An adequate supply of dissolved oxygen gas is essential for the survival of aquatic organisms. A deficiency in this area is a sign of an unhealthy river. There are a variety of factors affecting levels of dissolved oxygen. The atmosphere is a major source of dissolved oxygen in river water. Waves and tumbling water mix atmospheric oxygen with river water. Oxygen is also produced by rooted aquatic plants and algae as a product of photosynthesis.
There are physical factors that can lessen the amount of oxygen dissolved in the Cuyahoga. High temperatures, which may result from high turbidity, from the return of industrially used water to the river (the phenomenon of thermal pollution), or from dry periods, decrease the amount of gases that can be dissolved in water. Dry periods also decrease flow which reduces the amount of oxygen churned into the water.
In the navigation channel near the mouth of the Cuyahoga River, the river is dredged regularly to maintain sufficient depth for boats. This extra depth slows the river which hampers its mixing action. The navigation channel has particularly low dissolved oxygen levels.
Bacteria which decompose plant material and animal waste consume dissolved oxygen, thus decreasing the quantity available to support life. Ironically, it is life in the form of plants and algae that grow uncontrolled due to fertilizer that leads to the masses of decaying plant matter.
Too much dissolved oxygen is not healthy, either. Extremely high levels of dissolved oxygen usually result from photosynthesis by a large amount of plants. Great uncontrolled plant growth, especially algal blooms, is often the result of fertilizer runoff. This phenomenon is called cultural eutrophication.
Dissolved oxygen levels in sections of the river in which plants are the major contributor of oxygen fall sharply at night because photosynthesis ceases.
In the Cuyahoga, dissolved oxygen levels in one study of fourteen sites ranged from 1.5 to 90 percent saturation, with an average of 13.2 percent. 100 percent saturation is most desirable.
The pH of river water is the measure of how acidic or basic the water is on a scale of 0-14. It is a measure of hydrogen ion concentration. U.S. natural water falls between 6.5 and 8.5 on this scale with 7.0 being neutral. The optimum pH for river water is around 7.4. Water's acidity can be increased by acid rain but is kept in check by the buffer limestone. Extremes in pH can make a river inhospitable to life. Low pH is especially harmful to immature fish and insects. Acidic water also speeds the leaching of heavy metals harmful to fish.
The Cuyahoga River had a measured pH ranging from 6.0 to 8.0 in fourteen tests of a range of locations in September 1991. The lower values present a problem for most organisms with the exception of bacteria, which can survive pH's as low as 2.0. A pH of 8.0 should be sufficient to support most river life with the possible exception of snails, clams, and mussels, which usually prefer a slightly higher pH. The average pH in the study was 6.9, a value that is only sufficiently basic for bacteria, carp, suckers, catfish, and some insects.
Turbidity is the condition resulting from suspended solids in the water, including silts, clays, industrial wastes, sewage and plankton. Such particles absorb heat in the sunlight, thus raising water temperature, which in turn lowers dissolved oxygen levels. They also prevent sunlight from reaching plants below the surface. This decreases the rate of photosynthesis, so less oxygen is produced by plants. Turbidity may harm fish and their larvae. It is caused by soil erosion, excess nutrients, various wastes and pollutants, and the action of bottom feeding organisms which stir sediments up into the water.
In the Cuyahoga, the average turbidity from a study of twelve sites was 24.9 Nephelometer Turbidity Units (NTU), with a range of 60 units. A value of 24.9 indicates that a device called a Secchi disk can be seen underwater up to a depth of ten to twelve inches. An extreme recorded value of 60 NTU indicates water that is relatively clear to a depth of five inches, while at the other extreme, a value of zero NTU corresponds to water with visibility to five feet, which is the maximum depth that can be measured with this turbidity test.
Temperature impacts the rates of metabolism and growth of aquatic organisms, rate of plants' photosynthesis, solubility of oxygen in river water, and organisms' sensitivity to disease, parasites, and toxic materials. At a higher temperature, plants grow and die faster, leaving behind matter that requires oxygen for decomposition.
The temperature of the Cuyahoga as tested in September 1991 did not exceed 20 C or 68 F and thus did not create a climate for many fish diseases. It often exceeded 13 C or 55 F, creating a climate right for many fish, plants, insect nymphs and some fish diseases. Temperatures were recorded below this value, reducing plant life and fish diseases as well as indicating uninhabitable water for salmon. In the Cuyahoga, temperature changes radically in the spring and autumn. As a result, fish that are not indigenous to the region and not yet adapted for these shifts often die.
Source: Cuyahoga River Water Quality Monitoring Program, Cleveland State University