Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 79

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 N ITROGEN C YCLE

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 (NH 4 + /NH 3 ), nitrate (NO 3 − ), nitrite (NO 2 − ), nitrogen gas (N 2 ), and organic
nitrogen. These different forms of nitrogen, present in different oxidation states,
undergo oxidation and reduction reactions. Ammonia and oxidized forms of nitrogen
(NO 2 − , NO 3 − ) constitute dissolved inorganic nitrogen (DIN), which can be utilized
by phytoplankton for growth or by bacteria as an electron acceptor. Typical concen-
trations of NH 4 + and NO 3 − in coastal waters range from M,

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 (NH 4 + ) and unionized ammonia
(NH 3 ). 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

< 1-10 µ M and <2-25 µ

+ +

NH NH + H (4.1)

4 3

TABLE 4.1
Forms of Nitrogen and Their Oxidation States

Forms of Nitrogen Molecular Formula Oxidation State of N

Ammonium NH 4 + −3

Unionized ammonia NH 3 −3

Nitrogen gas N 2 0

Nitrite NO 2 − +3

NO 3 −

Nitrate +5