© Riccardo Roiter Rigoni
In the delicate context of the Venice Lagoon, the widespread process of erosion is a sign of hydrogeological and environmental degradation. Erosion results in a continuous retreat of salt marsh borders, associated with the loss of sediments transported by tidal currents to the sea. The surface of salt marshes continues to disappear, with significant alterations of the lagoon landscape. This is a self-amplifying process since the disappearance of a given salt marsh facilitates the erosion of the surrounding habitats, as a result of the lack of its protective and wave energy absorption functions. The extent of the surface occupied by salt marshes evolved from 170 km2 in 1901, to 104 km2 in 1932, to 47 km2 in 2003 .
A further important trend is the flattening of the bottom topography , a process linked to the salt marshes erosion process through the accumulation of eroded sediments on the surrounding tidal flat bottom. Only a part of these sediments is accumulated, while the rest is resuspended by the joint action of wave energy and tidal currents; once resuspended, sediments are transported to minor and then bigger channels, and from there out to the sea.
The consequences of erosion are several and are linked to the loss of the ecosystem services that salt marshes provide: support to biodiversity; pollutants abatement and hydrodynamic control; support to human activities such as fishing, hunting and eco-tourism given the many animal and plant species which live or spawn there; spiritual and recreational values; etc.
In the following paragraphs, we describe the features of the salt marsh system, its ecosystem functions and the multiple causes of its erosion.
The salt marsh system is one of the key morphological features of the Venice Lagoon: besides the salt marsh itself, the system includes the surrounding tidal flats and the network of channels and creeks that pass through and around it.
The salt marsh system is characterised by some intrinsic features. One of these is the regular phenomenon of submersion and emersion due to tide cycles, although complete flooding occurs only during pronounced high tide . The soil structure of salt marshes is composed of silty and clayey sediments with a high organic component. They are generally covered by halophyte, specialized and largely herbaceous vegetal species, adapted to the high soil salinity . There is a vertical pattern of vegetation changes, very sensitive to small variations in elevation. The sequence of vegetal species that develop in the vertical range between 0 and +40 cm is almost constant and very repetitive, and may be considered a pretty reliable indicator of altitude .
An important feature of this environmental system is the continuously evolving morphology that is the result of dynamic interactions between erosion and sedimentation processes, tidal currents, wave energy, and the presence of vegetation that consolidates the ground.
Salt marshes are considered to be transitional ecosystems not only in space, as they are located where the land meets the sea, but also in time, as the lagoon landscape is constantly changing. This concept should be at the basis of any engineering work that acts on the salt marsh system (e.g. to protect it); such a work should not be rigid, but rather flexible, easily reversible, modifiable, and biodegradable, to be harmonized with the dynamic nature of these habitats.
To analyse salt marshes and the ecological functions they perform, it is essential to understand their importance and the reasons why we aim to protect them. Some of the key functions carried out by the salt marsh ecosystem are now outlined.
Hydrodynamic function: this function is closely linked to the particular geomorphological structure of the salt marsh system, which contains a network of several channels of different sizes. This system promotes the circulation of water flows with different salinities, temperatures, and densities, and reduces tidal effects.
Buffer function: salt marshes are buffer zones in the lagoon; they dissipate current and wind energy, thus mitigating the erosive effect on other surrounding emerged surfaces and canal banks, and, as all wetlands, promote the natural removal of nutrients and pollutants such as those discharged from the inland.
Ecological function: the intrinsic transitional and variable nature of this ecosystem, with the continuous submersion and emersion processes and the sharp fluctuations in physical-chemical parameters, enhances the presence of specialized vegetation, with a vertical-sensitive development. All these features, combined with the peculiar morphology, promote the creation of ecological niches and the consequent fostering of terrestrial and marine biodiversity.
Socio-economic function: the uniqueness of this kind of habitat allows the reproduction and growth of valuable fish species, which support fishing activities and, consequently, local socio-economic development.
Spiritual function: there is definitely an intimate and emotional bond between this habitat and local communities who live and work within the lagoon.
These multiple services, meanings, and benefits salt marshes provide to society are often neglected or unknown. We believe they must be spread and disseminated, since only the full awareness of the societal value of such ecosystems can promote an adequate level of environmental protection and make it sustainable in the long term.
Salt marshes erosion is not consequence of one single contribution, but rather of a multiplicity of causes, both natural and anthropogenic.
Among the natural causes, we can identify the combined effect of tide currents and wind waves, which effect is the resuspention and transport of sediments out to the sea. The ability of the wind to generate waves, and therefore transmit shear stress to the bottom inducing resuspension, increases with wind speed and fetch length, which in turn depends on salt marshes, whose presence limits the fetch itself .
The causes of anthropogenic origin are numerous.
Over the past centuries, the major rivers flowing into the lagoon and carrying large amounts of sediments were diverted by the Serenissima Republic, with the purpose of inverting the historical morphological trend of the lagoon to be filled up (process that started already in the twelfth century). These measures were designed to ensure navigation up to the harbours, and permit economic and military activities . In the long term, these changes have led to a net increase in the process of erosion, mainly for two reasons: a lower contribution of suspended sediment transported by rivers, resulting in a change in the balance between sediments entering and exiting from the lagoon (the balance is now negative: sediment inputs to the lagoon are lower than outputs), and a lower input of fresh water, influencing the type of vegetation covering salt marshes and thus decreasing vegetation ability to consolidate the ground.
Other human actions that affect erosion and facilitate the exchange of sediments with the sea are the construction of the jetties at the lagoon inlets and the excavation of deep lagoon channels. Both actions were implemented to restore and enhance the navigability of the lagoon to allow large commercial ships to transit and reach the inland harbours. These measures have triggered important morphological effects in terms of hydrodynamics and morphodynamics, speeding up the salt marshes erosion intensity and the net loss of sediment to the sea .
Furthermore, there are causes related to the daily activities carried out in the lagoon, such as waves generated by motor boats which erode salt marsh borders, and invasive mollusc fishing which resuspends sediments and alters the microphytobenthic film that has a protective function for the lagoon bottom . Finally, salt marshes are negatively affected by the process of soil subsidence, that is the compaction of the alluvial and fine marine materials, such as silt and clay; this process can be caused either by the extraction of water from aquifers or by the natural process of soil consolidation. A similar effect has the rising sea level due to the global climate change.
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