of particles to scatter a light beam focused on them. A nephelometer measures the light that reaches a detector placed to the sides of the water column; the more light detected, the more particles are in the water.

To test the turbidity of water, people also use a Secchi Disk. A marine Secchi disk is a plain white disk 30 cm in diameter, while a freshwater version is 20 cm in diameter and divided into black and white fourths. The disk is lowered into the water, and the depth at which it is no longer visible is the measure of turbidity. Light can penetrate to a depth of about 2-3 times the Secchi Disk depth. This method is not always accurate due to factors like differences in eyesight and the sun’s glare.

Seasonal variations and lake turnover can change a lake's turbidity by releasing nutrients. Other sources of turbidity include gasoline or oil from roads, benthic organisms stirring up sediments, industrial wastes, and urban runoff. The effects of turbidity include an increase in water temperature, a decrease in photosynthetic rate, decreased growth, and more aesthetically displeasing water. Turbidity can also reduce the ability of fish gills to absorb dissolved oxygen. However, in some areas, high turbidity is necessary for ecosystem health. High levels of turbidity in drinking water correspond to an increased risk of developing gastrointestinal diseases, as turbidity can shield bacteria from certain types of water sterilization.

The standard for drinking water turbidity in the United States for systems using conventional or direct filtration methods is less than 1 NTU at the plant outlet. All samples for turbidity must be less than or equal to 0.3 NTU for at least 95 percent of the samples in any month. Other systems must follow state limits, where turbidity must never exceed 5 NTU. Many drinking water utilities strive to reach levels as low as 0.1 NTU. The World Health Organization establishes that the turbidity of drinking water should not be more than 5 NTU and should ideally be below 1 NTU. The US Environmental Protection Agency has also published water quality criteria for turbidity, which are scientific assessments used by states to develop their own water quality standards.

Dissolved Oxygen (DO)

Dissolved Oxygen (DO) is the amount of oxygen that is dissolved in a substance. DO is an important water quality indicator, as fish and other aerobic organisms require it for life. DO is usually measured in mg/L or parts per million (ppm), which are equivalent quantities. Percent dissolved oxygen is dependent upon many factors, including salinity and temperature. Most surface waters contain between 5 and 15 ppm of dissolved oxygen. If a stream or river has below 5 ppm of dissolved oxygen, aquatic life can be put under stress. Levels below 1-2 ppm for a few hours can kill large fish.

Should anoxic (low-oxygen) conditions continue for too long, the population of anaerobic organisms will increase relative to the population of aerobic organisms. Supersaturation of oxygen (DO levels over 100%) can occur naturally through photosynthetic organisms or through rapid environmental changes that occur too quickly for the system to reach equilibrium. Supersaturation can be harmful to organisms and can cause decompression sickness. Two very important factors affecting DO are temperature and atmospheric pressure; lower temperatures and higher atmospheric pressure result in higher levels of dissolved oxygen. The presence of dissolved or suspended solids can reduce the effectiveness of oxygen dissolving in water, which can be problematic for aquatic organisms.

Dry periods can lower stream discharge and raise water temperatures. From night to day, DO will fluctuate dramatically. Algal blooms can cause large fluctuations in DO, especially at night in areas with little current for aeration. Human activities can also cause DO fluctuation through sewage discharges, agricultural runoff, or over-baiting a fishing lake. DO enters the water by diffusion from the atmosphere, aeration from wind and waves, movement over rocks, and photosynthesis by aquatic plants. In areas with low DO levels, water aeration can be crucial. This can be achieved by infusing air into the bottom of the water body or agitating the surface to allow oxygen exchange. An increase in dissolved oxygen can support more fish and other aquatic organisms, generally increasing the health of the ecosystem.

Biochemical Oxygen Demand (BOD)

The biochemical oxygen demand (BOD) measures how fast organisms use up the oxygen in the water. Aerobic microbes use oxygen to oxidize the organic matter in the water, using the released energy for growth and reproduction, which creates a demand for DO. This demand is usually proportional to the amount of organic compounds available for oxidation. BOD is tested by measuring dissolved oxygen over a period of time to determine the rate of oxygen consumption, accounting for aeration and other factors. Oxygen used for decomposition robs other organisms of the oxygen needed to live. Organisms with low tolerance may die off and be replaced by organisms with more tolerance for low oxygen levels.

In some cases, microbes in an environment use oxygen faster than it can enter the water. This threatens those organisms, as the DO content will eventually become too low for them to survive, also resulting in the deaths of fish and other organisms. This can result in long-term DO shortages. BOD can be considered a measure of pollution in rivers and other bodies of water. Most pristine rivers should have a 5-day BOD below 1 mg/L. Moderately polluted rivers may have a BOD between 2 and 8 mg/L. Municipal sewage treated with a three-stage process would have a BOD of about 20 mg/L. Untreated sewage has a varying BOD but averages about 200 mg/L in the US, which is much lower than the world average due to greater water use per capita.

Water Temperature and Thermal Pollution

Water temperature in aquatic ecosystems is a very important quality indicator, as it affects other factors such as dissolved oxygen levels, photosynthesis of aquatic plants, and metabolic rates of aquatic organisms. Increases in water temperature are called thermal pollution. Thermal pollution increases the sensitivity of organisms to disease, parasites, and pollution. Small changes in temperature can adversely affect the reproductive systems of aquatic organisms like macroinvertebrates or fish.

Temperature change can be caused by natural seasonal changes, anthropogenic activities, industrial thermal pollution from the discharge of cooling water, or stormwater runoff from heated surfaces like streets. The amount of total solids in the water also affects its temperature; for example, soil erosion increases total solids, which absorb more sunlight and increase the temperature. The removal of trees and brush from a riverbank not only increases erosion but also increases the amount of sunlight that hits the water, further increasing the temperature. High temperatures also decrease the ability of water to hold DO, which has an even greater effect because temperature also increases the metabolic rates of aquatic organisms and their biochemical oxygen demand. This causes resources to be used faster and may increase the population of anaerobic bacteria relative to aerobic bacteria. Thermal pollution can also cause an overpopulation of plants by increasing their growth rates.

In some cases, thermal pollution can include the release of cold water, usually from the bottom of a reservoir. This can also be detrimental to the health of the ecosystem, altering its fauna and decreasing productivity. Abrupt changes in water temperature in any direction can be very harmful to ecosystem health and can kill fish or other organisms quickly through thermal shock. In very limited cases, thermal pollution may have no effect or may even increase ecosystem health, a phenomenon called thermal enrichment. An example is the manatee, which often uses power plant discharge sites during the winter. It is likely that manatee populations would decline if these sites were removed.

Aquatic Ecology Terminology

Algal Bloom

An overgrowth or rapid increase in populations of algae. The algae use up oxygen and create dead zones, causing hypoxia.

Allochthon

A large rock that has been moved from its original place of formation. Aquatic food webs that obtain things from it are called allochthonous; in some small bodies of water, allochthonous sources of carbon are the greatest source of carbon.

Bioluminescence

Biochemical light emission from organisms. 76% of ocean organisms have the ability to bioluminesce.

Thermophile

An organism with optimal growth above 45°C.

Acidophile

An organism that thrives at a pH under 3.

Xerophile

An organism that thrives at a water activity under 0.8.

Psychrophile/Cryophile

An organism with optimal growth at 15°C or below.

Alkaliphile

An organism that thrives at or above a pH of 9.

Cryptoendolith

An organism that lives in microscopic spaces under rocks.

Piezophile

An organism with optimal growth under hydrostatic pressures of 10 MPa (99 atm, 1450 psi).