Download as a PDF - CiteSeerX

During the past 20 years, it became clear that problems other than eutrophication
were also important and that inland lakes were not the only water bodies affected. A
National Science and Technology Council (2000) publication dealing with the effects
of nutrients on coastal waters noted that unusually high numbers of deaths of sea
lions and manatees, unusual patterns of coral reef destruction, widespread fish
kills, outbreaks of certain shellfish poisoning, disappearance of sea grasses, and
occurrence of so called ‘dead zones’ could all be linked in one way or another to
subtle changes in the absolute abundance of certain organisms near the very base
of the food web. The abundance of these organisms is related, directly or indirectly,
to nutrients flowing into the system from upstream watersheds. The water quality
problems of these water bodies have one unifying cause: they have been linked, to
some degree, to agricultural practices.
In the US and elsewhere, nitrate levels in high river flows in spring, after the application
of fertilizers, were closely correlated to the percent of the watershed under agriculture
(Figure 1). Because the concentration scale is logarithmic, the chart indicates that
nitrate concentrations begin to increase exponentially when the agricultural land use
percentage exceeds 40% t.
Figure 1: Nitrate concentration vs. % agricultural land use in the Willamette
River Basin, Oregon (from US Geological Survey, 1999). Forest was
the other major land use in this watershed
Currently, the 1991 Nitrate Directive of the European Community requires all Member
States too designate Nitrate Vulnerable Zones for protection of water resources
from agricultural pollution. The methodology for vulnerability assessment identifies
surface waters that are excessively polluted or could become polluted by nitrates.
The strategy is then a reduction of excessive current loads and prevention of future
increases that could impair the water body. The Nitrate Directive applies mostly to
agricultural point (animal husbandry) and non-point sources.
In the US, the US Environmental Protection Agency (USEPA) uses nutrient criteria
to assess vulnerability and target action. The Total Maximum Daily Load (TMDL)
4