Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 82

of NH 4 + that is needed by phytoplankton for growth. 17 The coupling of the nitrification
process with denitrification leads to loss of nitrogen from the atmosphere.
Nitrification can take place either in the water column or in the sediment.
However, nitrification in the water column of shallow marine and estuarine
systems appears to be relatively limited. 17 Nitrification rates in the water column
are at least in order of magnitude smaller than the nitrification rates per unit
volume in sediment. For example, in coastal waters, nitrification rates range from
only ~0.001-0.1 µ mol l −1 h −1 , whereas in coastal sediment nitrification rates are

often 20 mol l −1 h −1 . 18 Nitrification rates measured in coastal sediment are usually

on the order of 30-100

µ

mol m −2 h −1 . 17,10
Physico-chemical and biological factors regulating nitrification in coastal marine
sediment include temperature, light, NH 4 + concentration, dissolved oxygen concen-
tration, pH, dissolved CO 2 concentration, salinity, the presence of any inhibitory
compounds, macrofaunal activity, and the presence of macrophyte roots. 8,17
Temperature influences the metabolic activities of nitrification bacteria. The
optimum temperature is in the range of 25-35

µ

C in pure cultures. 17 Due to both
seasonal and diurnal changes in temperature in shallow coastal sediment, it is
expected that nitrifying bacteria would exhibit optimal growth and/or activity during
daytime and in the summer months when temperatures are maximum. 8 The effect
of temperature on nitrification rates in pure cultures is usually expressed through
Arrhenius type equations. 17 In addition, temperature also affects dissolved oxygen
solubility and therefore the process rates. Light may influence the nitrification
activity in shallow water sediment. Light availability and the penetration depth of
light into sediment may affect benthic nitrification. 17
Nitrification may be strongly impeded by hypoxia since it occurs only under
aerobic conditions. 19 Nitrifying bacteria, therefore, have to compete with other
heterotrophs for the limited supply of dissolved oxygen. The depth distribution of
nitrifying bacteria in sediment is ultimately constrained by the downward dissolved
oxygen diffusion, which is typically 1-6.5 mm. In Chesapeake Bay, U.S.A., 18,20
Étang du Prévost in southern France, 21 and Danish coastal zones, 8 O 2 penetration
into sediment declines due to increased temperature, organic inputs, and decreased
macrofaunal activity in summer. Consequently, thinning of the surficial oxidized
zone of sediment is responsible for the significant summer reduction in nitrification
rates in these systems. The reported dissolved oxygen concentrations, which inhibit
nitrification in sediment are in the range 1.1-6.2

°

M O 2 . 8
Salinity is another factor influencing nitrification. Although nitrifying bacteria
are able to acclimate to a wide range of salinities, such as those found in lagoon
systems, short-term fluctuations may have strong regulating effects on nitrifica-
tion. For example, a marine Nitrosomonas sp . isolated from the Ems-Dollard
estuary at 15% salinity was able to adapt to the entire salinity range (0-35%) and
grew at the same rate over the range after a lag phase of up to 12 days. 17 Rysgaard
et al. 21 reported in their study conducted with the sediment from the Randers
Fjord Estuary, Denmark that both nitrifying and denitrifying bacteria were phys-
iologically influenced by the presence of sea salt, showing lower activities at
higher salinities.

µ