Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 84

NH 4 + flux NO 3 − flux N 2 flux

WATER
SEDIMENT

settling

ammonification nitrification denitrification

NH 4 +

NO 3 −

N 2

Organic N

nitrate
ammonification

burial

FIGURE 4.2 Nitrification and denitrification in sediments.

continentally derived organic materials. Thus, most denitrification in marine sedi-
ment occurs in coastal regions rather than deep-sea environments. 22
Oxygen concentrations can also affect denitrification rates. Denitrification is gen-
erally considered to occur only under low oxygen or anaerobic conditions. 7 To explain
coupled nitrification-denitrification processes in sediment, it is often assumed that
these processes are separated vertically within the sediment. However, denitrification
can also occur within reduced microzones in the aerobic surface layer of sediment. In
both freshwater and marine systems, an oxygen concentration of 0.2 mg l −1 or less is
required for denitrification to occur in the water or sediment. 18 Bonin and Raymond 24
studied the kinetics of denitrification under different oxygen concentrations using
Pseudomonas nautica isolated from marine sediment. They reported that denitrification
can take place in the presence of oxygen. However, enzymes associated with denitri-
fication are affected by the presence of oxygen. Nitrate reductase enzyme was com-
pletely inhibited at oxygen concentrations greater than 4.05 mg l −1 , compared with
2.15 mg l −1 and 0.25 mg l −1 for nitrite and nitrous oxide reductase enzymes, respec-
tively. Yet, these results must not be generalized to all denitrifying strains because
some bacteria are inhibited by oxygen while other species are not.
In many coastal environments, seasonal trends in denitrification are determined
largely by availability of NO 3 − which is controlled by rates of nitrification. 20 The
response of denitrification rates in sediment slurries to increasing nitrate concentra-
tions can often be described by Michaelis-Menten type kinetics. The half-saturation
constant for marine sediment generally ranges from 27-53

18
Supplies of NO 3 − for denitrification in coastal marine sediment appear to be
derived almost exclusively from sediment nitrification. 17 Diffusion of nitrate from
the overlying water into the sediment is also a potential nitrate source for denitrifi-
cation, and its rate in the sediment is 3-4 orders of magnitude greater than that of
the overlying water. There is also evidence that the release rates of nitrate and
ammonium from sediment are greater than their diffusion rates into the sediment.
Nitrification is usually observed in the upper 5 cm of sediment, and the nitrate
produced diffuses either up to the water or down to the anoxic zone, where denitri-
fication takes place. 18,23

µ M NO 3 − .