Surveying a sensitive ecosystem

This exercise is a part of Educator Guide: Rising Carbon Dioxide Threatens Lake Food Webs / View Guide

Directions: After students have had a chance to review the article “Rising CO2 threatens lake food webs,” lead a classroom discussion based on the questions that follow.

Discussion questions:

1. Explain the pH scale. What is the benefit to it being a logarithmic-based scale?

pH = − log[H+], or the negative base-10 logarithm of the concentration of hydrogen ions in moles per liter (molarity of H+). For water at 2° Celsius, [H+]=1.0×10-7 M, so pH=7.0. An acid has more hydrogen ions and a lower pH. A base has fewer hydrogen ions and a higher pH. The scale is logarithmic, so a pH 6 solution has 10 times more hydrogen ions than neutral water with a pH of 7, a pH 5 solution has 100 times more hydrogen ions than neutral water, and so forth. There is a wide range of how acidic or basic substances are, meaning there is a wide range of hydrogen ion concentrations in solutions. Using a logarithmic-based scale to represent the hydrogen ion concentration allows for a wide range of acidic and basic strengths to be displayed on a scale of approximately 1 to 14 pH units (though solutions can have a pH lower than 1 or higher than 14).

2. What chemical reactions happen when carbon dioxide mixes with water? Explain why these reactions produce a more acidic environment?

CO2 + H2O <—> H2CO3 (carbonic acid)

H2CO3 (carbonic acid) <—> HCO3 (bicarbonate) + H+ (hydrogen ion)

The reactions produce a new source of hydrogen ions that add to, and increase, the overall hydrogen ion concentration in water. An increase in hydrogen ion concentration decreases the pH and increases the acidity of water.

3. What is Henry’s law and how does it relate to the amount of dissolved CO2 in a body of water. Can you think of another real-life example of Henry’s law?

Henry’s law (courtesy of the English chemist William Henry, 17741836) states that the solubility of a gas in a liquid is proportional to the partial pressure of that gas above the liquid at a specific temperature: Sgas = k Pgas, in which Sgas is the molar solubility of the gas in the liquid, k is the Henry’s law constant that depends on the specific conditions and Pgas is the partial pressure of the gas above the solution. 

Henry’s law shows that at a particular temperature, as the partial pressure of CO2 in the atmosphere increases, the molar solubility of the gas will increase proportionally — meaning if the body of water was already fully saturated with CO2 at the previous partial pressure, even more could be dissolved. In another example, carbonated soda beverages are bottled under more than one atmosphere of carbon dioxide pressure, which increases the solubility of carbon dioxide in the liquid beverage. The drink retains that atmosphere and that solubility as long as it is not opened. Once the soda is opened, the beverage is exposed to the normal atmosphere with a much lower carbon dioxide partial pressure, making the carbon dioxide solubility in the drink decrease, bubbles come out of the liquid, and the drink gradually goes flat if not drunk or resealed in a timely fashion. If you do drink it, the carbon dioxide comes out of the solution in you, and you burp.

4. How does temperature affect the solubility of solids or gases in liquids?

Increasing temperature generally increases the solubility of most (but not all) solids in liquids. From chemistry class to cooking to washing dishes, it is easier to get solids (salt, sugar, starch) to dissolve in hot water than cold water. In contrast, increasing temperature decreases the solubility of gases in liquids. For example, as cold water heats up, air that had been dissolved in the water forms bubbles and begins to rise to the top, and warm soda bubbles and goes flat much more quickly than cold soda.

5. What are arthropods and the related families?

Arthropods are invertebrate animals with segmented bodies, pairs of jointed appendages and a hard exoskeleton made of chitin (a very rugged and relatively indigestible polymer of glucose). Fossilized trilobites from around 500 million years ago indicate that arthropods originated in the sea. Many arthropods remained in the water and are modern crustaceans: water fleas, shrimp, lobsters, crabs, crayfish and barnacles. Some arthropods started living on land back during the Paleozoic Era before the dinosaurs and became insects (six legs), arachnids (eight legs) or myriapods (many legs).

6. What is a food web?

A food web is an interconnected and overlapping diagram of all food chains in a single ecosystem. Within a given ecosystem, every living thing is part of many food chains. A single food chain will indicate “who eats whom.” Microorganisms and plants are typically at the bottom: Herbivorous animals eat them. Carnivorous animals eat herbivores, and when animals die, they decompose with the aid of microorganisms back at the bottom of the food web. A food chain indicates a possible path for energy and nutrient flow through an ecosystem.

7. What are chemical defense signals? Why would these defenses be selected for over time?

Prey animals are typically sensitive to the presence of their natural predators. Many prey have adapted to sense chemical signals that are naturally given off by their predators or by other threatened individuals. When some prey are attacked, they release chemical signals that warn other prey to hide or to raise their defenses. Similar processes operate at the microscopic level as well. When a cell in your body is invaded by a virus, it typically produces interferon or other cell-signaling molecules to warn other uninfected cells to activate their natural antiviral defenses. Such chemical defense signals increase the probability that a group of organisms, or the cells within an organism, will survive to pass down their genes.

Extension prompts:

8. Based on the normal mode of oxygen and carbon dioxide transport in animals, how might increased CO2 make animals less alert?

Animals typically use hemoglobin to transport oxygen into tissues and carbon dioxide out of those tissues. If CO2 levels are higher than normal, there is a possibility that CO2 could interfere with the process of hemoglobin binding to and transporting oxygen. The brain and sensory receptors need a considerable amount of oxygen to function, and would become impaired if oxygen was less readily available.

9. For this week’s article, make a diagram of how each event causes or interacts with other events, starting with removal of fossil fuels from the ground.

Student answers will vary, but might include fossil fuel hydrocarbons from ground burned with oxygen (a fossil-fuel power station) ®  CO2 production into the atmosphere ®  increased CO2 concentration in the atmosphere will increase the CO2 concentration in oceans and in freshwater (according to Henry’s law) ® reaction of CO2 and H2O yields carbonic acid and additional hydrogen ions ®  decreased pH in water. Predator organisms try to eat prey organisms. Prey organisms (water fleas in the article) try to protect themselves from predators by forming shells in response to chemical warning signals. Increased water CO2 concentration and decreased pH can both harm organisms by reducing their alertness, making it harder to build and maintain shells, or harming eggs or newly hatched organisms.  

Discussion questions:

1. What additional aquatic organisms could be tested for sensitivity to climate change?

Algae, plants, bacteria, fungi, worms, mollusks, other arthropods, fish, amphibians, reptiles, mammals or birds could all be tested for sensitivity to climate change.

2. Based on current climate trends and general human behavior, what variables could be tested for these organisms?

Increased CO2, decreased pH, increased temperature, increased concentrations of other dissolved ions in water due to possibly increased drought and evaporation, changes in concentrations of heavy metals or other waste products in the water due to possible changes in runoff to lakes/rivers/oceans, altered seasons’ effects on mating and other behavior and effects on predators/parasites/pathogens due to less freezing in winters or harsher summers.

Extension prompts:

3. Design a simple procedure that would demonstrate the reaction of CO2 and water yielding a more acidic solution?

To prove that hydrogen ions are produced (the acidity of the solution increases) when CO2 reacts with water, add a few drops of universal pH indicator solution to a clear cup of water. Mix it gently and notice the color. Humans can make good CO2 sources. Take a deep breath, hold your breath as long as you comfortably can and then exhale through a straw to make bubbles in the water. (Be very careful not to suck up any liquid through the straw.) Notice the color. Use the color key that came with the indicator to estimate how much the pH changed. Swirl the water gently for several minutes and notice how long it takes it to return to its normal color as the CO2 gradually leaves the water.

4. Shells are made of calcium carbonate (CaCO3), the same chemical as over-the-counter antacids like Tums, chalk, limestone and marble. How can you demonstrate the effect of a mildly acidic liquid on calcium carbonate?

Vinegar is a mild acid. Drop one or two antacid tablets into a clear cup containing a small amount of vinegar. The tablets should start to fizz immediately and should dissolve over the course of several minutes. Use pH strips to measure the pH before and after adding the antacid tablets. This demonstration shows why calcium carbonate is used to neutralize excess stomach acid. Antacid tablets are designed to be fairly powdery so they will dissolve easily. Chalk, limestone and marble are the same material but are less powdery and take much longer to dissolve.

5. How could you reduce or counteract the effects of rising CO2 on water and water-dwelling organisms?

Reduce CO2 production by energy conservation or switching to energy sources that do not produce the greenhouse gas. Remove more CO2 from atmosphere and water by growing more plants or algae or by artificially extracting CO2. 

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