Analyses on salt-marsh vegetation composition changes in the Venice lagoon in the last twenty years

Coastal salt-marshes are important eco-geomorphic features of coastal landscapes providing valuable ecosystem services, but unfortunately, they are among the most vulnerable ecosystems around the world. Their survival is mainly threatened by sea-level rise, wave erosion and human pressure. Halophytic vegetation distribution and dynamics control salt-marsh erosional and depositional patterns, critically determining marsh survival through complex bio-morphodynamic feedbacks. Although a number of studies have proposed species-classification methods and analyzed halophytic vegetation species distribution, our knowledge of the temporal evolution of species composition remains limited. To fill these gaps and better describe vegetation composition changes in time, we developed a novel classification method which is based on the Random Forest soft classification algorithm, and applied the method to two multi-spectral images of the San Felice marsh in the Venice lagoon (Italy) acquired in 2001 and 2019. The Random Forest soft classification achieves high accuracy (0.60 < R2 < 0.96) in the estimation of the fractional abundance of each species in both images. We also determined the local dominant species, i.e. the species with the highest fractional abundance in each pixel. Our observations on the dominant species in 2001 and 2019 show that: 1) the area dominated by Juncus and Spartina decreased dramatically in such period; 2) the area dominated by Limonium almost maintained constant; 3) a noticeable decrease in the bare-soil area occurred due to the encroachment of Salicornia between 2001 and 2019. We also noticed that the probability distribution of the dominant patch area of each species is consistent with a power-law distribution, with different slopes for different vegetation species at different times. We suggest that vegetation composition changes are related to sea-level rise and to the species-specific inundation tolerance.

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Assessing the Fractional Abundance of Highly Mixed Salt-Marsh Vegetation Using Random Forest Soft Classification

Remote Sensing ◽

10.3390/rs12193224 ◽

2020 ◽

Vol 12 (19) ◽

pp. 3224

Author(s):

Zhicheng Yang ◽

Andrea D’Alpaos ◽

Marco Marani ◽

Sonia Silvestri

Keyword(s):

Salt Marsh ◽

Random Forest ◽

Near Infrared ◽

Salt Marsh Vegetation ◽

Marsh Vegetation ◽

Classification Approach ◽

Plant Associations ◽

Fractional Abundance ◽

Soft Classification ◽

Vegetation Species

Coastal salt marshes are valuable and critical components of tidal landscapes, currently threatened by increasing rates of sea level rise, wave-induced lateral erosion, decreasing sediment supply, and human pressure. Halophytic vegetation plays an important role in salt-marsh erosional and depositional patterns and marsh survival. Mapping salt-marsh halophytic vegetation species and their fractional abundance within plant associations can provide important information on marsh vulnerability and coastal management. Remote sensing has often provided valuable methods for salt-marsh vegetation mapping; however, it has seldom been used to assess the fractional abundance of halophytes. In this study, we developed and tested a novel approach to estimate fractional abundance of halophytic species and bare soil that is based on Random Forest (RF) soft classification. This approach can fully use the information contained in the frequency of decision tree “votes” to estimate fractional abundance of each species. Such a method was applied to WorldView-2 (WV-2) data acquired for the Venice lagoon (Italy), where marshes are characterized by a high diversity of vegetation species. The proposed method was successfully tested against field observations derived from ancillary field surveys. Our results show that the new approach allows one to obtain high accuracy (6.7% < root-mean-square error (RMSE) < 18.7% and 0.65 < R2 < 0.96) in estimating the sub-pixel fractional abundance of marsh-vegetation species. Comparing results obtained with the new RF soft-classification approach with those obtained using the traditional RF regression method for fractional abundance estimation, we find a superior performance of the novel RF soft-classification approach with respect to the existing RF regression methods. The distribution of the dominant species obtained from the RF soft classification was compared to the one obtained from an RF hard classification, showing that numerous mixed areas are wrongly labeled as populated by specific species by the hard classifier. As for the effectiveness of using WV-2 for salt-marsh vegetation mapping, feature importance analyses suggest that Yellow (584–632 nm), NIR 1 (near-infrared 1, 765–901 nm) and NIR 2 (near-infrared 2, 856–1043 nm) bands are critical in RF soft classification. Our results bear important consequences for mapping and monitoring vegetation-species fractional abundance within plant associations and their dynamics, which are key aspects in biogeomorphic analyses of salt-marsh landscapes.

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Venice lagoon salt marsh vulnerability and halophytic vegetation vertical migration in response to sea level rise

10.5194/egusphere-egu2020-14404 ◽

2020 ◽

Author(s):

Zhicheng Yang ◽

Sonia Silvestri ◽

Marco Marani ◽

Andrea D’Alpaos

Keyword(s):

Salt Marsh ◽

Sea Level Rise ◽

Sea Level ◽

Salt Marshes ◽

Current Knowledge ◽

Venice Lagoon ◽

Sea Level Changes ◽

Human Pressure ◽

Sea Levels ◽

Vegetation Species

Salt marshes are biogeomorphic systems that provide important ecosystem services such as carbon sequestration and prevention of coastal erosion. These ecosystems are, however, threatened by increasing sea levels and human pressure. Improving current knowledge of salt-marsh response to changes in the environmental forcing is a key step to understand and predict salt-marsh evolution, especially under accelerated sea level rise scenarios and increasing human pressure. Towards this goal, we have analyzed field observations of marsh topographic changes and halophytic vegetation distribution with elevation collected over 20 years (between 2000 and 2019) in a representative marsh in the Venice lagoon (Italy).Our results suggest that: 1) on average, marsh elevation with respect to local mean sea level decreased , (i.e. the surface accretion rate was lower than the rate of sea level rise); 2) elevational frequency distributions are characteristic for different halophytic vegetation species, highlighting different ecological realized niches that change in time; 3) although the preferential elevations at which different species have changed in time, the sequence of vegetation species with increasing soil elevation was preserved and simply shifted upward; 4) we observed different vegetation migration rates for the different species, suggesting that the migration process is species-specific. In particular, vegetation species colonizing marsh edges (Juncus and Inula) migrated faster facing to changes in sea levels than Limonium and Spartina , while Sarcocornia was characterized by delayed migration in response to sea level changes. These results bear significant implications for long-term biogeomorphic evolution of tidal environments.

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Characterizing marshland compressibility by an in-situ loading test: design and set-up of an experiment in the Venice Lagoon

Proceedings of the International Association of Hydrological Sciences ◽

10.5194/piahs-382-345-2020 ◽

2020 ◽

Vol 382 ◽

pp. 345-351

Author(s):

Pietro Teatini ◽

Cristina Da Lio ◽

Luigi Tosi ◽

Alessandro Bergamasco ◽

Stefano Pasqual ◽

...

Keyword(s):

Salt Marsh ◽

Sea Level ◽

Scale Effects ◽

Venice Lagoon ◽

Depth Interval ◽

Deformation Model ◽

High Porosity ◽

Loading Test ◽

In Situ Conditions

Abstract. The fate of coastal marshlands in the near future will strongly depend on their capability to maintain their elevation above a rising mean sea level. Together with the deposition of inorganic sediments during high tides, organic soil production by halophytic vegetation, and organic matter decomposition, land subsidence due to natural soil compression is a major factor controlling the actual elevation of salt-marsh platforms. Due to their high porosity and compressibility, the marsh sedimentary body undergoes large compression because of the load of overlying more recent deposits. The characterization of the geotechnical properties of these deposits is therefore of paramount importance to quantify consolidation versus accretion and relative sea level rise. However, undisturbed sampling of this loose material is extremely challenging and lab tests on in-situ collected samples are not properly representative of in-situ conditions due to the scale effects in highly heterogeneous silty soils such as those of the Venice lagoon. To overcome this limitation, an in-situ loading test was carried out in the Lazzaretto Nuovo salt-marsh in the Venice Lagoon, Italy. The load is obtained by a number of plastic tanks that are filled with seawater, reaching a cumulative load of 40 kN applied on a 2.5×1.8 m2 surface. Specific instrumentations were deployed before positioning the tanks to measure soil vertical displacement at various depths below the load (0, 10, and 50 cm) and distances (0, 40, and 80 cm) from the load centre. Moreover, six pressure transducers were used to record overpressure dissipation over time. The collected datasets will be interpreted through a 3-D flow-deformation model that, once calibrated, provides reliable estimates of the compressibility values for each monitored depth interval.

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Characterization of organic matter decomposition in the Venice Lagoon using the Tea Bag Index

10.5194/egusphere-egu21-3355 ◽

2021 ◽

Author(s):

Alice Puppin ◽

Marcella Roner ◽

Alvise Finotello ◽

Massimiliano Ghinassi ◽

Laura Tommasini ◽

...

Keyword(s):

Organic Matter ◽

Salt Marsh ◽

Sea Level ◽

Salt Marshes ◽

Litter Quality ◽

Venice Lagoon ◽

Organic Matter Decomposition ◽

Tea Bag Index ◽

Decomposition Processes ◽

Surface Accretion

Salt-marsh evolution importantly depends on complex feedbacks between hydrodynamic, morphological, and biological processes. These crucial ecogeomorphic structures support a diverse range of ecosystem services, including coastal protection and biodiversity increase. In addition, they are among the most carbon&#8208;rich ecosystems on Earth, as their high primary production coupled with rapid surface accretion results into the ability to sequester atmospheric carbon at high rates. However, salt-marsh future is at risk today, due to the effects of climate changes and local anthropogenic disturbances, in particular sea-level rise and reduced fluvial sediment delivery to the coasts. The organic matter captured and stored by salt marshes results from the balance between inputs and outputs and may contribute to marsh surface accretion, which determines their ability to keep pace with sea-level rise. Therefore, a better understanding of the processes regulating organic matter dynamics on salt marshes is a critical step to elucidate their carbon sink potential and to address salt-marsh management and conservation issues. Toward this goal, we analysed organic matter decomposition processes within salt-marsh ecosystems by burying 712 commercially available tea bags within different marshes in the Venice Lagoon (Italy), following the Tea Bag Index protocol. The process provides the values of two key parameters: the decomposition rate (k) and litter stabilisation factor (S). Based on standardized litter bag experiments, the Tea Bag Index focuses on the effects of abiotic conditions, neglecting litter-quality influences. The mean values of the decomposition metrics from our analyses are in general consistent with previous results and indicate a quite fast decomposition of the organic matter with a remaining mass of about 34% of the initial labile mass after 90 days. We next explore the possible dependence of k and S on environmental drivers. Temperature showed the most significant relationship with decomposition processes, suggesting an organic-matter decay acceleration with warming temperature, in line with previous literature. Moreover, the statistical analysis indicated some significant trends of the decomposition rate also with surface elevation and distance from the marsh edge. This suggests that, at the marsh scale, higher and probably less frequently flooded sites are exposed to faster decomposition, likely due to greater oxygen availability enhancing microbial respiration. In conclusion, the organic matter decay we observed is rapid enough to consume all the labile material before it can be buried and stabilized, hence increased global temperatures may not have a significant effect in increasing organic matter decomposition in coastal marshes. Therefore, we argue that, at least in the short term, the remaining mass of the organic matter contributing to carbon sequestration and marsh accretion, strongly depends on the initial litter quality, recalcitrant or labile, which may differ considerably between different species and plant parts and may be affected by climate change effects.

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Coastal flooding protection will change salt-marsh sedimentation dynamics

10.5194/egusphere-egu21-1639 ◽

2021 ◽

Author(s):

Davide Tognin ◽

Andrea D'Alpaos ◽

Marco Marani ◽

Luca Carniello

Keyword(s):

Salt Marsh ◽

Sea Level Rise ◽

Sea Level ◽

Storm Surge ◽

Sedimentation Rate ◽

Urban Areas ◽

Storm Surges ◽

Venice Lagoon ◽

Water Levels ◽

Sedimentation Dynamics

Coastal wetlands lie at the interface between submerged and emerged environments and therefore represent unique yet delicate ecosystems. Their existence, resulting from complex interactions between hydrodynamics and sediment dynamics, is challenged by increasing rates of sea-level rise, lowered fluvial sediment input as well as an increasing anthropogenic pressure. The future survival of these peculiar morphologies is becoming even more complicated, because of the construction and planning of coastal defence structures designed to protect urban areas from flooding. Important examples are the flood protection systems built to protect New Orleans (USA), the river Scheldt Estuary (The Netherlands) and Venice (Italy). In this context, understanding the physical processes on which coastal marshes are grounded and how engineering measures can alter them is of extreme importance in order to plan conservation interventions.To understand marsh sedimentation dynamics in flood-regulated environments, we investigated through field observations and modelling the effect of the storm-surge barrier designed to protect the city of Venice, the so-called Mo.S.E. system, which has in fact become operational since October 2020.Sedimentation measurements in different salt marshes of the Venice lagoon carried out in the period October 2018-October 2020 show that more than 70% of yearly sedimentation accumulates during storm-surge conditions, despite their short duration. Moreover, the sedimentation rate displays a highly non-linear increase with marsh inundation intensity, due to the interplay between higher water levels and greater suspended sediment concentration. Barrier operations during storm surges to avoid flooding of urban areas will reduce water levels and marsh inundation. Therefore, we computed sedimentation in a flood-regulated scenario for the same observation period, using the relation we obtained between tidal forcing and sedimentation rate. Our results show that some occasional closures during intense storm surges (70 hours/year on average) suffice to reduce the yearly sedimentation of the same order of magnitude of the relative sea-level rise rate experienced by the Venice lagoon during the last century (2.5 mm/y).We conclude that storm-surge barrier operations can dangerously reduce salt-marsh vertical accretion rate, thus challenging wetland survival in face of increasing sea-level rise.

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