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Publication year : 0
Thematic : Coastal Biodiversity
Language : English
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In marine sediments, the major anaerobic mineralization
processes are Fe(III) oxide reduction and sulfate reduction.
In this article, we propose that the two alternative
microbial mineralization pathways in sediments exert
decisively different impacts on aquatic ecosystems. In
systems where iron reduction dominates in the recently
deposited sediment layers, the fraction of Fe(III) oxides
that is dissolved to Fe(II) upon reduction will ultimately be
transported to the oxic layer, where it will be reoxidized.
Phosphorus, which is released from Fe(III) oxides and
decomposing organic matter from the sediment, will be
largely trapped by this newly formed Fe(III) oxide layer.
Consequently, there are low concentrations of phosphorus
in near-bottom and productive water layers and
primary production tends to be limited by phosphorus
(State 1). By contrast, in systems where sulfate reduction
dominates, Fe(III) oxides are reduced by sulfides. This
chemical reduction leads to the formation and permanent
burial of iron as solid iron sulfides that are unable to
capture phosphorus. In addition, the cycling of iron is
blocked, and phosphorus is released to overlying water.
Owing to the enrichment of phosphorus in water, the
nitrogen : phosphorus ratio is lowered and nitrogen tends
to limit algal growth, giving an advantage to nitrogenfixing
blue-green algae (State 2). A major factor causing a
shift from State 1 to State 2 is an increase in the flux of
labile organic carbon to the bottom sediments; upon
accelerating eutrophication a critical point will be reached
when the availability of Fe(III) oxides in sediments will be
exhausted and sulfate reduction will become dominant.
Because the reserves of Fe(III) oxides are replenished
only slowly, reversal to State 1 may markedly exceed the
time needed to reduce the flux of organic carbon to the
sediment. A key factor affecting the sensitivity of a
coastal system to such a regime shift is formed by the
hydrodynamic alterations that decrease the transport of
O2 to the near-bottom water, e.g., due to variations in
salinity and temperature stratification.
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Encoded by : Pauline Carmel Joy Eje
