Effects of Changing Environmental Conditions on Lagoon Ecology - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 191

5.1 INTRODUCTION

Coastal lagoon ecosystems are dynamic and open systems, dominated and subsi-
dized by physical energies, and characterized by particular features (such as
shallowness, presence of physical and ecological boundaries, and isolation) that
distinguish them from other marine ecosystems.

Shallowness usually provides a
lighted bottom, and the wind affects the entire water column, promoting resus-
pension of materials, nutrients, and small organisms from the sediment to the
surface layer. The large number of boundaries (between water and sediment,
pelagic and benthic communities, and among lagoon, marine, freshwater, and
terrestrial systems and with the atmosphere) involve the existence of intense
gradients and, consequently, a high potential to do work.

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( Figure 5.1) . Because
of that, coastal lagoons are usually among the marine habitats with the highest
biological productivity.

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Nutrient input from both run-off and irrigated land waters
and from currents through tidal channels contribute to increase the primary pro-
ductivity affecting the structure of the communities. On the one hand, due to their
relatively high degree of isolation, outlets usually have a total surface of less than
20% of the barrier closing the lagoon,

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and the water exchange between lagoons
and the open sea is limited, resulting in a series of physical, chemical, and
hydrodynamic boundaries.

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On the other hand, the generated environmental stress
regulates the structure of biological assemblages and leads to complex interactions
among physical (light, temperature, mixing, flow), chemical (organic and inor-
ganic carbon, oxygen, nutrients), and biological parameters and processes (nutri-
ents uptake, predation, competition).
As a consequence of high levels of biological productivity, lagoons play an
important ecological role among the coastal zone ecosystems, providing a collection
of habitat types for many species

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and maintaining high levels of biological diversity.
Most lagoons are subjected to human exploitation through fishing, aquaculture,
tourism, and urban, industrial and agricultural developments, inducing changes that
affect their ecology.
Under the designation of lagoons a high diversity of environments can be
found. Size can vary from a few hundred square meters to extensive areas of
shallow coastal sea. The salinity range can go from nearly fresh to hyperhaline
waters, with concentrations of salt reaching three times the salinity of the adjacent
sea.

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Salt balance relies on several factors such as the exchange of water with the
open sea, the inputs of continental waters from rivers, watercourses and ground-
water, and on the rainfall-evaporation balance. The variability of salinity can also
be observed inside the lagoon both spatially and temporally. From a hydrographical
point of view, most of this variability between lagoons can be summarized by a
set of quantitative parameters or indexes that describe both lagoon orientation and
structure, as well as spatial variability and the potential sea influence ( see Chapter 6
for details).
In biological terms, heterogeneity can be applied to both the structure (species
composition and abundance) and functioning (productivity, trophic webs, and
fluxes) of the lagoon ecosystem at a wide range of spatial and temporal scales,
from biogeographic (thousands of kilometers) to regional (hundred to thousands

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