Lesson 4: Marine Ecosystems and Estuaries


Activity 1: Water and Estuaries (Saltwater Activity)

Materials: eggs prepared in Lesson 2, pen, saline water, scale (kit), slotted spoon, small clear cups (kit), tap water, tape
For this activity, you are going to investigate the following hypothetical question: How does the saltwater in an estuary impact the organisms living within it? Prior to conducting this experiment, you will want to think about the question and consider the following hypothetical situations.

A marine (saltwater) organism is moved to a freshwater environment.
A freshwater organism is moved to a marine environment.

Predict what will happen in each situation and record your predictions on the data sheet located on the "Saltwater Activity" page.
Next, follow these steps. (Safety note: Because of the risk of salmonella with raw eggs, be sure to wash your hands frequently with soap during this experiment.)
  1. Prepare the following and label them using the pen and tape: two glasses containing tap water at room temperature and one glass containing saline solution at room temperature (3.5% solution is preferred). NOTE: Use enough water/saline solution to completely cover the egg you will be placing in each glass.
  2. Find the two cups of vinegar and eggs that you prepared at the end of Lesson 2. Carefully lift the eggs out of the vinegar using a slotted spoon. NOTE: Use care, as the membranes on the eggs are fragile. Do not rinse off the eggs.
  3. Weigh one egg using the scale from the kit and record its weight on the "Saltwater Activity" page. Place that egg in one of the glasses of tap water.
  4. Weigh the other egg, record its weight, and place that egg in the saline solution.
  5. In the "Prediction" box for Egg One, predict whether the egg's mass will increase or decrease after sitting in tap water. For Egg Two, predict whether the egg's mass will increase or decrease after sitting in saline solution.
  6. After one hour, remove Egg One from its glass and weigh it. After weighing, place the egg back in the glass. Repeat with Egg Two. Record the weights on the activity page.
  7. Repeat the weighing procedure after 24 hours and again after 48 hours. Be sure to record this information on the activity page.
  8. After the eggs have been measured and all the information is collected, predict what will happen when the egg from the saline solution is placed in tap water; record your prediction on the activity page.
  9. Place the egg from the saline solution into the second glass of tap water. You do not have to weigh the egg -- just make note of what happens.
Estuaries are a place where the fresh water of rivers and streams come into contact with the salt water of the oceans. Organisms that live in estuaries require the same things that other organisms require plus the ability to handle salt water. You saw in the experiment that the eggs were affected by the solution that they were placed in.
Student Activity Page
While the reading has covered the variety of marine ecosystems, the importance of the estuary warrants special attention as a unique component of the hydrosphere. For this activity, the goal is to have your child consider the role of nutrients or minerals in the estuary, specifically salt. The presence of minerals and nutrients will influence the activities and type of life within the estuary, and this activity will give your child a hands-on look at how a mineral can influence an organism. Also, the activity gives your child exposure to a very significant process for life in general, equilibrium. Below are the questions to ponder with the activity. Also included is the "Saltwater Activity" table with appropriate answers (note that actual mass readings will vary depending on the egg).
How does the saltwater in an estuary impact an organism living within it?
Solution NamePrediction
OneWill vary depending on egg.Tap waterSlight increase (after 1st and 2nd days; little increase with initial hour)
TwoWill vary depending on egg.Saline solutionSignificant decrease (after 1st and 2nd days; will see little decrease after 1st hour)
For the activity, your child considered the following questions and answered them based on her results. Discuss her answers with her. Below are some suggested answers:
  • Prior to placing the eggs in the solution, which one had more salt? Both had equal amounts of salt; there is no difference.
  • Based on what happened to the egg placed in the saline solution, explain what would happen if a freshwater organism moved to a saltwater environment. An organism that is moved from freshwater to saltwater will lose a lot of its water and shrink. Because of the loss of the water, the organism will die or must find a way to remove the salt from its body to survive.
  • Based on what happened to the egg placed in the tap water, explain what would happen if a freshwater organism moved to a freshwater environment. The organism should remain about the same size. It may grow some as it absorbs some water, but it should not shrink. The organism would neither gain nor lose water. Because of this, the organism should be able to survive.
  • Based on what happened to the egg removed from the saline solution and placed in tap water, explain what would happen if a saltwater organism moved to a freshwater environment. The egg moved from the saline to the tap water should expand or increase in size. Water will move from an area of higher concentration to an area of lower concentration. Saltwater generally will have a lower concentration of water than the egg, so there should be an increase in size when an egg soaked in salt solution is put in a freshwater solution or when an egg soaked in vinegar is soaked in freshwater; conversely, an egg placed in saltwater from freshwater or a vinegar solution will shrink. For an organism, the loss or gain of water will cause harmful effects. The organism, though surrounded by water, needs freshwater within its body. If the freshwater is lost because of the environment (moving from freshwater to saltwater) the organism can die. If the difference in concentration is very small (freshwater to 0.1% saltwater), the organism will not be harmed. The ocean's concentration is approximately 3.3%, and an estuary has a salt concentration of up to 3.3% (at high tide) and decreases with the lower tides (ranges unknown).