Figure 2.25 Ocean acidification. CÓ, HO H₂CO H₂CO₂H+ HCO3 HACO HCOS CO₂ + Ca CaCO₂ Scientific Skills Exercise Interpreting a Scatter Plot with a Regression Line How Does the Carbonate lon Concentration of Seawater Affect the Calcification Rate of a Coral Reef? Scientists predict that acidi- fication of the ocean due to higher levels of atmospheric CO, will lower the concentration of dissolved carbonate ions, which living corals use to build calcium carbonate reef structures. In this exercise, you will analyze data from a controlled experiment that examined the effect of carbonate ion concentration ([CO-]) on calcium carbonate deposition, a process called calcification. Now the Experiment Was Done For several years, scientists conducted research on ocean acidification using a large coral reef aquarium at Biosphere 2 in Arizona. They measured the rate of calcification by the reef organisms and examined how the calcification rate changed with differing amounts of dissolved carbonate ions in the seawater Data from the Experiment The black data points in the graph form a scatter plot. The red line, known as a linear regression line, is the best fitting straight line for these points. These data are from one set of experiments, in which the pH, temperature, and calcium ion concentration of the seawater were held constant. INTERPRET THE DATA 1. When presented with a graph, first determine what each axis repre- sents. (a) in words, what is shown on the x-axis? (Include the units) (b) What is on the y-axis? (c) Which variable is the independent variable the one that was manipulated by the researchers? (d) Which is the dependent variable-the one that responded to the treatment, which was measured by the researchers? (For additional in- formation about graphs, see the Scientific Skills Review in Appendix F.) Some carbon dioxide (CO₂) in the atmo- sphere dissolves in the ocean, where it reacts with water to form carbonic acid |(HCO Carbonic acid dissociates into hydrogen ions (H") and bicarbonate ions |(HCO3). The added H combines with carbonate ions (CO), forming more HCO3. Less CO²- is avail- able for calcification -the formation of calcium carbonate (CaCO3)-by marine organisms such as corals. Calcification rate Immo Cacom day 0 220 240 260 CO₂] (umol/kg of seawater) 280 Data from C.Langdon et af, Effect of calcium carbonate saturation state on the calification rate of an experimental coral reet, Global Biogeochemical Cycles 14:539-654 (2000) 2. Based on the data shown in the graph, describe in words the rela- tionship between carbonate ion concentration and calification rate. 3. If the seawater CO2 concentration is 270μmol/kg, estimate the rate of calcification and how many days it would take 1 square meter of reef to accumulate 30 mmol of CaCO, 4. (a) Which step of the process in Figure 2.25 is measured in this experiment? (b) Are the results of this experiment consistent with the hypothesis that increased atmospheric [CO₂] will slow the growth of coral reefs? Why or why not? Instructors: A version of this Scientific Skills Exercise can be assigned in Mastering Biology.