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important as the shelf edge. However, the localised nature of river sources means
that the nutrient concentrations in the adjacent coastal waters can be very large,
resulting in eutrophication and significant water quality problems arising from the
phytoplankton growth that they fuel. We will discuss eutrophication in more detail
in Chapter 9 .
Nitrogen can also be supplied from the atmosphere, in the form of ammonium
NH 4 Þ
ð
from the treatment of animal wastes and nitric oxide (NO x ) from the com-
bustion of fossil fuels (Jickells, 2006 ). The impact of atmospheric ammonium inputs
tends to occur close to the coast, while the reactions in the atmosphere required to
transform nitric oxides to nitrate can result in long transport times before deposition
to the ocean. Down-wind of heavily populated regions the atmospheric source of
nitrogen can rival river inputs. However, unlike riverine nutrient sources, there is so
far no evidence of atmospheric sources triggering excessive phytoplankton growth
(Jickells, 2006 ).
Finally, bacterial action within the sediments of the seabed will remineralise
organic material, such as dead phytoplankton, and supply the water column with
nutrients. Measurements of nitrogen fluxes out of sediments suggest that nitrogen
is released mainly in the form of ammonium in shallow waters, with the
importance of nitrate increasing in deeper shelf waters (Rowe and Phoel, 1992 ;
Hopkinson et al., 2001 ). Nitrate fluxes out of the sediments of the Irish Sea have
been measured between 0.3 and 0.8 mmol m 2 d 1 (Trimmer et al., 2003 ). This
sediment source of nitrogen to the overlying water can be important in regions
with extensive shelves, where the weak residual flows (see Section 3.7 ) are likely to
limit the direct influences of either fluxes across the shelf edge or nutrients supplied
by rivers.
The discussion above has mainly concentrated on nitrate as the source of
nitrogen to the phytoplankton. Ammonium is generally found in very small quan-
tities, partially because of the oxidation process but also because it is energetically
cheaper for phytoplankton to assimilate nitrogen from ammonium than from
nitrate: when using nitrate, phytoplankton have to first convert it to ammonium,
so if ammonium is available they will preferentially take it up. If it is not used
quickly, for instance when organic material is being recycled in the sediments,
ammonium is oxidised by bacteria to form nitrite
NO 2 Þ
, and further oxidised to
yield nitrate, a process known as nitrification. Another obvious source of nitrogen
to seawater is nitrogen gas from the atmosphere, which is dissolved in seawater and
forms by far the most abundant form of nitrogen in the marine environment. The
chemical bonds holding the N 2 molecule together are very strong, making it
inaccessible to most of the photo-autotrophs. However, there are specialised
organisms in the ocean, the nitrogen-fixers or diazotrophs, which are able to fix
atmospheric nitrogen to ammonium. The most important of the marine diazo-
trophs is the cyanobacterium Trichodesmium (Capone, Zehr, et al., 1997 ), a species
which occurs mainly in the near-surface waters of the deep ocean where nitrogen is
severely limiting in the surface layer. Nitrogen fixation also occurs in coastal
environments as seen, for example, in the western English Channel (Rees et al.,
2009 ). This source of atmospheric nitrogen to the ocean is vital. Up until the
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