Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 83

Salinity has another, nonphysiological effect on nitrification processes. As a
consequence of higher salinity, the concentration of cations also increases. These
compete with NH 4 + for adsorption on the sediment. As a result, the residence time
of NH 4 + within the sediment decreases, and the NH 4 + flux from the sediment
increases. At higher salinities, NH 4 + might diffuse out of the sediment before nitri-
fication can take place. 21
Rooted benthic macrophytes might also influence nitrification-denitrification pro-
cesses in deeper sediment because they release O 2 via their roots. This release could
stimulate nitrification and thus provide an additional NO 3 − source for denitrification. 15
Sulfide, the product of anaerobic sulfate reduction, is quantitatively the most
important toxic sulfur compound in marine sediment. 17 Sulfide concentrations can
significantly reduce the activity of nitrifying bacteria by lowering the redox potential, 20
and concentrations between 0.9 and 40 M can inhibit nitrification completely. 18 HS −

concentrations in estuarine sediment commonly range from 7-200

µ

µ M, which is much

lower than those for organic-rich sediment ( 1 mM). The range of HS − concentration

in freshwater sediment pore water is much lower (0-30

>

M). 22
The presence of nitrifying bacteria in anaerobic sediment at depths well below
the zone into which oxygen can penetrate is attributable to macrofaunal irrigation
of sediment by physical resuspension and bioturbation. 15,20

µ

4.1.1.3 Denitrification

Denitrification is the microbiological reduction of nitrate to nitrogen gas, where
facultative heterotrophic organisms use nitrate as the terminal electron acceptor
under anoxic conditions:

− −

→→→→

NO NO NO N O N (4.5)

3 2 2 2

Nitrogen gas is largely unavailable to support primary production; therefore, deni-
trification removes a substantial portion of the biologically available nitrogen and
represents a mechanism for partial buffering against coastal eutrophication. 18,22
The nitrification and denitrification processes taking place in the sediment and
in the sediment-water interface are schematically shown in Figure 4.2. Several
factors affect denitrification rate, including temperature, pH, redox potential, as well
as concentrations of oxygen, nitrate, and organic matter. 7,8,13,14,18
Denitrification rate is highly temperature dependent and generally increases with
increasing temperature. 7 However, because of other factors such as nitrification rate
and oxygen concentration, which also are temperature dependent it is difficult,
especially in sediment, to separate the effect of temperature alone. 18
The rate of denitrification decreases with acidity. 13,23 The pH range, where
denitrifiers are most active, is given as 5.8-9.2. 7
In marine systems, one of the most important environmental factors favoring
denitrification is the availability of organic matter. 14 Simple organic compounds,
such as formate, lactate, or glucose, usually serve as the electron donor in addition
to their assimilation. Coastal marine environments act as centers of deposition for