Lesson 8: Water Quality

Getting Started

So far in this unit you have learned about the hydrosphere and some of its properties. You have learned that an ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving, physical components of the environment with which the organisms interact, such as air, soil, water, and sunlight. You have looked at some of the biological communities in the hydrosphere and investigated the importance of water's characteristics. You have also studied pollution and its impact on the environment and explored a little about laws and policies that are meant to protect the environment. Now, once again, you will look more specifically at water and its contents.

In this lesson, the goal is to look at two important components of the hydrosphere — oxygen and nitrogen — and to examine how reservoirs behave. The two elements are not as primary as water, yet their presence is crucial for life in the hydrosphere's ecosystems.

Oxygen plays a very active role in the chemistry and biology of coastal waters, and its concentration is a major indicator of water quality — where oxygen levels are good, life thrives. The second element, nitrogen, is required for photosynthesis by marine plants. Nitrogen gas in the atmosphere and dissolved in sea water cannot be used as is; instead, to become a useful nutrient, it must be converted into forms that plants can use, such as nitrate or ammonia. Still, without the presence of nitrogen, major issues will arise for ecosystems in the hydrosphere. As you progress through this lesson, keep in mind the role of oxygen and nitrogen and factors that may or may not impact the presence of both in the hydrosphere.

Stuff You Need

  • Painless Earth Science by Edward J. Denecke, Jr. (green edition)
  • colored pencils
  • ruler or straight edge

Ideas to Think About

  • How are levels of elements in oceans or other reservoirs important?
  • What can impact the level of elements in oceans or other reservoirs?
  • What prevents an ecosystem in the hydrosphere from being fatally harmed by a natural or human disaster?

Things to Know

  • Water quality is a measure of the condition of water relative to the requirements of the organisms it contains or the organisms that use it as a resource.
  • Phytoplankton are microscopic organisms that utilize sunlight to make food; although they use photosynthesis, phytoplankton are not considered plants.
  • Equilibrium is the condition of a system in which influences are balanced; the tendency in a system is to regulate itself to acceptable balances.
  • Steady state is a characteristic of a system in which properties are unchanging over time.


  • Analyze evidence to explain observations, make inferences and predictions, and develop the relationship between evidence and explanation. (S)
  • Analyze and evaluate information from a scientifically literate viewpoint by reading, hearing, and/or viewing scientific texts, articles, and events in the popular press. (S)
  • Analyze hydrospheric data over time to predict the health of a water system, including dissolved oxygen. (S)
  • Recognize that the good health of environments and organisms requires monitoring of the hydrosphere, water quality standards, methods of water treatment, maintaining safe water quality, and stewardship. (S)

Introducing the Lesson

In this lesson, your child will go through two activities that look at the presence of dissolved oxygen and pollution. In the first activity, the goal is for your child to understand data given in order to produce a graph that can be effectively interpreted. The second part of Activity 1 has your child looking at a graph that tracks both dissolved oxygen and pollutants in an unnamed body of water. The goal of this portion is for your child to begin to take a next step and understand the relationship between dissolved oxygen and pollutants.

In the second activity, your child will be asked to read and understand an excerpt from a newspaper article that focuses on a natural disaster. The goal of the activity is to recognize the challenges associated with a disaster and to see that reservoirs in the hydrosphere are not totally helpless.
Reading and Questions
Materials: Painless Earth Science by Edward J. Denecke, Jr. (green edition)
Read from page 105 to the top of page 108 in Painless Earth Science. This reading will describe how oxygen and nitrogen are balanced against each other in oceans. While you are reading, focus on the question, "What makes these elements important for life?"
  1. What is salinity, and how is it measured? What is the average salinity of ocean water?
    Salinity is a measure of how much of certain substances are dissolved in water. It is measured in parts per thousand in 1 kilogram of seawater. The average salinity of ocean water is 35 parts per thousand.
  2. How does water evaporating from the oceans change the salinity of the water?
    Only the water evaporates, leaving behind slightly less water. This makes the ratio of water to dissolved substances slightly lower and making the remaining water more saline.
  3. Where does organic and inorganic matter come from that is dissolved in ocean water? What about gases?
    Inorganic matter comes from minerals, rocks, and sediment that runs off into the water. Organic matter comes from animal waste and the decaying bodies of dead animals. Gases come from the atmosphere or from photosynthesis by phytoplankton.
  4. What do you think would happen if the steady state of a couple of elements, specifically oxygen and nitrogen, were disrupted?
    Organisms would be more likely to die because they wouldn't be able to obtain an amount necessary for survival. It might also change what other organisms move into the ecosystem or how they thrive in that ecosystem, throwing off the balance of predators and prey.