A comparison of 1927, 1970 and 2002 bathymetric surveys in the Lagoon of Venice was used to reconstruct historical changes in sedimentation. A detailed GIS-based analysis of the charts revealed the ...timing and pattern of geomorphic changes and allowed calculation of sediment deposition and erosion for the entire lagoon and each of its four sub-basins: Treporti, Lido, Malamocco and Chioggia.
Two main developments are discernible from comparative observation of the areal distribution of the main elevation ranges: the diminution in area of the saltmarshes, which decreased by more than 50%, from 68
km
2 in 1927 to 32
km
2 in 2002, and the progressive deepening of the lagoon, with a huge increase in the area of subtidal flats (between −0.75 and −2.00
m depth), from 88 to 206
km
2 during the same period.
Generally, the lagoon showed a clear-cut change in the most frequent depths (modal depth) from a value of −0.62
m in 1927 to −0.88
m in 2002. The deepening of the lagoon affected mostly the lagoonal sub-basins south of the town of Venice, where modal depth increased from −0.65 to −1.12
m in Lido, from −0.64 to −1.75
m in Malamocco and from −0.39 to −0.88
m in Chioggia.
Large changes in lagoonal morphology were caused by human-induced subsidence, the dredging of navigation channels between 1927 and 1970, and intense natural erosion enhanced by sediment re-suspension due to Manila clam fishing between 1970 and 2002. There was a net loss of about 110
Mm
3 of sediment from the lagoon, most of which (73
Mm
3, ca.70%) was in the earlier period. A significant amount was lost by dredging and direct disposal outside the system, either on land or at sea, and there was a net loss of 39
Mm
3 from the lagoon to the sea through the inlets, at an annual rate of 0.5
Mm
3.
Comparison of erosion rates in the two periods revealed an alarming acceleration, from a net sediment loss of 0.3
Mm
3
yr
−1 in the period 1927–1970 to 0.8
Mm
3
yr
−1 in 1970–2002. Deterioration caused a shift from a highly differentiated lagoon morphology in the 1930s to a sediment-starved and subsidence-dominated structure in the 1970s, and from there to the high-energy and more open (bay-like) lagoon of today.
The results demonstrate the potential application of GIS to reconstruct the recent chronology of sediment distribution and to improve the understanding of the geomorphic processes shaping the seafloor, whilst providing an insight into the possible impacts of environmental changes induced by natural and anthropogenic forcing.
A comparison was made of shallow water sediments from the Lagoon of Venice (LV) and the Lagoon of Cabras (LC), comparing depositional environments and exploring the relationships between ...hydrodynamics and sedimentological parameters
. The two water bodies are very different in size (LV: 360 km
2; LC: 22 km
2), and the sediments predominantly consist of silty-clay (LV: Mz
≈
26 μm; LC: Mz
≈
6 μm). However, there are large differences between the two lagoons with respect to sand (LV: mean 19%; LC: mean ~
3%) and clay (LV: mean 20%; LC: mean 45%) contents. The Lagoon of Venice (mean depth ~
1 m) can be considered a tidal basin, whereas the Lagoon of Cabras (mean depth ~
2 m) has the character of a coastal lake in which wind is the main hydrodynamic forcing factor. A comparison of sediment grain-size distributions with water circulation patterns in different parts of the lagoons highlighted some interesting differences. Grain-size analyses of samples reveal a deficiency of particles around 8 μm in the LC, which is interpreted as reflecting the transition between cohesive flocs/aggregates and non-cohesive coarser silt particles, while the transition limit in the LV is ~
20 μm. Thus, particles are cohesive below 8 μm in the LC and below ~
20 μm in the LV. This is probably because of the differences in the clay/silt ratio, which is much lower in the LV (~
0.3) than in LC (~
1), conferring a “silt-dominated network structure” on most of the LV sediments.
The hydrographical data used were root mean square velocity (RMSV) and water residence time (WRT), computed under the main forcing conditions. The results show a general correlation between RMSV and sortable silt in the LC, and between RMSV and coarser sediments (63–105 μm) in the LV. Some significant differences between the lagoons were detected in the degree of correlation between WRT and grain size. Root mean square velocity (~
7 cm s
−
1
in the LV and ~
3 cm s
−
1
in the LC) was a greater forcing factor in the LC than in the LV. Conversely, WRT, which is on average ~
16 days in the LV and ~
19 days in the LC, has more influence in the LV. This study highlights the usefulness of comparing environments with different hydrodynamic energies, e.g., tidal and/or wind-driven currents, to elucidate and thereby improve our understanding of the processes governing the spatial distribution of sedimentological features, the transport mechanisms of sediments, and the relationship between them. The results demonstrate that the approach outlined in this study has the potential to provide a universal hydro-sedimentological classification scheme.
First available data on atmospheric fall-out were provided by sampling monthly bulk depositions in four sites inside the Lagoon of Venice (550 km2). Sampling was carried out monthly during the period ...July 1998-July 1999, in one site near an industrial area (Porto Marghera; site D), another site in the city of Venice (site A), and the remaining two in the southern- and northernmost ends of the Lagoon (Valle Figheri, site C; Valle Dogà site B). The following determinations were carried out for each samples: pH, conductivity, grain-size, particulate load, and dissolved nutrients (N, P). Samples were then subdivided into soluble and insoluble fractions, and Al, Ca, Na, K, Mg, Si, Mn, Fe, Zn, Ni, Cr, Cu, Pb, Cd, As, Hg, Ti, V, S, P, Se and Sb were analysed on both fractions. Total organic micropollutants (PAH, PCB, HCB, DDT, PCDD/F) were measured. As regards particle size distribution, there was great variability among sampling sites. The percentage of the < or =2 microm grain-size fraction was higher in the southern and northern ends of the Lagoon. Small differences were found among sites for major elements, whereas higher variability was observed for inorganic and organic micropollutants, with standard deviations between 20% and 60% of the fluxes measured. Major differences in annual fluxes between the most polluted sites (mostly D and A) and background (site B) were seen for Cd (0.26 vs. 0.06 mg m(-2) year(-1)), Hg (41 vs. 15 microg m(-2) year(-1)), PCB ( approximately 2500 vs. approximately 500 ng m(-2) year(-1)) and HCB ( approximately 8000 vs. approximately 1000 ng m(-2) year(-1)). Comparisons with previous data, collected in the periods 1993-1994 and 1995-1997, were only available for a few trace metals. A definite decline in the annual Pb flux in the city of Venice was detected, from 18 to 13 mg m(-2) in 1996/1997 and 1995/1996 respectively, to approximately 5 mg m(-2) in the present study. Total annual deposition was calculated by means of two different methods, which gave very similar results: (i) the mean value of deposition in the four sites was multiplied by lagoon area (550 km2); (ii) the monthly rain isopleths were combined to normalize deposition values. The figures are: 15-34 kg of Hg and Sb, approximately 200 kg of As, approximately 100 kg of Cd and PAH, 0.7-1.3 tons of Cr, Ni and V, more than 2 tons of Cu and Pb, 17 of Zn, 55 of total P, approximately 200 of Al, and 3900 of DIN. Total fluxes of organics inside the lagoon were: PAH approximately 100 kg; HCB approximately 1 kg; DDT approximately 0.4 kg. PCB and PCDD/F fluxes were approximately 500 g and approximately 10 g, corresponding respectively to 0.1 and 0.4 g I-TE. The correlations between fluxes of inorganic micropollutants and grain-size were significant. Multivariate statistical analysis was applied to investigate more accurately relationships between the insoluble and dissolved fractions of inorganic micropollutants and grain-size fractions. In particular, significant correlations were highlighted between the dissolved fraction of As and the < or =1 mum particle size fraction. Relations between levels of SigmaPCDDF, SigmaPCDD, PCB and PAH congeners and grain-size revealed significant correlation coefficients for the remote sites (B, C), and none in the urban and industrial sites (A, D). In particular, significant correlations were highlighted between SigmaPCDDF, SigmaPCDD and particle size fraction < or =2 mum, and between benzo(a)pyrene and PCB 167 and particle size fraction 4-8 mum.
Inputs of biogenic carbonate sediment from
Posidonia oceanica seagrass meadows to four beaches of the Sinis peninsula (Sardinia, western Mediterranean) were evaluated.
Beach and continental shelf ...sediment samples were analysed for grain size distribution and composition, biogenic vs. siliciclastic, in order to identify the provenance of beach sediments and sediment transport pathways. Seabed mapping was carried out in order to identify the distribution of meadows and sediment deposits offshore.
Shelf sediments were collected in unvegetated sites and in
P. oceanica meadows. Sediments from unvegetated sites were coarse sands and gravel, mainly siliciclastic (biogenic carbonate content is 3–7%). Sediments from
P. oceanica meadows were coarse sand, mainly biogenic (carbonate contents varying between 60 and 90%).
Beach sediments showed bimodal grain size distribution (59% of samples) resulting from mixing of coarser siliciclastic with finer biogenic materials in variable proportions. Biogenic carbonate contents in beach sediments range from 0 to 90%, reaching the highest values in offshore samples.
Analysis of grain size and compositional trends from shelf to beach sediments highlighted that the latter originate from two different sources: erosion of granitic outcrops, providing the siliciclastic component, and export of sediments from
P. oceanica meadows, providing biogenic material.
P. oceanica meadows also influence shore by contributing towards maintaining the beach sediment budget.
Data are presented for a number of parameters for aerosols and rainwaters collected at a station on Sardinia. The findings are interpreted with special reference to Saharan dusts, and are compared to ...other data on these dusts obtained from a variety of sites around the Mediterranean Sea. At the Sardinia site the particle size distribution of the Saharan outbreaks exhibits a bimodal structure, the two modes being between 2 and 4 μm and 15 and 30 μm. The presence of giant particles strongly affects the deposition velocities of the Saharan aerosols. Source markers for the Saharan dusts are palygorskite, kaolinite, calcite, dolomite and rounded quartz grains.
The input of Saharan dust has important effects on the chemistry of the Mediterranean aerosols. These include: (i) increases in the atmospheric concentrations and sea surface fluxes of crust-controlled trace metals (e.g. Al, Fe); (ii) decreases in the EFcrust values of non-crust-controlled trace metals (e.g. Cu, Zn and Pb) in the aerosols, and (iii) changes in the solid state speciation of Cu, Zn, and Pb, which decrease their solubilities in seawater. The Saharan dusts also affect the composition of rainwater by raising the pH, following the dissolution of calcium, and by decreasing the solubility of trace metals such as Cu, Zn and Pb.
Wet deposition controls the flux of Saharan dust to the Mediterranean Sea, but dry deposition can also be important. The dust transport occurs in the form of “pulses”, and the annual dust flux can be controlled by a few episodes of Saharan outbreaks, e.g. sometimes a single outbreak can account for 40–80% of the flux. Saharan dust deposition fluxes range from 2 to 25 g m−2 (average ≅10) in the west Mediterranean between 39° and 42°N, from 6 to 46 g m−2 (average ≅20) in the east Mediterranean, and from 0.4 to 1.0 g m−2 over the Alps on continental Europe. The present day Saharan dust fluxes (≈ 1 mg cm−2 year−1) account for about 10–20% of the recent deep-sea sedimentation in the western Mediterranean (3–15 mg cm−2 year−1).
A comparative study was performed of three instruments used to measure the grain‐size distribution of thirty sediment samples from shallow lagoonal flats: the hydrometer, the Sedigraph 5100 and the ...CIS‐1. The hydrometer and Sedigraph are based on sedimentation whereas the CIS‐1 uses the time of transition. The percentage of the samples accounted for by the <8 μm fraction was not affected by the technique used, but this was not the case with the clay fraction (<2 μm). Due to its relative independence from the analytical method applied, the <8 μm fraction can be used in ternary diagram classifications. This fraction also has an environmental significance in coastal lagoons in terms of hydrodynamics, organic enrichment and macrozoobenthos assemblages. The linear relationships obtained in this study may provide useful operational indications for similar studies.
Data on atmospheric fall-out of polychlorinated dibenzo-
p-dioxins (PCDDs) and dibenzofurans (PCDFs) were provided by collecting bulk deposition in four stations inside the Lagoon of Venice. A total ...of 44 monthly samples was collected during the period July 1998–July 1999 in one site near an industrial area (Porto Marghera), one site in the city of Venice, and two sites in the southern- and northernmost ends of the Lagoon. Fluxes of PCDD/Fs were between 0.1 and 470 pg
m
−2
d
−1, corresponding to 0–9.2 pg of 2,3,7,8-TCDD equivalents (TEQ) m
−2
d
−1, with a gradient increasing from remote to urban/industrial stations. Thus, annual deposition of PCDD/Fs to the Lagoon (total area=550 km
2), calculated with various methods, turned out to be ∼12 g, corresponding to ∼400 mg TEQ. Significant differences were found among the stations, with a clear fingerprinting signature (PCDF/PCDD
>
1) of the deposition collected near Porto Marghera, and a reversed pattern (PCDF/PCDD
<
1) in the rest of the Lagoon, which pattern was similar to the sediments collected in the same locations. Lastly, the amount of bulk ∑TEQ of all stations was compared with the guide values for dioxins in depositions proposed by De Fré et al. Organohalogen Compounds 45 (2000) 324.
PDF
