Coastal Lagoons - Chapter 4 docx

Biogeochemical Cycles

Melike Gürel, Aysegul Tanik,

Rosemarie C. Russo, and I. Ethem Gönenç

CONTENTS

4.1 Nutrient Cycles

4.1.1 Nitrogen Cycle

4.1.1.1 Uptake of Nitrogen Forms

4.1.1.2 Nitrification

4.1.1.3 Denitrification

4.1.1.4 Nitrate Ammonification

4.1.1.5 Mineralization of Organic Nitrogen

(Ammonium Regeneration)

4.1.1.6 Ammonia Release from Sediment

4.1.1.7 Nitrogen Fixation

4.1.2 Phosphorus Cycle

4.1.2.1 Uptake of Phosphorus

4.1.2.2 Phytoplankton Death and Mineralization

4.1.2.3 Phosphorus Release from Sediment

4.1.2.4 Sorption of Phosphorus

4.1.2.5 Significance of N/P Ratio

4.1.3 Silicon Cycle

4.1.3.1 Uptake of Silicon

4.1.3.2 Settling of Diatoms

4.1.3.3 Dissolution of Silica

4.1.4 Dissolved Oxygen

4.1.4.1 Processes Affecting the Dissolved Oxygen

Balance in Water

4.1.4.1.1 Reaeration

4.1.4.1.2 Photosynthesis—Respiration

4.1.4.1.3 Oxidation of Organic Matter

4.1.4.1.4 Oxidation of Inorganic Matter

4.1.4.1.5 Sediment Oxygen Demand

4.1.4.1.6 Nitrification

4.1.4.2 Redox Potential

4.1.5 Modeling of Nutrient Cycles

4.1.5.1 Modeling Nitrogen Cycle

4.1.5.1.1 Phytoplankton Nitrogen

4

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4.1.5.1.2 Organic Nitrogen

4.1.5.1.3 Ammonium Nitrogen

4.1.5.1.4 Nitrate Nitrogen

4.1.5.1.5 Organic Nitrogen (Benthic)

4.1.5.1.6 Ammonia Nitrogen (Benthic)

4.1.5.1.7 Nitrate Nitrogen (Benthic)

4.1.5.2 Modeling of Phosphorus Cycle

4.1.5.2.1 Inorganic Phosphorus

4.1.5.2.2 Phytoplankton Phosphorus

4.1.5.2.3 Organic Phosphorus

4.1.5.2.4 Organic Phosphorus (Benthic)

4.1.5.2.5 Inorganic Phosphorus (Benthic)

4.1.5.3 Modeling of Silicon Cycle

4.1.5.4 Modeling of Dissolved Oxygen

4.1.5.4.1 Dissolved Oxygen

4.1.5.4.2 Dissolved Oxygen (Benthic)

4.1.5.4.3 Sediment Oxygen Demand

4.2 Organic Chemicals

4.2.1 Sources of Organic Chemicals

4.2.2 Classification of Organic Chemicals That Might Appear

in Aquatic Environments

4.2.3 Fate of Organic Chemicals in Aquatic Environments

4.2.3.1 Volatilization

4.2.3.2 Ionization

4.2.3.3 Sorption

4.2.3.4 Hydrolysis

4.2.3.5 Oxidation

4.2.3.6 Photolysis

4.2.3.7 Biodegradation

4.2.4 Governing Equations of Reactions To Be Used in Modeling

4.2.4.1 Volatilization

4.2.4.2 Sorption

4.2.4.3 Computation of Partition Coefficients

4.2.4.4 Hydrolysis

4.2.4.5 Oxidation

4.2.4.6 Photolysis

4.2.4.7 Biodegradation

Acknowledgments

References

4.1 NUTRIENT CYCLES

Among the most productive ecosystems in the biosphere, coastal lagoons cover 13%

of world’s coastal zone1 and constitute an interface between terrestrial and marine

environments.2,3 Nutrient loadings coming from both boundaries to lagoon ecosys￾tems have increased considerably in recent years, and they have a major impact on

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water quality and ecology.4,5 Control of nutrients is thus one of the major problems

faced by those responsible for the management of these sensitive ecosystems. In

order to develop appropriate modeling strategies for making scientifically sound

approaches to reduce the risk of environmental degradation of these ecosystems, a

better understanding of nutrient cycles is required.

In this section, nutrient cycles and their associated mechanisms and major

reactions in coastal marine environments are described. Additional information on

eutrophication caused by nutrient loading will be presented in Chapter 5.

4.1.1 NITROGEN CYCLE

Among nutrients, nitrogen is of particular importance because it is one of the major

factors regulating primary production in coastal marine environments.6–8 Nutrients

are imported to coastal lagoons via atmosphere, agricultural lands, forests, rivers,

urban and suburban run-off, domestic and industrial wastewater discharges, ground￾water, and the sea. Nutrients are exported via tidal exchange, sediment accumulation,

and denitrification. An additional source is nitrogen fixation. Internal sources of

nitrogen include benthic and pelagic regeneration. In general, little is known about

the supply of nutrients from the atmosphere and groundwater to coastal lagoons.9

The nitrogen forms that are important in aquatic environments are ammonia/

ammonium (NH4

+/NH3), nitrate (NO3

−), nitrite (NO2

−), nitrogen gas (N2), and organic

nitrogen. These different forms of nitrogen, present in different oxidation states,

undergo oxidation and reduction reactions. Ammonia and oxidized forms of nitrogen

(NO2

−, NO3

−) constitute dissolved inorganic nitrogen (DIN), which can be utilized

by phytoplankton for growth or by bacteria as an electron acceptor. Typical concen￾trations of NH4

+ and NO3

− in coastal waters range from <1–10 µM and <2–25 µM,

respectively.10 The various nitrogen compounds and their oxidation states, together

with their molecular formulas, are given in Table 4.1.

Ammonia exists in two forms: ammonium ion (NH4

+) and unionized ammonia

(NH3). The latter form is toxic to aquatic organisms and is in equilibrium with the

ammonium and hydrogen cations. The concentrations of these forms vary consid￾erably as a function of pH and temperature in natural water bodies. The method of

calculation of the percent of total ammonia that is unionized at different pH and

temperature is given in Emerson et al.11

(4.1)

TABLE 4.1

Forms of Nitrogen and Their Oxidation States

Forms of Nitrogen Molecular Formula Oxidation State of N

Ammonium NH4

+ −3

Unionized ammonia NH3 −3

Nitrogen gas N2 0

Nitrite NO2

− +3

Nitrate NO3

− +5

NH NH H 4 3

+ +
+

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Nitrogen compounds can be classified into organic and inorganic nitrogen. Organic

nitrogen in water bodies can be found in both dissolved and particulate forms. The

particulate organic nitrogen (PON) is composed of organic detritus particles and

phytoplankton and has two possible fates. Dead plant cells lyse and bacteria degrade

the resulting dissolved organic nitrogen (DON) or protozoa/zooplankton to consume

PON.12 Most of the DON in seawater is still chemically uncharacterized, and its

chemical and biological properties are becoming better known.7 Except for amino

acids and urea, which comprise only a small fraction of DON, most of the DON

may be resistant to decomposers.10 Excretion by animals also releases dissolved

nitrogen. Zooplanktons excrete free amino acids, ammonia, and urea. Fish excrete

ammonia, urea, and other organic compounds.7

In aquatic ecosystems, a very complex biogeochemical nitrogen cycle is

observed (Figure 4.1). The following sections give information about the processes

involved in the biogeochemical cycling of nitrogen in the aquatic environment.

4.1.1.1 Uptake of Nitrogen Forms

Primary production in coastal waters is largely regulated by the availability of NH4

+

and NO3

− for growth. Ammonium is preferred by phytoplankton, as its oxidation

FIGURE 4.1 Nitrogen cycle.

denitrification

uptake

excretion

excretion

grazing

grazing

death

death

Fish

Sediment

anoxia

settling

Zoo￾plankton

Phyto￾plankton

Organic

Detritus

uptake

N2

N2 fixation

denitrification

nitrification

nitrification

ammonification

ammonification

NO2

− NO3

−

NH4

+

mineralization

death

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state is equivalent to that of cellular nitrogen (−3) and thus requires the least energy

for assimilation.12,13 Ammonia concentrations above 1–2 µM tend to inhibit assim￾ilation of other nitrogen species.10 On the other hand, if nitrate is to be assimilated

for the synthesis of cellular materials, it should be reduced to ammonia with the aid

of several enzymes including nitrate reductase (enzyme catalyzed reduction) within

the cell. This reduction process is called “assimilatory nitrate reduction” and requires

energy.7,14

Nitrogen uptake can be an important process. For example, in Basin d’Arcachon

in southern France, due to the high nitrogen uptake rates of the seagrass Zostera

noltii, nitrogen uptake is quantitatively more important than denitrification as a

nitrogen sink.15

In shallow water systems, biological organisms larger than phytoplankton

turn over slowly, and their metabolism is lower. Nevertheless, these organisms

store large amounts of nitrogen, because a substantial amount of nitrogen is tied

up in their biology. Thus, nitrogen concentrations in the shallow systems tend to

be lower.16

Nutrient assimilation by macrophytes can be significantly different from that by

phytoplankton because macrophytes have the ability to grow for long periods on

stored nutrients. Rooted seagrasses can assimilate nutrients from sediment and

possibly serve as nutrient pumps10 (see Chapter 5 for details).

4.1.1.2 Nitrification

Nitrification is the microbiological oxidation of ammonium to nitrite and then to

nitrate under aerobic conditions, to satisfy the energy requirements of autotrophic

microorganisms. Much of the energy released by this oxidation is used to reduce

the carbon present in CO2 to the oxidation state of cellular carbon, during the

formation of organic matter.

As indicated previously, the first step in nitrification is oxidation of ammonium

to nitrite, which is accomplished by Nitrosomonas bacteria.

(4.2)

The second step is oxidation of nitrite to nitrate by Nitrobacter. This is a faster

process.

(4.3)

The overall nitrification reaction is therefore

(4.4)

The nitrification process can influence marine primary production by competing with

heterotrophs for the limited supply of dissolved oxygen and by decreasing the amount

NH O H NO H O 4

1

2 2 2

+ + − + ++ 1 2
2

NO O NO 2 3

1

2 2

− − +

NH O NO H O H 4 32 2 2 2 + − + + ++

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of NH4

+ 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 µmol l−1 h −1, whereas in coastal sediment nitrification rates are

often 20 µmol l−1 h−1.

18 Nitrification rates measured in coastal sediment are usually

on the order of 30–100 µmol m−2 h−1.

17,10

Physico-chemical and biological factors regulating nitrification in coastal marine

sediment include temperature, light, NH4

+ concentration, dissolved oxygen concen￾tration, pH, dissolved CO2 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°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 O2 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 µM O2.

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.

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Salinity has another, nonphysiological effect on nitrification processes. As a

consequence of higher salinity, the concentration of cations also increases. These

compete with NH4

+ for adsorption on the sediment. As a result, the residence time

of NH4

+ within the sediment decreases, and the NH4

+ flux from the sediment

increases. At higher salinities, NH4

+ 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 O2 via their roots. This release could

stimulate nitrification and thus provide an additional NO3

− 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 µM can inhibit nitrification completely.18 HS−

concentrations in estuarine sediment commonly range from 7–200 µM, 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 µM).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

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:

(4.5)

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

NO NO NO N O N 3 2 22

− − → →→ →

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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 Raymond24

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 NO3

− 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 µM NO3

−.

18

Supplies of NO3

− 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

FIGURE 4.2 Nitrification and denitrification in sediments.

N2 flux

settling

Organic N NH4

+

NH4

+ flux

NO3

−

NO3

− flux

N2

nitrate

ammonification

burial

ammonification nitrification denitrification

WATER

SEDIMENT

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Macrophytes, benthic algae, and certain macrofauna have been shown to influ￾ence denitrification rates in both freshwater and marine sediment by affecting the

oxygen and/or the nitrate distribution in the sediment.18

A wide range of experimental methodologies has been developed to estimate

denitrification rates in shallow marine environments. These techniques are based on

different assumptions; therefore, care must be taken when comparing denitrification

rates obtained using these different techniques. Seitzinger18 has given the ranges of

denitrification rates as 50–250 µmol N m−2 h−1 in estuarine and coastal marine

sediment, 2–171 µmol N m−2 h−1 in lake sediment, and 0–345 µmol N m−2 h−1 in

river and stream sediment. In low oxygen hypolimnetic lake waters, denitrification

rates were generally 0.2–1.9 µmol N l−1d−1. The higher rates were from systems that

receive substantial amounts of anthropogenic nutrient input. Groundwater is another

nitrate source.18

4.1.1.4 Nitrate Ammonification

Denitrification is widely accepted as the dominant process of nitrate reduction in

most shallow marine sediment. An alternate pathway to denitrification is nitrate

ammonification, which is the reduction of NO3

− to NH4

+ by heterotrophic bacteria.

In contrast to denitrification, nitrogen is not lost from the system but converted to

a readily available nitrogen form.8 Nitrate ammonification can occur occasionally

under anaerobic conditions.2

Nitrate ammonification is also called dissimilatory nitrate reduction, and it has been

described as an important process in marine sediment.24 In both Bassin d’Arcachon and

Étang du Prévost, two coastal lagoons in southern France, rates of nitrate ammonifica￾tion were quantitatively as important as denitrification.15

4.1.1.5 Mineralization of Organic Nitrogen (Ammonium Regeneration)

The process of transforming organic compounds back to inorganic compounds

is generally referred to as mineralization.25 Through the mineralization of organic

nitrogen compounds, nitrogen recycling is accomplished. Recycled nitrogen is

primarily in the form of ammonia and urea (a dissolved organic nitrogen com￾pound). Urea is rapidly broken down to ammonia by bacteria or by the extracel￾lular enzyme urease.8,23,25 Ammonium is regenerated from organic compounds by

animal excretion and by microbial decomposition of organic matter. It is presumed

that excretion contributes to the largest part of NH4

+ regeneration in the water

column, while decomposition of organic matter is the most important in the

sediment.10

It is widely accepted that shallow coastal sediments are important sites for the

mineralization of organic matter. The difference of shallow coastal waters compared

with open seas is that a much larger fraction of the organic matter is mineralized

on the bottom rather than in the water column.2,26 Because of the shallow depth of

coastal areas (e.g., 2–20 m) and the relatively rapid settling rates, a significant portion

of the primary production is transferred to the sediment. Thus, much of the miner￾alization of nutrients occurs in the upper layer of the sediment.2,18,27,28

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Organic compounds are mineralized through both aerobic and anaerobic res￾piration processes. Aerobic respiration, which takes place in the surface sediment

layers (typically 0–5 mm depth), results in a rapid depletion of oxygen. In the

sediment, bacteria oxidize a significant fraction of the organic matter using terminal

electron acceptors other than oxygen (e.g., nitrate, manganese and iron compounds,

sulfate, and carbon dioxide).8,29 The two dominant anaerobic processes are dissim￾ilatory sulfate reduction and methanogenesis (methane production). Generally,

sulfate reduction precedes methanogenesis because sulfate-reducing bacteria out￾compete methanogens for substrates. Freshwater has lower sulfate concentrations

(10–200 µM) than estuarine water (30 mM).22 Decomposition through sulfate

reduction occurs deeper in the sediment column (>10 cm) and provides an addi￾tional source of NH4

+.

26 Sulfate reduction, and subsequent inhibition of nitrification

and denitrification by HS−, should lead to enhanced ammonium regeneration during

summer, when sulfate reduction rates are high compared with those in winter.22 In

all cases, the mineralization of organic nitrogen compounds results in the produc￾tion of NH3/NH4

+.

All living matter contains nitrogenous macromolecules, which become available

to decomposer organisms upon the death of cells. Depending upon the structural

complexity of the organic matter, mineralization can either be a simple deamination

reaction or a complex series of metabolic steps involving a number of hydrolytic

enzymes. Thus, mineralization rates depend on the degradability of the organic

matter; i.e., whether it is labile or highly refractory. For example, seagrass detritus

that has 25–30% lignin containing fibers, has a lower mineralization rate than

phytoplankton cells, which contain more labile nitrogenous material.8 Another

parameter affecting the mineralization rate of organic matter is temperature.7 Sea￾sonal patterns of benthic nutrient regeneration generally exhibit strong summer

maxima, which correlate well with water temperature. The effects of temperature

can be represented by Arrhenius type expressions.27

Mineralization of organic nitrogen plays a central role in nitrogen recycling in

coastal marine environments. Regeneration from the sediment regulates all produc￾tivity since inorganic nutrients are the limiting factors for primary production,30 and

much of the primary production of many coastal marine systems is supported by

nutrient recycling rather than by nutrient inputs alone.26 In shallow water ecosystems,

benthic recycling may account for 20–80% of the nitrogen requirements of the

phytoplankton.8,27 Nixon26 reported that nutrient inputs to Narragansett Bay, U.S.A.

(without being recycled) could support, at the most, only 24–50% of the annual

production, depending on the nutrient considered.

Ammonium produced during the deamination of organic nitrogen in sedi￾ment is not totally available to the primary producers; some of the ammonium

remains dissolved in interstitial water, some is adsorbed and buried into deeper

sediment,7 some is consumed by benthic algae for cell synthesis,13 and a fraction

undergoes nitrification in the surficial oxic zone of the sediment.8 Denitrification

following nitrification produces gaseous forms of nitrogen (N2, N2O) essentially

unavailable to most coastal phytoplankton.2,31 Thus, the coupled processes of

nitrification–denitrification represent a sink that shunts nitrogen away from recy￾cling pathways.20

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4.1.1.6 Ammonia Release from Sediment

Most of the nitrogen mineralized in the sediment is recycled by diffusion from the

sediment to the overlying water as NH4

+ or NO3

−. Some nitrogen can also be released

as urea and dissolved organic nitrogen, but the quantities of these fluxes are

unknown.2 There are a few important factors influencing the quantity of NH4

+ and

NO3

− release, which are explained below.

As stated previously, benthic regeneration is a function of temperature. NH4

+

regeneration rates and pore-water concentrations tend to increase with temperature.26

Factors such as sediment grain size and physical circulation also influence this

process. It has been demonstrated that the activity of (meio- and macro-) fauna

enhances the rate of NH4

+ release from sediment.10

The amounts of NH4

+ and NO3

− depend greatly on seasonal conditions. For

example, the major part of nitrogen released from the sediment is NH4

+ in summer

when the mineralization rate is high and the aerobic zone depth is generally small.

On the other hand, nitrification dominates during winter and spring when the aerobic

zone is deeper, and, therefore, NO3

− is released from the sediment.

In the presence of anaerobic conditions, redox potential decreases significantly,

resulting in the termination of nitrification in the sediment. The loss of the oxic

microzone between the sediment and overlying water under anoxic conditions also

causes a considerable decrease in adsorption capacity of the sediment, producing a

significant increase in the release of NH4

+ from the sediment.

Salinity is another factor influencing ammonium release from sediment. Ambient

exchangeable ammonium concentrations in freshwater sediment are generally consid￾erably greater than those reported for marine sediment.32 Fluctuating salinity plays a

major role in controlling the NH4

+ adsorption capacity of the sediment.21 Specifically,

the total amount of cations (primarily Na+, Mg2+) increases with salinity, resulting in

greater molecular competition with ammonium for the sediment cation exchange

sites.21,32,33

The greater ammonium adsorption in freshwater sediment relative to marine

sediment increases the amount of ammonium that can be nitrified. Seitzinger et al.32

reported in their study that a larger percentage of net ammonium produced in aerobic

freshwater sediment (Toms River, U.S.A.) was nitrified and denitrified (80–100%)

compared with that in marine sediment (40–60%) (Barnegat Bay, U.S.A.).

Postma et al.10 reported an NH4

+ flux range of 50–800 µmol m−2 h−1 for estuarine

and coastal sediment. According to Day et al.27 the annual mean value of nutrient

regeneration ranges from 20–300 µmol m−2 h−1 for NH4

+ in estuarine sediment. The

rate of release of ammonia by a wide variety of marine sediment during summer was

given as 13–710 µmol m−2 h−1 by Nixon.26 NH4

+ fluxes in Chesapeake Bay, U.S.A.,

were reported to be 46 µmol m−2 h−1 in April, increasing to 753 µmol m−2 h−1 in

August.20 The calculated benthic NH4

+ flux at Thau Lagoon, France, for a period of

10 days in August during anoxia was 600 µmol m−2 h−1.

28

4.1.1.7 Nitrogen Fixation

The process that converts atmospheric nitrogen gas (N2) into organic nitrogen com￾pounds is known as nitrogen fixation. Most organisms, due to the significant amount

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