Coastal Lagoons - Chapter 4

4 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 © 2005 by CRC Press 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 © 2005 by CRC Press 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 NH4+ NH3 + H+ (4.1) TABLE 4.1 Forms of Nitrogen and Their Oxidation States Forms of Nitrogen Ammonium Unionized ammonia Nitrogen gas Nitrite Nitrate Molecular Formula NH4+ NH3 N2 NO − NO3− Oxidation State of N −3 −3 0 +3 +5 © 2005 by CRC Press 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 c i n NO− denitrification N2 NO− N2 fixation Phyto-plankton grazing NH+ excretion Zoo- death plankton Organic Detritus grazing Fish Sediment FIGURE 4.1 Nitrogen cycle. © 2005 by CRC Press 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. NH4+ +12 O2 2H+ + NO2− +H2O (4.2) The second step is oxidation of nitrite to nitrate by Nitrobacter. This is a faster process. NO2− + 2 O2 NO3− (4.3) The overall nitrification reaction is therefore NH4+ +2O2 NO3− +H2O+2H+ (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 © 2005 by CRC Press ... - tailieumienphi.vn