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macrophyte meadows display patch distributions related to the nature of the bottom,
the physical and chemical composition of sediment, temperature ranges, wave-
energy, or hydrodynamism and depth rather than only horizontal gradients.
The horizontal zonation and the structure of lagoon communities are, therefore,
the result of interactions between biotic and abiotic factors. Species survival depends
on physiological adaptations to these extreme and fluctuating environments, and
such adaptations involve energy costs that limit their growth and reproductive rates.
It also must be taken into account that for many species the ionic composition of
water can be a factor.
114,115
Another aspect that must be emphasized is the role played by the immigra-
tion of species from coastal open ecosystems into the structure of the lagoon
communities, and the advantages and difficulties of different ecological and
reproductive strategies (e.g.,
r
vs.
K
) in an environment that can be considered
stable from some points of view but stressed from others. In this sense, Pérez-
Ruzafa
89
and Pérez-Ruzafa and Marcos
112,113
redefined the term confinement as
the capability of open-sea organisms to colonize lagoon environments on the
basis of their arrival rates depending on water renewal rates and the circulation
model (i.e., as planktonic or juvenile stages and other passive dispersal forms)
and swimming capabilities, and survival, reproductive, and competition capa-
bilities, depending on environmental conditions inside the lagoon. In this con-
text, in the equations for modeling the population dynamics of two competitors
based on Lotka-Volterra type models, the disadvantage in the competition of
allocthonous species can be compensated if their immigration rates are high
enough (see Levinton
116
). On the other hand, models of species equilibrium
(MacArthur and Wilson
117
) also could be applied to explain seasonal changes
in specific richness.
Given that water exchange with the coastal ocean is the main factor affecting
both hydrological characteristics and biological structure of a lagoon's assemblages,
Kjerfve's classification
118
into three types (choked, restricted, and leaky;
see Chapter
6 for additional details)
is useful here to explain lagoon variability.
5.3.1
C
HOKED
L
AGOONS
Choked lagoons are those connected to the open sea by a single long narrow entrance
channel with reduced tidal oscillations when compared to the adjacent coastal sea.
They are characterized by long flushing times, dominant wind forcing, and inter-
mittent stratification events that result from changes in the hydrological balance
(mainly evaporation and run-off ),
119
which also determines the salinity range. This
type of lagoon is characteristic of physically controlled ecosystems with strong
fluctuations in environmental parameters depending on weather conditions. It cor-
responds to zones V and VI in
Figure 5.8,
with faunal composition characterized by
a few, but very abundant, small-sized opportunistic species. Under such extreme
environmental conditions food chains are shortened and competitive weakness
In extreme isolation faunal and flora assemblages are highly impoverished.
However, with moderate renewal rates, it is possible to find maximum concentrations
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