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The peaks and valleys within the overall upward trend generally
reflect years with higher rainfall (peaks) and those with
less rainfall (valleys). In wet years, increased runoff from
land surface carries more nitrogen into streams, increasing
nitrogen loads; the reverse is true in dry years.
Discussion Higher values for both loads
and yields reflect greater “leakage” of nitrogen
from a watershed, with potentially significant downstream
effects, particularly on marine ecosystems. Total nitrogen
is the preferred form for reporting on the amount of nitrogen
delivered from the U.S. landscape to our coastal waters, but
because the historical record for it for the Mississippi River
is short, we chose instead to present river nitrate loads.
Nitrate is the largest component of total nitrogen and serves
as a strong indicator of total nitrogen loads. The longer
historical record for nitrate reveals the significant increases
that have occurred over the past few decades. Future reports
may present loads of total nitrogen.
Other indicators report on the amount of nitrate dissolved
in streams or groundwater in farmlands,
forests, grasslands
and shrublands, and urban
and suburban areas.
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