This chapter discusses nutrients, specifically nitrogen and phosphorus, which produce many stormwater problems because of urban and agricultural land use contributions. The discussion covers both low terrestrial delivery rates, at which microbial populations react with higher metabolic rates, much like fertilizing garden plants, and high rates that exceed uptake resulting in nutrient export to streams. The chapter describes how excessive nutrients entering surface waters lead to algal blooms, subsequent bacterial decay, depressed oxygen levels, and dead fish. In oxygen-depleted soils, certain microbes use nitrate in a way similar to oxygen while consuming organic matter. This chapter also discusses denitrification, the microbial process that converts nitrate to nitrogen gas, removing it from the biosphere. It also discusses land use issues with the overall message that pristine forested watersheds export a few kilograms of nitrogen per hectare each year, whereas agricultural and urban areas export tens of kilograms.
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