TEACHING ALL VOLUMES SUBMIT WORK SEARCH TIEE
VOLUME 1: Table of Contents TEACHING ISSUES AND EXPERIMENTS IN ECOLOGY
Issues : Figure Sets

Figure Set 3: How have human activities in watersheds changed N export from rivers?

Purpose: Help students better understand how nitrogen loading to watersheds affects input of nitrogen to receiving waters.
Teaching Approach: "pairs share"
Cognitive Skills: (see Bloom's Taxonomy) — comprehension, interpretation, analysis
Student Assessment: take home quiz

BACKGROUND


Over the last decade or so several synthetic studies have documented the very large effects of - fertilizer use in agriculture, other agricultural activities, and increased human population in coastal watersheds - on N concentrations in surface waters and estuaries. These include papers by Cole et al. (1993), Howarth et al. (1996), Jordan and Weller (1996), and Nixon (1995).

Rivers receive mobile ions such as nitrate 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 have greatly increased worldwide. For instance, according to Vitousek et al. (1997) nitrate has more than doubled in the Mississippi since 1965 and concentrations of nitrate in majors rivers in the northeast has increased 3-10x since the early 1990's.

Howarth et al. (1996) and Jordan and Weller (1996) correlate riverine nitrate discharge to population density both in the U.S. and worldwide. The positive relationship between total N flux as nitrate and human population density is to be expected given the mobility of nitrate. Still, the validity of the relationship for rivers over the globe is striking.

Jordan and Weller (1996) show that total net anthropogenic input to watersheds is a much better predictor of U.S. nitrate discharge than any individual component. The r2 values they report are: net anthropogenic N input (0.76), agricultural biotic N fixation (0.62), atmospheric deposition (0.61), livestock waste (0.44), N fertilizer (0.36), human density (0.23), import of agricultural-product N (0.22), and percent cropland in watershed (0.08).

Recent collaborative work of scientists worldwide has resulted in data such as Figure 3. 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). The correlation between net anthropogenic inputs of N and total N fluxes for these data is quite high (r2 = 0.73; P=0.002; Howarth et al. 1996). For most of the regions around the North Atlantic fertilizers account for most anthropogenic inputs but atmospheric deposition of oxidized N is greatest for the northeast U.S. and Saint Lawrence River and Great Lakes Basin (Vitousek et al. 1997a).

Several of the articles in Links (below) also discuss organic loss export from rivers plus large losses in watersheds as a result of denitrificiation. The Rabalais testimony shows effects of huge N loadings to the Gulf of Mexico (hypoxia, anoxia) from the Mississipi River.

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Literature Cited


Links

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