STUDENT INSTRUCTIONS

In this "pairs-share" you first work on your Figure by yourself. Use the step one-step two approach you have practiced in class.* Then address the questions posed to you. After this you will pair up with a student who has focused on the other data set and the two of you will explain the figures/table to each other and also address the questions.

Some Background Information and Questions

Over the last decade or so scientists have tried to document the very large effects of human activities on N concentrations in surface waters and estuaries. (Human-caused effects are called "anthropogenic"). Increasing rates of "N loading" to rivers and to the coast has resulted in severe problems such as massive algal growth followed by oxygen depletion and accompanying "fish kills." (Why?) This phenomenon is eutrophication. The N increases are a result of synthetic fertilizer use in agriculture, other agricultural activities, sewage, and N in precipitation.

Figure 3A

Rivers receive mobile anions (negatively charged ions that easily leach out of soils) such as nitrate (NO₃^-) from the watersheds that they drain. Given factors such as increases in fertilizer use, nutrient inputs from sewage treatment plants, and N deposition as precipitation it is not surprising that nitrate concentrations in water bodies have greatly increased worldwide. According to Vitousek et al. (1997) nitrate concentration has more than doubled in the Mississippi River since 1965 and in majors rivers in the northeast they have increased 3-10x since the early 1990's. These are huge changes in a very short time!

Figure 3A is an example of an ecosystem approach to the study of human alteration of global N cycling. Note the log-log scale and make sure you understand what log-log means. What is the question that the scientists who constructed this figure were asking?

Questions

• What is meant by the term "export"? Where is the nitrate exported to?


• Why does nitrate export in rivers increase with increasing human population? What human activities result in increases in river nitrate concentrations?


• What are some effects of high nitrate concentration in water?

Figure 3B

Recent collaborative work of scientists worldwide has resulted in data such as Figure 2B. In the Atlantic Ocean Basin total dissolved N into rivers is estimated to have increased by 2-20x since pre-industrial times (Vitousek et al. 1997b). For most of the regions around the North Atlantic fertilizers account for most anthropogenic inputs but atmospheric deposition of oxidized N (such as nitrous oxide) is greatest for the northeast U.S. and Saint Lawrence River and Great Lakes Basin (Vitousek et al. 1997a).

Questions

• The data are labeled as flux (or input) per area? Per area of what? What does "flux" mean? What is total N (as opposed to nitrate for instance)?


• The data points are export of nitrate from watersheds to the North Atlantic Ocean. What is the watershed for the data point labeled NE U.S.? For the North Sea? How does the water actually enter the Atlantic Ocean - where does it come from and where would you go to see "it" enter?


• Synthesis of these data was an enormous undertaking for scientists from many nations. Why are these scientists so interested in the relationship between N flux and net anthropogenic input?


• Vitousek, P. M., J. D. Aber, R.W. Howarth, G. E. Likens, P. A. Matson, D. W. Schindler, W. H. Schlesinger, and D. G. Tilman, 1997a. Human alteration of the global nitrogen cycle: sources and consequences. Ecological Applications 7: 737-750.


• Vitousek, P. M., J. D. Aber, R.W. Howarth, G. E. Likens, P. A. Matson, D. W. Schindler, W. H. Schlesinger, and D. G. Tilman, 1997b Human alteration of the global nitrogen cycle: causes and consequences. Issues in Ecology 1: 1-15.

* In step one you first figure out how the figure or table is set up (e.g. what the labels on the axes mean). You also need to have a pretty good idea of the experimental design - how the researchers set up their study - and the hypotheses the study address. In step two you can go on to interpreting the data. For both steps write down any questions you have.