Roberto Zonta

Heavy metal and grain-size distributions in estuarine shallow water sediments of the Cona Marsh (Venice Lagoon, Italy)

Anthropogenic heavy metals and grain size were determined in 15 surface sediment samples from the Cona Marsh, an estuarine area of the Venice Lagoon (Italy). The investigation is based on separate analyses of both metal concentrations obtained with two acid extractions and particle-size percentages by laser light scattering. The grain size was also measured in sample aliquots previously submitted to the organic matter removal to disperse mineral-organic aggregates, visualized by scanning electron microscopy. These aggregates apparently shift the particle-size spectrum toward larger diameters, and the intimate association between metals and sediment particles is more evident after their dispersion. The comparison of the two distributions showed a strong correlation between heavy metals and finer particles content in the sediment (d < 7.8 μm). The investigation finally illustrated a spatial zoning of the marsh into three sectors with different degrees of pollution, in relation to the water circulation determined by river discharge and tidal forcing.

Study on redox state and grain size of sediments in a mud flat of the Venice Lagoon

An extensive investigation based on the redox potential and grain size distributions was made on the sediment of a Venice Lagoon mud flat subjected to excessive growth of macroalgae. Redox potential and grain size measurements are proved useful “tracers” for, respectively, oxygen bearing and consuming processes in the water-sediment column and hydrodynamical behavior inside the mud flat. Depth measurements and a considerable number of sites with respect to the size of the area studied are needed to obtain an outline of the behavior of the water body in response to stress conditions caused by human activities. With respect to the top 15-cm-thick sediment layer where EH variations occur, the mud flat is subdivisible into sectors with different characteristics. Positive or near-zero EH values were recorded in zones characterized by sparse macroalgae growth and a high content of coarse sediments (diameter ⩾44 µm). On the contrary, very negative EH values were found in zones affected by overabundant macroalgae bloom and with a higher presence of fine-grained sediment (diameter ⩽44 µm). The clear relationships between algae presence in the mud flat and both the redox potential and grain size characteristics of the sediment emphasize the hydrodynamics as a “critical factor” determining the variations of the environmental conditions in the ecosystem.

Sediment pollution and dynamic in the Mar Piccolo of Taranto (southern Italy): insights from bottom sediment traps and surficial sediments

Major and trace element, PAH, and PCB concentrations were measured in surface sediments and particles from sediment traps collected in the First and Second Basin of the Mar Piccolo (Gulf of Taranto) in two periods (June–July and August–September, 2013). The aim of the study was to evaluate pollution degree, sediment transport and particle redistribution dynamic within the area. Results confirm the higher contamination of sediments from the First Basin observed by previous researches, particularly for Cu, Hg, Pb, total PAHs, and total PCBs. Advective transport from the First to the Second Basin appears to be the leading transfer mechanism of particles and adsorbed contaminants, as evidenced by measured fluxes and statistical analyses of contaminant concentrations in surficial sediments and particles from sediment traps. Long-range selective transports of PAHs and microbial anaerobic degradation processes for PCBs have been also observed. These results are limited to a restricted time window but are consistent with the presence of transport fluxes at the bottom of the water column. This mechanism deserves further investigation and monitoring activities, potentially being the main responsible of pollutant delivering to the less contaminated sectors of the Mar Piccolo.

Heavy metals behaviour during resuspension of the contaminated anoxic sludge of the venice canals

A field study on the water column in a canal of Venice was perfomed to investigate heavy metals behaviour during dredging interventions on the contaminated anoxic sludge. Concentrations in the dissolved and particulate phases were analysed following a specifically designed filtration scheme. The variations of physico‐chemical parameters, monitored during the sampling, allowed the comparison between the trend of the variables characterising sludge resuspension and pollutant concentrations in the water column. Dissolved nutrient species and hydrogen sulphide were also analysed. Due to the presence of a large amount of reducing species, the sludge resuspension induces rapid variations in the oxidation‐reduction conditions in the overlying water. In response to these changes, two distinct behaviours were observed for dissolved species. Some elements such as Zn, Cu, Pb and As are scavenged by resuspended amorphous sulphides, while others (Mn and Ni) undergo a mobilisation by dilution of pore‐water concentrations.

Mercury in the food chain of the Lagoon of Venice, Italy

Sediments and biota samples were collected in a restricted area of the Lagoon of Venice and analysed for total mercury, monomethyl mercury (MMHg), and nitrogen and carbon isotopes. Results were used to examine mercury biomagnification in a complex food chain. Sedimentary organic matter (SOM) proved to be a major source of nutrients and mercury to primary consumers. Contrary to inorganic mercury, MMHg was strongly biomagnified along the food chain, although the lognormal relationship between MMHg and δ(15)N was less constrained than generally reported from lakes or coastal marine ecosystems. The relationship improved when logMMHg concentrations were plotted against trophic positions derived from baseline δ(15)N estimate for primary consumers. From the regression slope a mean MMHg trophic magnification factor of 10 was obtained. Filter-feeding benthic bivalves accumulated more MMHg than other primary consumers and were probably important in MMHg transfer from sediments to higher levels of the food chain.

Extraction of anthropogenic heavy metals from reduced sediments: Indeterminations due to authigenic sulphides and clay minerals

Abstract A mineralogical and micro‐chemical study was performed by X‐ray diffraction and SEM/EDS techniques in a reduced sediment sample from the Venice Lagoon to investigate selectivity and significance of two widely used acid extractions (cold IN HCl and hot 8N HNO3) for the evaluation of anthropogenic metal concentrations. Hydrochloric acid determines an incomplete and variable solubilization of authigenic iron sulphides, yielding a partial extraction of the associated metals (Hg, Mn, Zn and less Ni and Cu). Nitric acid is effective in solubilizing all sulphides, however, it leads to an overestimate of the anthropogenic metal content since it partially dissolves the lattice of some clay‐minerals, particularly chlorites. When these acid extractions are used to asses metal pollution in aquatic ecosystems, the possible over or under‐estimate of the anthropogenic concentration must be taken into account, considering the mineralogical composition of the sediment.

Useful tracer parameters to investigate the environmental conditions in areas of the Venice Lagoon

Grain-size and redox potential distributions in sediments were used as tracers to investigate environmental conditions in a shallow water area of the Venice Lagoon subjected to summer anoxic events. Data are presented showing different environmental characteristics within the study area. The results illustrate the reliability of these tracers to acquire a preliminary knowledge of the aquatic ecosystem behavior. Even small differences in morphology and hydrodynamics are observable because of the marked influence they exert on sediment parameters.

A filtration system for the size separation of freshwater samples

Abstract A system to filter freshwater samples in situ in the field is described, using polycarbonate membranes. It was developed and tested with respect to the conventional filtration method, both for its efficiency and for the effect on heavy metal concentrations in the filtrates. A marked reduction of filtration time is obtained and the possibility to carry out one stage filtrations that otherwise would need the filter to be changed. PIXE analysis generally shows an increase of heavy metal concentrations in the filtrates; this is accompanied by a smaller data spread in sub-samples obtained by repeated filtrations, indicating a higher capability in maintaining the nominal membrane pore size during operations, which produces a better dimensional selection. The system is suitable to perform immediate filtration of numerous fresh water samples to be treated in heavy metal pollution assessments.

Tracing the effects of eutrophication on molluscan communities in sediment cores: outbreaks of an opportunistic species coincide with reduced bioturbation and high frequency of hypoxia in the Adriatic Sea

Abstract. Estimating the effects and timing of anthropogenic impacts on the composition of macrobenthic communities is challenging, because early twentieth-century surveys are sparse and the corresponding intervals in sedimentary sequences are mixed by bioturbation. Here, to assess the effects of eutrophication on macrobenthic communities in the northern Adriatic Sea, we account for mixing with dating of the bivalve Corbula gibba at two stations with high accumulation (Po prodelta) and one station with moderate accumulation (Isonzo prodelta). We find that, first, pervasively bioturbated muds typical of highstand conditions deposited in the early twentieth century were replaced by muds with relicts of flood layers and high content of total organic carbon (TOC) deposited in the late twentieth century at the Po prodelta. The twentieth century shelly muds at the Isonzo prodelta are amalgamated but also show an upward increase in TOC. Second, dating of C. gibba shells shows that the shift from the early to the late twentieth century is characterized by a decrease in stratigraphic disorder and by an increase in temporal resolution of assemblages from ~ 25–50 years to ~ 10–20 years in both regions. This shift reflects a decline in the depth of the fully mixed layer from more than 20 cm to a few centimeters. Third, the increase in abundance of the opportunistic species C. gibba and the loss of formerly abundant, hypoxia-sensitive species coincided with the decline in bioturbation, higher preservation of organic matter, and higher frequency of seasonal hypoxia in both regions. This depositional and ecosystem regime shift occurred in ca. A.D. 1950. Therefore, the effects of enhanced food supply on macrobenthic communities were overwhelmed by oxygen depletion, even when hypoxic conditions were limited to few weeks per year in the northern Adriatic Sea. Preservation of trends in molluscan abundance and flood events in cores was enhanced by higher frequency of hypoxia that reduced bioturbation in the late twentieth century.