III. Water and Sediments
 
 
 

1. Composition and balance are important factors in the chemistry of water.
   
    
   1. Dissolved gases
      1. Oxygen available to organisms in water bodies is known as dissolved oxygen.
          
      2. Dissolved oxygen in water bodies comes from the atmosphere and photosynthesis.
          
      3. Dissolved oxygen is used by organisms to breathe and is also used in the decaying process of organic material.
          
      4. Generally, levels of dissolved oxygen remain constant in a water column for a stream. However, in estuaries, lakes, and ponds, dissolved oxygen levels usually decrease with depth.
          
      5. The amount of dissolved oxygen available to organisms is dependent upon the temperature: cold water can hold more dissolved oxygen than warm water.
          
   2. Nutrients
      1. Of the many nutrient elements in nature, the most common and important are nitrogen and phosphorus.
          
      2. Nutrients are essential for plant growth.
          
      3. Both phosphorus and nitrogen are essential nutrients for the plants and animals that make up the aquatic food web. Because phosphorus is the nutrient in short supply in most fresh waters, even a modest increase in phosphorus can, under the right conditions, set off a whole chain of undesirable events in a stream, including accelerated plant growth, algae blooms, low dissolved oxygen, and the death of certain fish, invertebrates, and other aquatic animals.
          
      4. Nutrients can come from precipitation, sewage discharges, other organic material, or poor management of fertilizers.
          
      5. Excessive nutrients in a waterway can be damaging by causing excessive plant growth. The decaying process of the plant material utilizes precious dissolved oxygen and can cause raised water temperatures.
          
   3. Toxic chemicals
      1. Soil and water contain a wide variety of toxic chemicals, such as inorganic salts, trace elements, and heavy metals. Chemicals used in pesticides and herbicides are examples.
          
      2. Toxic chemicals are harmful to organisms that come into contact with the toxin. Toxic chemicals can cause lesions, interrupt the reproductive system, or even kill organisms.
          
   4. Acidity and alkalinity
      1. The term pH is used to indicate the alkalinity or acidity of a substance as ranked on a scale from 1.0 to 14.0. Water’s acidity increases as the pH gets lower. Alkalinity is a measure of the capacity of water to neutralize acids.
          
      2. Many chemical and biological processes take place in the water. For example, different organisms flourish within different ranges of pH.
          
      3. The majority of aquatic animals prefer a range of 6.5–8.0. If pH falls outside this range, it reduces the diversity in the stream because it stresses the physiological systems of most organisms and can decrease reproduction.
          
      4. Low pH can also allow toxic elements and compounds to become mobile and “available” for uptake by aquatic plants and animals.
          
      5. Measuring alkalinity is important in determining a stream’s ability to neutralize acidic pollution from rainfall or wastewater. It is one of the best measures of the sensitivity of the waterway to acid inputs.
          
2. Turbidity is a measure of water clarity, or how much the material suspended in water decreases the passage of light through the water.
    
   1. Composition
      1. Dissolved solids usually consist of calcium, chlorides, nitrate, phosphorus, iron, sulfur, and other ions.
          
      2. Suspended solids include silt and clay particles, plankton, algae, fine organic debris, and other particulate matter.
          
      3. Buoyant and semi-buoyant litter, such as drink cans, plastic six-pack rings, and empty ice bags, can decrease the passage of light through water.
          
   2. Effects
      1. The concentration of total dissolved solids affects the water balance in the cells of aquatic organisms, influencing their ability to keep position in the water column. Depending on the concentration of dissolved solids, an organism might float up or sink down to a depth to which it is not adapted.
          
      2. Suspended solids may have traces of pesticides or herbicides attached to the particles. These pesticides and herbicides may be toxic substances for organisms that come into contact with them.
          
      3. Suspended sediments may also clog the gills of the aquatic animals such as fish and stonefly larvae that use gills to obtain oxygen from the water.
          
      4. Total solids may affect water clarity, reducing the passage of light through water. This phenomenon can slow photosynthesis in plants living in the water.
          
      5. Total solids may also affect the temperature of the water, elevating the temperature. The presence of total solids may also lower the water’s freezing point.
          
      6. Total solids—and suspended sediments specifically—may clog the physical components of a stream and change the natural channel of the stream.
          
3. Salinity is a physical property of water.
   1. Like the ocean, some streams, lakes, and estuaries have dissolved salts in the water. These waters are called saline or brackish waters.
       
   2. The closer to the ocean, the greater the concentration of salt in the water. This area is known as the “salt wedge” because of the shape assumed by the volume of denser salt water as the fresh water moves over it.
       
   3. The amount of dissolved salts (or salinity) affects the ability of organisms to live and reproduce in water. Some organisms may not be able to tolerate certain levels of dissolved salts.
       
4. The rates of biological and chemical processes depend on temperature.
   1. Organisms require certain water temperatures to live and reproduce.
       
   2. Certain organisms, such as trout and stonefly larvae, require colder waters found in headwater or first and second order streams.
       
   3. Temperature affects the oxygen content of the water (oxygen levels become lower as temperature increases); the rate of photosynthesis by aquatic plants; the metabolic rates of aquatic organisms; and the sensitivity of organisms to toxic wastes, parasites, and diseases.
       
   4. If water temperatures get very high or very low, organisms react by moving to waters with more suitable water temperatures, going into a state of dormancy (such as hibernation), or in extreme cases, dying.
       
   5. Causes of temperature change include weather, removal of shading stream-bank vegetation, impoundments (a body of water confined by a barrier, such as a dam), discharge of cooling water, urban storm water, and groundwater inflows to the stream.

Resources
“Studying the Effect of pH on Aquatic Organisms.” Water on the Web. Natural Resources Research Institute. University of Minnesota Duluth.
<http://wow.nrri.umn.edu/wow/student/ph/study.html>.

“Water and Sediments.” Chesapeake Bay: Introduction to the Ecosystem. Chesapeake Bay Program.
<http://www.chesapeakebay.net/info/ecointr3.cfm>.

Water Science for Schools. U.S. Geological Survey.
<http://ga.water.usgs.gov/edu/>.

Next: IV. Elements of a Watershed

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