Clean Coastal Waters Understanding and Reducing the Effects of Nutrient Pollution (2000) / Chapter Skim
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5 Sources of Nutrient Inputs to Estuaries and Coastal Waters
5 Sources of Nutrient Inputs to Estuaries and Coastal Waters
Pages 113-162
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From page 113... ...
Although point source nutrients are the major problem for small watersheds adjacent to major population centers, these inputs are relatively easy to minimize with tertiary wastewater treatment processes. In contrast, nutrients from nonpoint sources have become the dominant and least easily controlled component of nutrients transported into coastal waters from large watersheds, and especially from watersheds with extensive agricultural activity or atmospheric nitrogen pollution.
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From page 114... ...
1997~. By 1995, the global use of inorganic nitrogen fertilizer was again growing rapidly, with much of the growth driven by increased use in China (Figure 5-1~.
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From page 115... ...
SOURCES OF NUTRIENT INPUTS TO ESTUARIES AND COASTAL WATERS 115 90 80 70 60 50 40 an an ~ TV o z Q o 20 10 o 1960 1965 1970 1975 1980 1985 1990 1995 Year FIGURE 5-1 Annual global nitrogen fertilizer consumption for 1960-1995 (1 Tg = 1012 g; data from FAO 1999~. The rate of increase was relatively steady until the late 1980s, when collapse of the former Soviet Union reduced fertilizer use in Russia.
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From page 116... ...
116 CLEAN COASTAL WATERS Denitrified? NH3 Building up in soil?
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From page 117... ...
1997) , matching the natural rate of biological nitrogen fixation on all the land surfaces of the world (Vitousek et al.
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From page 118... ...
Human activity is estimated to have increased nitrogen inputs to the coastal waters of the northeastern United cat r a.
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From page 119... ...
Therefore, if China's population continues to grow it may once again be forced to import food from the United States and other developed countries, leading to more use of nitrogen fertilizer here. WASTEWATER AND NONPOINT SOURCE INPUTS Traditionally, most water quality management emphasizes control of discharges from wastewater treatment plants and other point sources.
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From page 120... ...
1996~. Nonpoint sources also dominate for phosphorus inputs to surface waters in the United States (Sharpley and Rekolainen 1997; Carpenter et al.
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From page 121... ...
. For tropical regions, rates of biological nitrogen fixation and natural deposition of nitrogen from the atmosphere are far higher, and so nitrogen export to downstream ecosystems from undisturbed ecosystems may also be greater (Howarth et al.
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From page 122... ...
Assuming a baseline flux of 133 kg N key yr-~ for an undisturbed temperate landscape, human activity has increased the nitrogen flux in the Mississippi River by more than 4-fold, in the rivers of the northeastern United States by 8-fold, and in the rivers draining to the North Sea by 11-fold (Howarth 1998~. In an independent analysis for Chesapeake Bay, Boynton et al.
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From page 123... ...
In the United States, the value is somewhat lower, but still 25 percent of the inorganic nitrogen fertilizer that is used is volatilized to the atmosphere (Holland et al.
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From page 124... ...
The rate of use of inorganic nitrogen fertilizer, on the other hand, continued to rise rapidly until the early 1980s (Figure 5-4~. This relative gain in nitrogen use over phosphorus use resulted primarily from favorable crop yield responses, especially corn, to nitrogen fertilizers.
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From page 125... ...
In those areas with an excess of nitrogen and phosphorus relative to crop needs, there is a greater risk of nutrient export from agricultural watersheds to surface and ground waters (Figure 5-8~. This excess of nutrients in manure tends to occur in areas where downstream export is likely due to relatively wet climates, since high water availability is conducive to animal feeding operations.
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From page 126... ...
26 CLEAN COASTAL WATERS N Frog e n Phosphorus PD~h~ ~~ C~U~TV In Tho~shnds ~BStlRn 104 ~ ]
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From page 127... ...
SOURCES OF NUTRIENT INPUTS TO ESTUARIES AND COASTAL WATERS N itrogen 127 rr~r~t or err Taken L1p and RDm~d mat Owed be supply bV Nltromr~ In IVIB refire r~r~terthen 1W~ 50%to low n 5~* brl~ FIGURE 5-6 Potential for nitrogen and phosphorus available from animal manure to meet or exceed plant uptake and removal on non-legume, harvested cropland, and hayland (modified from Lander at al.
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From page 128... ...
Unfortunately, many of these areas of soil phosphorus build-up tend to occur in areas with phosphorus-sensitive water resources or major drainage ways, such as the inland waters of the Carolinas, Florida Everglades, Lake Okeechobee, Great Lakes, and the Mississippi River basin (Figure 5-9~. Although this survey of soil phosphorus includes only samples sent for analysis and does not represent a complete survey of all soils in the United States, it does highlight some of the effects of long-term changes on agricultural production systems.
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From page 129... ...
began to shape patterns of fertilizer application. These patterns of nutrient distri
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From page 130... ...
button and accumulation have come about by a number of complex and interrelated factors, not merely independent farmer decisions (Lanyon 1999~. Farmers could do little to increase nutrient supplies on their farms when nitrogen and phosphorus fertilizers were scarce.
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From page 131... ...
1995~. In the eastern United States, conversion of land from forests to agriculture between 1700 and 1900 resulted in a 10-fold increase in soil erosion and a presumed similar increase in phosphorus export to coastal waters, even without any addition of phosphorus fertilizer (Meade 1988; Howarth et al.
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From page 132... ...
However, they can contribute to eutrophication of downstream aquatic ecosystems. While phosphorus export from agricultural systems is usually dominated by surface runoff, important exceptions occur in sandy, acid organic, or peaty soils that have low phosphorus adsorption capacities and in soils where the preferential flow of water can occur rapidly through macropores (Sharpley et al.
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From page 133... ...
1996~. For typical farming practices in the United States, the percentage of fertilizer that leaches to ground and surface waters varies between 10 and 40 percent for loam and clay soils, and 25 and 80 percent for sandy soils (Howarth et al.
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From page 134... ...
1999~. Virtually all the nitrogen volatilized from agricultural fields is eventually redeposited back onto the landscape and can reach estuaries and coastal waters (Howarth et al.
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From page 135... ...
found that estimates of nitrogen consumption by domestic animals were far better as predictors of nonpoint source nitrogen fluxes in rivers than were rates of application of inorganic nitrogen fertilizer. Fate of Nitrogen in Atmospheric Deposition Reactive nitrogen in the atmosphere includes both reduced compounds (NHy)
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From page 136... ...
However, because of the large natural flux of nitrogen from the deepwater of the North Altantic Ocean onto the continental shelf off the eastern U.S., this atmospheric deposition probably contributes less than 10 percent of the total input of nitrogen to the surface waters of the continental shelf (Howarth 1998~. Much of the reactive nitrogen deposited from the atmosphere falls onto terrestrial ecosystems.
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From page 137... ...
forests, Johnson (1992) found no relationship between nitrogen inputs and nitrogen losses to downstream ecosystems the percentage of nitrogen deposition that was retained varied among forests from nearly none to virtually all.
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From page 138... ...
The fate of atmospheric nitrogen deposited onto urban and suburban landscapes appears to be virtually unstudied, although the nitrogen content of stormwater runoff from urban environments is high (EPA 1983~. Deposition onto urban landscapes is high, as expected since much of the reactive nitrogen in the atmosphere is deposited near sources, and it is reasonable to expect that the export of this deposition to coastal waters is also high.
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From page 139... ...
1999~. Uncertainty over the extent of nitrogen deposition in urban environments is one of the greatest uncertainties in the nitrogen budget for the United States (Holland et al.
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From page 140... ...
Krug (1993) has shown that in southern Sweden, the conversion of the last 10 percent to 15 percent of land into agricultural use disproportionately destroyed fringing wetlands and therefore doubled nitrogen inputs to streams.
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From page 141... ...
However, much insight into nitrogen fluxes to coastal waters has been gained recently by analyzing fluxes at relatively large spatial scales. Over the past six years, the International SCOPE Nitrogen Project has been analyzing nitrogen fluxes at the scale of large regions, such as the combined watersheds of the North Sea, the combined watersheds of the northeastern United States from Maine through the Chesapeake Bay, and the Mississippi River basin.
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From page 142... ...
1996~. For imports, the analysis carried out by the International SCOPE Nitrogen Project considered application of nitrogen fertilizer, nitrogen fixation by agricultural crops, deposition from the atmosphere of oxidized forms of nitrogen (which are presumed to come primarily from fossil-fuel combustion in the temperate zone; Holland et al.
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From page 143... ...
(A) Total nitrogen fluxes in rivers and in sewage treatment plants.
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From page 144... ...
FIGURE 5-13 The relationship between population density and the export of nitrogen in rivers to the coast for temperate regions surrounding the North Atlantic Ocean. Each point represents one region.
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From page 145... ...
Lawn , SW European Coast Ah Canad,a' · , 0 1 000 2000 3000 4000 5000 6000 Net Anthropogenic Nitrogen Inputs Per Area (kg km-2 yr1 ) y= 102.5+0.2x r2=0.73 p = 0.002 7000 8000 FIGURE 5-14 A comparison of human-controlled inputs of nitrogen to a region and nitrogen export from the region to the coast in rivers, for temperate regions surrounding the North Atlantic Ocean.
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From page 146... ...
146 o vat o To o g o c., Xt ~ .
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From page 147... ...
1999~. The best regression fit for the export of nitrogen from nonpoint sources for the temperate regions of the North Atlantic Ocean results from using the sum of NOy deposition and ammonium deposition to predict nitrogen export (Figure 5-15A-D; r2 = 0.92; Howarth 1998~.
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From page 148... ...
~ High (> 1000) em .J No data FIGURE 5-17 Predicted local total nitrogen yield in hydrologic cataloging units of the conterminous United States.
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From page 149... ...
1997~. For the portions of the United States where total nitrogen export was over 1,000 kg N km-2 yaw, the SPARROW model estimated land-water delivery factors of 0.24 for livestock wastes, 0.32 for fertilizer application, and 1.62 for atmospheric deposition (Smith et al.1997~.
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From page 150... ...
and 0.20 for nitrogen fertilizer application plus nitrogen fixation by agricultural crops leads to the conclusion that nitrogen from depositional sources is about two-fold more mobile in the landscape than is nitrogen running off agricultural fields. This conclusion is consistent with that from the regional analysis of the International SCOPE Nitrogen Project discussed earlier, which also demonstrated the greater mobility of nitrogen from NOy deposition (Howarth et al.
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From page 151... ...
Methodologies for determining the sources of nutrients and the magnitude of the load contributed by each are poorly developed at the scale of individual estuaries, and there is an urgent need for developing better approaches, particularly with regard to atmospheric deposition of nitrogen onto the landscape. The large-scale and regional analyses discussed above (the International SCOPE Nitrogen Project and the SPARROW
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From page 152... ...
In many other estuaries, budgets estimate the importance only of direct deposition onto the surface waters of the estuary itself (and generally only wet deposition, not dry deposition) , and do not estimate deposition onto the landscape with subsequent export to the estuary.
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From page 153... ...
Both the SPARROW model and regressions comparing nitrogen flux in rivers to sources of nitrogen across landscapes (used by the International SCOPE Nitrogen Project) represent examples of statistical approaches that appear to provide reliable estimates of the portion of the nitrogen deposition retained in the landscape versus what is exported to rivers and coastal areas.
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From page 154... ...
1996~. In contrast, estimates from the process-based model indicated 9 percent of the nitrogen flux of the Hudson-Raritan total nitrogen load comes from nitrogen deposition onto the landscape.
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From page 155... ...
Note that, on average, nitrogen is exported in foods and feedstocks Tom the Mississippi basin and imported to Me northeastern United States.
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From page 156... ...
estimates that between 10 percent and 40 percent of the total nitrogen input to estuaries comes from atmospheric deposition, including deposition directly onto the water surface and onto the watershed (EPA l999c)
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From page 157... ...
Consequently, human activity has almost tripled nitrogen input to the Gulf of Mexico, but has increased nitrogen inputs to the waters on the continental shelf of the northeastern United States by only 28 percent (Table 5-1~. Of course, much of this input in the northeastern United States is concentrated in the plumes of a few rivers, such as that of the Hudson River, and these waters may therefore be experiencing eutrophication (Howarth 1998~.
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From page 158... ...
1997~. Interestingly, most of the increase in nitrate in the Mississippi River was due to increased use of nitrogen fertilizer (Goolsby et al.
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From page 159... ...
Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution' ...............
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From page 160... ...
While sewage inputs dominate in some estuaries, nonpoint sources dominate nationally. Insufficient effort has been expended on controlling nonpoint sources of nitrogen and phosphorus, and there are few comprehensive plans for managing nutrient enrichment of the nation's coastal waters, particularly from nonpoint sources.
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From page 161... ...
For instance, nitrogen deposition and fate in urban and suburban areas is poorly known, and wet nitrogen deposition in coastal areas is poorly understood. There is only a limited understanding of dry deposition in any environment, and understanding this in coastal areas and over water is challenging.
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From page 162... ...
62 CLEAN COASTAL WATERS the producers; the larger costs, such as the external cost of nutrient exports to estuaries, remain unaddressed. As is discussed further in Chapter 9, a balanced and cost-effective nutrient management strategy will require an understanding of both the relative importance of various sources of nutrients, and the economic costs associated with reducing the loads attributable to each.
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