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turn, determine the residence time of nutrients in the lagoon. For example, discharging
the same amount of nutrients into a leaky lagoon with strong tidal currents will not
have the same local effects as will a similar discharge into a choked lagoon with a
low water exchange ( see Chapter 6 for classifications of lagoons).
Four successional stages can be identified in the eutrophication process: olig-
otrophic, mesotrophic, eutrophic, and hypertrophic. Nixon
provides some ranges
of carbon supply (primary production) in the ecosystem for each stage (Table 5.2).
Abundant seagrasses (such as Eelgrass
11
Zostera, Cymodocea, Posidonia,
or
Thalassia
) and transparent water at relatively low nutrient concentrations generally
characterize the oligotrophic state of coastal lagoons. The mesotrophic state, charac-
terized by moderate nutrient concentrations, is associated with the presence of benthic
macroalgae at the bottom level and some higher phytoplankton concentration in the
water column. At this stage, complex interactions among these primary producers
(macroalgae and phytoplankton) and with primary consumers (grazers) lead, in some
systems, to cycles of alternate dominance by either submerged vegetation or phy-
toplankton. These cycles can be relatively stable. However, a large disturbance, with
the ability to affect different parts of the ecosystem, can override the self-stabilizing
capacities, causing a shift from a benthic to a planktonic dominated system.
15,16
A lagoon is considered eutrophic when high nutrient concentrations can be found
in the water column. The biomass and production of phytoplankton communities that
are greatly stimulated with nutrients produce highly turbid waters until the point that
the phytoplankton biomass becomes dense enough to limit light access to the bottom,
17
thus preventing growth of benthic vegetation seagrasses. Benthic vegetation is then
restricted to shallower areas, mostly disappearing in the deepest zones. Oxygen con-
sumption from degradation of produced organic material increases, especially in the
sediment, thus causing anoxic periods. The lack of oxygen and production of toxic
gasses, such as hydrogen sulphide, due to the anaerobic condition in the sediment
( see Chapter 4), has detrimental effects on the bottom-living fauna and in the recruit-
ment of species (mainly fishes and crustaceans) that enter into the lagoon as larvae
and juvenile stages. Hypereutrophy is generally considered an extreme case of eutro-
phy in which the above-mentioned characteristics are heavily enhanced. An idealized
sequence of the main features of the eutrophication processes is summarized in
Figure 5.2 and will be described in the following sections of this chapter.
TABLE 5.2
Successional Stages in Eutrophication Processes Related
to Organic Carbon Supply
Successional Stages
Organic Carbon Supply (g C m
2
y
1
)
Oligotrophic
<100
Mesotrophic
100-300
Eutrophic
301-500
Hypertrophic
>500
Source
: Nixon, S.W.,
Ophelia
, 41, 199, 1995. With permission.
 
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