As human population increases in coastal zones, watersheds discharge greater amounts of nutrients (nitrogen and phosphorus), accelerating eutrophication of coastal ecosystems. Researchers at the Smithsonian Environmental Research Center are developing regional methods for predicting nutrient discharges based on watershed characteristics. The tools they are developing may help in assessing the relative importance of reducing nitrogen or phosphorus discharges.
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The scientists developed budgets that quantify the balance between human-mediated imports and exports of phosphorus in the agricultural and human food systems for every county in the Chesapeake Bay drainage. They used geographic information system tools to relate those budgets to other watershed characteristics and to predict phosphorus delivery from major rivers to the Bay.
For the entire Chesapeake watershed, people introduce more phosphorus than can be accounted for in the harvest, transport, and effective use of crops, animal feed, and human food. This means that phosphorus is available to accumulate in the landscape or to pollute the water. The annual excess averages 5.41 kilograms of phosphorus per hectare of watershed. The largest phosphorus excesses are found in areas with intense agricultural activities or urban and suburban development.
The net phosphorus balance in a county can be well predicted from three variables: human population density, livestock density, and the percentage of land used for row crops. Together, these factors explain 93 percent of the variability among counties in net phosphorus balance.
Around 10 percent of the phosphorus brought into the Chesapeake Bay watershed by people is discharged to the Bay. In contrast, similar past studies of nitrogen balances show that a larger percentage (20 to 40 percent) of the net amount of nitrogen introduced by people is delivered to coastal waters. Measured phosphorus discharges from major rivers into Chesapeake Bay are strongly correlated with phosphorus inputs to developed lands.
The study helps identify the sources of phosphorus in the watershed and demonstrates the utility of the net phosphorus balance as a predictor of phosphorus discharges. Comparing phosphorus and nitrogen balances for an area can help prioritize which of these major pollutants requires stronger management action.