Geology Reference
In-Depth Information
consist of an amorphous, hydrated silicate, SiO
2
nH
2
O, often called
opaline silica.
Photosynthetic carbon fixation depends on the availability of the
nutrients, and of course, it ceases when one of the nutrient supplies is
exhausted. The limiting nutrient in the ocean is usually nitrogen, but
there are conditions under which Fe may limit biological productivity.
Biological productivity is greatest in regions with the greatest supply of
nutrients, such as coastal regions and near zones of upwelling. Moreo-
ver, biological activity is seasonal at most latitudes, being influenced by
PAR coupled with the depletion of nutrients from surface waters that
become thermally stratified due to summer warming.
The biogeochemical cycling of nitrogen in the marine environment is
quite complex, largely owing to the diversity of forms in which it is
present, coupled with the ease with which organisms can either assim-
ilate or transform the various nitrogenous species. Nitrogen exhibits the
range of oxidation states, from N(V) in nitrate to N(
III) in NH
3
and
numerous organic compounds. Also, N occurs as dissolved gaseous
molecules, including N
2
and N
2
O; dissolved inorganic nitrogen (DIN)
such as NO
3
and NO
2
; dissolved organic nitrogen (DON) present in
organic compounds like amino acids; and particulate organic nitrogen
(PON) in organisms and detritus. Few marine organisms are capable of
assimilating N
2
directly, (known as nitrogen fixation) despite its relative
abundance in seawater. The nitrogenous species more commonly used
are nitrate, nitrite, urea and ammonium.
Bacteria play an important role in the redox chemistry of nitrogen
species in seawater. Starting with PON, the first step is remineralisation
in which PON is converted to DON. The breakdown of some of the
DON to DIN follows with the first product being NH
3
; the process is
relatively rapid and known as ammonification. NH
3
is protonated to a
limited extent in seawater, giving rise to NH
4
1
ions. Nitrification is the
stepwise oxidation of NH
4
1
to NO
2
and eventually to NO
3
. Denitri-
fication, the reduction of nitrogen species to N
2
, can occur under
conditions of hypoxia or anoxia. In such cases, bacteria respire organic
material using NO
3
and NO
2
as electron acceptors.
A depth profile of nitrate, phosphate and silicate in the North Pacific
Ocean is presented in Figure 11. Nutrients behave much like
P
CO
2
and
are removed in the surface layer, especially in the photic zone. Thus,
concentrations can become quite low, and indeed sufficiently low to limit
further photosynthetic carbon fixation. The organisms sink following
death. The highest concentrations occur where respiration and bacterial
decomposition of the falling organic material are greatest, that is at the
oxygen minimum. The nutrients,
including silica, are consequently
