important role in these processes. Of particular importance
are also wind waves that induce sediment resuspension.
Although sediments are extensively reworked, especially
by storms, lagoons primarily function as net sediment
sinks in which the accumulation rates adjust to submergence (Nichols and Allen, 1981; Boyd et al., 1992;
Nichols and Boon, 1994; Brito et al., 2012). On the other
hand, submerged aquatic vegetation and salt marsh vegetation as well microphytobenthos may exert a strong influence on the morphology of the lagoons bottom by
bio-stabilization of sediment (Widdows and Brinsley,
2002). Generally speaking, lagoonal sediments are a mixture of components, derived either from external sources
(river, sea, shore, barrier) or from internal sources
(organic production, chemical precipitation, erosion of
older deposits) (Nichols and Boon, 1994).
From an ecological perspective, coastal lagoons are
highly productive ecosystems, 10–15 times higher than
continental shelves (Valiela, 1995), excluding, of course,
some hypersaline lagoons. Thus, many of them can also
provide a wide range of natural services, very important
for the development of society. Anthony et al. (2009) perceive a wide range of social values of lagoons, in such categories as pragmatic, scholarly, aesthetic, and tactic,
especially highlighting commercial, recreational, and
tourism uses of lagoons. A comprehensive and reliable
overview of some European semi-enclosed coastal systems, including lagoons, mainly from the ecological and
socioeconomic perspective, has been given by Newton
et al. (2014).
Coastal lagoons act as buffers of the land-sea interface,
providing valuable ecosystem services such as nutrient
recycling, decomposition of organic matter, and removal
of pollutants. Lagoons are regions of restricted exchange,
subject to anthropogenic pressures that result in problems
such as eutrophication. Lagoons are extremely vulnerable
to climate changes. Climate change, especially sea-level
rise and global warming, is likely to affect shallow coastal
lagoons and to increase their vulnerability to eutrophication (Brito et al., 2012).
In arid regions, lagoonal sediments are formed primarily as a result of chemical precipitation and biological
extraction of calcium carbonate. Hot and dry climate creates favorable conditions for the deposition of evaporites.
Thus, carbonates, chlorides, and sulfates can accumulate
in the lagoonal environment. Such processes are observed,
for example, on the southern coast of the Persian Gulf,
where lagoonal waters have a salinity of up to 67 %. In
the local lagoons, e.g., the Khor al Bazam, many variations of carbonate sediments occur, such as pelleted lime
muds, oolites, and pellet aggregates, as result of high
organic production (Alsharhan and Kendall, 2003). Their
distribution and the characteristics of individual grain
types are controlled largely by their hydrodynamic
setting, with interactions between wave and current
action, sediment mobility, burial rate, and cementation.
Coastal shallows are covered by cyanobacterial mats and
mangrove swamps, and the supratidal salt flats (sabkhas)
are covered by gypsum-salt deposits (Kendall and
Alsharhan, 2011).
There are some conceptual models of lagoon evolution.
One of the earliest concepts of lagoon evolution, known as
“Lucke model,” assumes that coastal lagoons evolve from
a marine embayment to a relatively deep, partially
enclosed, back-barrier lagoon (Lucke, 1934). In developing this concept, Oertel et al. (1992) point out that at
wave-dominated coasts, the floors of barrier lagoons are
initially smooth after lagoon formation, and basin infilling
is dependent on changes in capacity relative to sea-level
rise and sediment input from siltation, runoff, inlets, and
cross-island transport. At tide-dominated coasts, the
lagoon floors are irregular and reflect the antecedent
topography of pre-transgressed landscape. The interfluve
areas of the topography produced very shallow areas in
the lagoon that form tidal flats or are colonized by
marshes. In “Lucke model,” the open-water embayment
represents the first stage in the evolutionary sequence,
while a model proposed by Oertel et al. (1992) assumes
that the open-water embayment may become
end-member of the evolutionary cycle. However, it should
be considered that “Lucke model” was developed for stationary sea-level conditions. A model of transgressive
evolution of lagoons was presented by Nichols (1989),
based upon the interplay between sedimentation rate and
relative sea-level rise. The model assumes three different
types of lagoons: a “deficit lagoon,” a “surplus lagoon,”
and an “equilibrium lagoon.” In a “deficit lagoon,”
sea-level rise exceeds sediment supply and lagoon
becomes deeper. In a “surplus” lagoon, sediment supply
exceeds sea-level rise and lagoon becomes increasingly
filled with sediments. In “equilibrium lagoon,” the rate
of sedimentation is in equilibrium with the rate of submergence from sea-level rise, thus lagoon morphology
remains unchanged.
In general, the evolution of many coastal lagoons is a
result of the combined effects of marine and fluvial processes leading to trapping and infilling of semi-enclosed
coastal systems, including coastal lagoons, and the
reshaping of seaward boundaries (Kjerfve and Magill,
1989). However, there is wide variation in the evolution
of coastal lagoons. Their development can progress in a
variety of ways and with different results. Thus, lagoons
can be found at various stages of development in an evolutionary sequence.
Overall, lagoon development is the result of the balance
between those processes which act to reduce the size of a
lagoon and those which act to increase it. It is mainly
related to the balance between the supply of sediment
and the rate of sea-level rise. The response of a barrier to
sea-level rise has a direct bearing on lagoonal evolution.
If a transgressive lagoonal barrier stabilizes against a
break in slope or topographic high and accretes upward
rather than landward, then the volume of the lagoon may
increase if sea-level rise exceeds fluvial sediment supply.
If the barrier continues to move landward as sea-level
rises, then the actual lagoon volume may remain constant.
LAGOONS
389
are also wind waves that induce sediment resuspension.
Although sediments are extensively reworked, especially
by storms, lagoons primarily function as net sediment
sinks in which the accumulation rates adjust to submergence (Nichols and Allen, 1981; Boyd et al., 1992;
Nichols and Boon, 1994; Brito et al., 2012). On the other
hand, submerged aquatic vegetation and salt marsh vegetation as well microphytobenthos may exert a strong influence on the morphology of the lagoons bottom by
bio-stabilization of sediment (Widdows and Brinsley,
2002). Generally speaking, lagoonal sediments are a mixture of components, derived either from external sources
(river, sea, shore, barrier) or from internal sources
(organic production, chemical precipitation, erosion of
older deposits) (Nichols and Boon, 1994).
From an ecological perspective, coastal lagoons are
highly productive ecosystems, 10–15 times higher than
continental shelves (Valiela, 1995), excluding, of course,
some hypersaline lagoons. Thus, many of them can also
provide a wide range of natural services, very important
for the development of society. Anthony et al. (2009) perceive a wide range of social values of lagoons, in such categories as pragmatic, scholarly, aesthetic, and tactic,
especially highlighting commercial, recreational, and
tourism uses of lagoons. A comprehensive and reliable
overview of some European semi-enclosed coastal systems, including lagoons, mainly from the ecological and
socioeconomic perspective, has been given by Newton
et al. (2014).
Coastal lagoons act as buffers of the land-sea interface,
providing valuable ecosystem services such as nutrient
recycling, decomposition of organic matter, and removal
of pollutants. Lagoons are regions of restricted exchange,
subject to anthropogenic pressures that result in problems
such as eutrophication. Lagoons are extremely vulnerable
to climate changes. Climate change, especially sea-level
rise and global warming, is likely to affect shallow coastal
lagoons and to increase their vulnerability to eutrophication (Brito et al., 2012).
In arid regions, lagoonal sediments are formed primarily as a result of chemical precipitation and biological
extraction of calcium carbonate. Hot and dry climate creates favorable conditions for the deposition of evaporites.
Thus, carbonates, chlorides, and sulfates can accumulate
in the lagoonal environment. Such processes are observed,
for example, on the southern coast of the Persian Gulf,
where lagoonal waters have a salinity of up to 67 %. In
the local lagoons, e.g., the Khor al Bazam, many variations of carbonate sediments occur, such as pelleted lime
muds, oolites, and pellet aggregates, as result of high
organic production (Alsharhan and Kendall, 2003). Their
distribution and the characteristics of individual grain
types are controlled largely by their hydrodynamic
setting, with interactions between wave and current
action, sediment mobility, burial rate, and cementation.
Coastal shallows are covered by cyanobacterial mats and
mangrove swamps, and the supratidal salt flats (sabkhas)
are covered by gypsum-salt deposits (Kendall and
Alsharhan, 2011).
There are some conceptual models of lagoon evolution.
One of the earliest concepts of lagoon evolution, known as
“Lucke model,” assumes that coastal lagoons evolve from
a marine embayment to a relatively deep, partially
enclosed, back-barrier lagoon (Lucke, 1934). In developing this concept, Oertel et al. (1992) point out that at
wave-dominated coasts, the floors of barrier lagoons are
initially smooth after lagoon formation, and basin infilling
is dependent on changes in capacity relative to sea-level
rise and sediment input from siltation, runoff, inlets, and
cross-island transport. At tide-dominated coasts, the
lagoon floors are irregular and reflect the antecedent
topography of pre-transgressed landscape. The interfluve
areas of the topography produced very shallow areas in
the lagoon that form tidal flats or are colonized by
marshes. In “Lucke model,” the open-water embayment
represents the first stage in the evolutionary sequence,
while a model proposed by Oertel et al. (1992) assumes
that the open-water embayment may become
end-member of the evolutionary cycle. However, it should
be considered that “Lucke model” was developed for stationary sea-level conditions. A model of transgressive
evolution of lagoons was presented by Nichols (1989),
based upon the interplay between sedimentation rate and
relative sea-level rise. The model assumes three different
types of lagoons: a “deficit lagoon,” a “surplus lagoon,”
and an “equilibrium lagoon.” In a “deficit lagoon,”
sea-level rise exceeds sediment supply and lagoon
becomes deeper. In a “surplus” lagoon, sediment supply
exceeds sea-level rise and lagoon becomes increasingly
filled with sediments. In “equilibrium lagoon,” the rate
of sedimentation is in equilibrium with the rate of submergence from sea-level rise, thus lagoon morphology
remains unchanged.
In general, the evolution of many coastal lagoons is a
result of the combined effects of marine and fluvial processes leading to trapping and infilling of semi-enclosed
coastal systems, including coastal lagoons, and the
reshaping of seaward boundaries (Kjerfve and Magill,
1989). However, there is wide variation in the evolution
of coastal lagoons. Their development can progress in a
variety of ways and with different results. Thus, lagoons
can be found at various stages of development in an evolutionary sequence.
Overall, lagoon development is the result of the balance
between those processes which act to reduce the size of a
lagoon and those which act to increase it. It is mainly
related to the balance between the supply of sediment
and the rate of sea-level rise. The response of a barrier to
sea-level rise has a direct bearing on lagoonal evolution.
If a transgressive lagoonal barrier stabilizes against a
break in slope or topographic high and accretes upward
rather than landward, then the volume of the lagoon may
increase if sea-level rise exceeds fluvial sediment supply.
If the barrier continues to move landward as sea-level
rises, then the actual lagoon volume may remain constant.
LAGOONS
389