NUTRIENTS - Marine Pollution: What Everyone Needs to Know - page 20

coastal waters worldwide. Nitrogen is an essential nutrient
and a fertilizer that is important for agricultural productivity,
but when too much of it gets into the water it is a pollutant.
The ability to synthesize N into fertilizer on an industrial
scale increased crop yields throughout the twentieth century.
Synthetic fertilizer not only fueled this growth, it also sup-
ported human population growth, providing a steady and
cheap supply of grains. Synthetic fertilizer was a benefit in
terms of crop yield but is an ongoing environmental problem,
primarily because of nutrient runoff into aquatic ecosystems.
The increased use of commercial fertilizers has increased N
inputs by tenfold in many parts of the world. Only about 18%
of the N in fertilizer actually gets into the produce; the rest is
absorbed in the soil, runs off into the water, or enters the atmo-
sphere. The amount of manure produced by huge herds of live-
stock may exceed the ability of the croplands to absorb it, so the
rest runs off into the streams that lead eventually to estuaries.
Nutrients also come from the atmosphere—N released
from the burning of fossil fuels returns and gets deposited on
the land or in the water. The burning of fossil fuels, which emit
nitrogen oxides into the atmosphere, initially creates acid rain
and air pollution, followed by water pollution once it comes
down in precipitation. These nutrients cause algal blooms, fol-
lowed by hypoxia (low oxygen) in deeper waters, a process
called eutrophication (Figure 2.1).
The global rise in eutrophication is due to increases in
intensive agriculture, industrial activities, and the human
population. There are variations in the importance of each
source among regions. For example, in the United States
and Europe, agricultural sources (animal manure and fertil-
izers) are generally the primary contributors, while sewage
and industrial discharges (both of which are regulated and
usually receive treatment prior to discharge) are a secondary
source. Atmospheric sources are also a significant contributor
of N in coastal areas. In the Chesapeake Bay, for example, the
atmosphere is a major source of all controllable N that enters