Texas A&M University
Department of Oceanography
Nitrogen's sources and pathways
Pools and cycling of nitrogen in the northwest gulf
by Diego Lopez-Veneroni
The
Mississippi River delivers a considerable amount of nutrients
into the northwest Gulf of Mexico from more than 40 percent of
the continental United States land.
Fertilizers, industrial wastes, and other
nutrients enhance phytoplankton production in the coastal region,
which in turn promote animal and bacterial growth. The respiration
and breakdown of the produced organic matter consume oxygen,
and the coastal region may become hypoxic (low in oxygen concentration).
My study focused on the sources and biogeochemical
pathways of nitrogen in different reservoirs of the northwest
Gulf of Mexico using samples from 10 cruises between 1989 and
1993 on board R/V Gyre. I analyzed the concentrations and stable
isotopes of nitrogen in dissolved and particulate material, zooplankton
and sediments, to trace the sources and cycling of this element
in the region.
The purpose of this research was to determine
the fate of nitrogen that flows from the Mississippi River onto
the adjacent shelf. My key question was: What happens to the
high dissolved inorganic nitrogen concentration after it disappears
within a few miles from the Mississippi River outflow region?
Nitrogen is essential to life because organisms
use it to grow or as an energy source. This element is ubiquitous
in the environment and has a wide range of oxidation states which
favor its biogeochemical cycling (its transport resulting from
biological, chemical and geological processes). In the biosphere,
nitrogen cycling consists of three steps: (1) the uptake of its
inorganic forms, mainly nitrate and ammonium, by phytoplankton,
or microscopic aquatic plants; (2) its transfer as organic nitrogen
to other levels in the trophic food web; and (3) its remineralization
back to the inorganic nitrogen pool by bacteria. Dissolved organic
nitrogen consists of compounds produced by organisms such as
ammino acids and urea, which are released when organisms prey
on others, or via exudation which is slowly released via organisms'
pores and orifices.
Previous research showed that dissolved inorganic
nitrogen from the river is consumed by plankton within a few
miles from the outflow region while the particulate pool (plants,
animals, bacteria and detritus) increases. In this study, it
was shown that the breakdown of particulate nitrogen produces
dissolved organic nitrogen; thus, an important fraction of the
river's nitrogen is transported along the shelf and offshore
waters as dissolved organic nitrogen.
Dissolved organic nitrogen can thus be used
as a tracer of the mixing of riverine nitrogen onto the shelf.
Furthermore, a fraction of dissolved organic nitrogen is continually
breaking down, which suggests that terrestrial nitrogen can indirectly
promote growth of plants and animals throughout the shelf. Stable
isotopes were also used to trace the nitrogen sources for particulate
nitrogen and the food chain among zooplankton.
Nitrogen has two stable isotopes which differ
in the number of neutrons in their nucleus. The much more abundant
lighter isotope (denoted by 14N) has identical chemical but different
physical properties (velocity of reaction, for example) to the
heavier isotope (15N).
These small but measurable differences can
be used to trace sources and cycling pathways of nitrogen.
From nitrogen concentrations and stable isotopes
a model was developed to determine nitrogen inputs and outputs
of the northwest Gulf of Mexico continental shelf. Its results
suggest that approximately 40 percent of the nitrogen on the
shelf originates from the land, while the remaining 60 percent
results from the upwelling of nitrogen-rich oceanic subsurface
waters.
Nitrogen outputs include advection (large-scale
transport) as dissolved organic nitrogen into the Gulf's upper
waters and burial into the shelf's sediments as depositing particulate
nitrogen. Less than 10 percent of the total nitrogen is denitrified
(lost to the atmosphere as nitrogen gas). Thus, an increase in
nitrogen derived from fertilizer use and industrial wastes would
not contribute significantly to atmospheric nitrogen (and possibly
greenhouse gases) because most of it would be buried.
Diego Lopez-Veneroni graduated with a Ph.D. in August 1998.
He is currently working for the Secretaria de Marina (Secretariat
of the Navy) in Mexico City.
Figure:
Average stable nitrogen isotope values
(in delta notation) of dissolved nitrate, particulate nitrogen
(PN) and zooplankton groups in northwest Gulf of Mexico coastal
plume waters. Trophic position increases with increasing isotope
value. H. and C. Copepoda are herbivorous and carnivorous copepods.
http://oceanography.tamu.edu/Quarterdeck/1999/1/lopez.html
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