Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 147

Figure 5.9 A schematic of the nitrogen
cycle. Each of the curved arrows
within the cycle represents a process
carried out by bacteria.

Atmosphere

Nitrogen
fixation

N 2

Denitrification

Ammonium
NH 4

Nitrate
NO 3

Nitrification

Nitrite
NO 2

Nitrification

invention of the Haber process, 2 nitrogen fixation by marine and terrestrial plants
was the dominant source of fixed inorganic nitrogen to the ocean (Galloway et al.,
2004 ), and on very long timescales the nitrogen-fixing bacteria are thought to
govern the global pool of inorganic nitrogen available to the photosynthetic
autotrophs (Karl et al., 2002 ).
The reduction of nitrate back to nitrogen gas, N 2 , by bacteria is called
denitrification. This sink for nitrate generally takes place in regions depleted in
oxygen. It is of particular importance in estuaries, removing much of the nitrate
before it reaches the shelf seas, and also within sediments and the water column on
the shelf (Nixon et al., 1996 ; Hydes et al., 2004 ). You can think of the nitrogen cycle
as beginning with atmospheric N 2 being fixed by the diazotrophs, with different
species of bacteria then controlling the transfers of nitrogen from ammonium,
through to nitrite and nitrate, and eventually denitrification returning N 2 to the
atmosphere. This is summarised in the schematic illustration in Fig. 5.9 . This bac-
teria-mediated cycle is independent of the rest of the marine primary production. The
autotrophic phytoplankton simply tap into the nitrogen cycle, temporarily diverting
some of the fixed nitrogen to aid their own growth.
One final point to note concerns the supply of silicate to the ocean. About two-
thirds of the silicate that enters the ocean is from the weathering of rocks on land and
subsequent transport down rivers into the coastal ocean, with another one-third from
volcanism and hydrothermal vents (Demaster, 1981 ). The cycling of silicate within
the shelf seas, and the physical transports towards and across the shelf edge, are thus
important controls on the amount of silicate that reaches the open ocean.

2 The Haber process, developed by the German chemist Fritz Haber in early 20th century, is a vitally
important industrial method of fixing atmospheric nitrogen to ammonium in the production of
agricultural fertilisers. Today global anthropogenic fixation of nitrogen by the Haber process, along with
a smaller contribution from fossil fuel burning, fixes a similar amount of nitrogen as all marine and
terrestrial plant nitrogen fixers. See J. N. Galloway et al., ( 2004 ). Used to grow crops, this large amount
of fixed nitrogen eventually finds its way into rivers and the coastal ocean, leading to eutrophication.