Interannual heavy element and nutrient concentration trends in the top sediments of Venice Lagoon (Italy)

https://doi.org/10.1016/j.marpolbul.2014.10.036Get rights and content

Highlights

  • Heavy element concentrations in sediments of a transitional system were studied.

  • Element contamination in the recent years has generally declined.

  • Mercury and cadmium contamination still remain above the limits in hot spots.

  • The role of sediment resuspension due to anthropogenic activity is discussed.

  • A basic knowledge to assess the impact of the MOSE construction is provided.

Abstract

The elemental composition of surficial sediments of Venice Lagoon (Italy) in 1987, 1993, 1998 and 2003 were investigated. Zn and Cr concentrations resulted in higher than background levels, but only Cd and Hg were higher than legal quality standards (Italian Decree 2010/260 and Water Framework Directive 2000/60/EC). Contaminants with similar spatial distribution are sorted into three groups by means of correlation analysis: (i) As, Co, Cd, Cu, Fe, Pb, Zn; (ii) Ni, Cr; (iii) Hg. Interannual concentrations are compared by applying a factor analysis to the matrix of differences between subsequent samplings. A general decrease of heavy metal levels is observed from 1987 to 1993, whereas particularly high concentrations of Ni and Cr are recorded in 1998 as a consequence of intense clam fishing, subsequently mitigated by better prevention of illegal harvesting. Due to the major role played by anthropogenic sediment resuspension, bathymetric variations are also considered.

Introduction

Heavy metals, generally described as elements with a density >5 g/cm3, such as lead (Pb), cadmium (Cd), zinc (Zn), mercury (Hg), chromium (Cr), copper (Cu), and iron (Fe), are among the main environmental threats to human health, being responsible for harmful effects depending on the type of metal, exposure time, concentration and degree of bio-accumulation in the food chain. Moreover, high contamination levels can seriously affect ecosystem equilibrium, compromising metabolic activities and community diversity. Trace concentrations of such elements are essential for organism physiology, but the development of industry and mining activities since the mid 19th century has rapidly increased metal pollution (Järup, 2003) to levels which may have adverse effects on biota and ecosystem. Therefore its reduction is a serious challenge, necessary in order to protect human health and to favour environmental conservation and recovery. However, background concentrations (naturally occurring) depend on local geological features and an universal threshold value cannot be established for all ecosystems (Ridgway et al., 2003). This renders the assessment of actual pollution levels and the determination of appropriate management policies very difficult. Despite the recent increasing efforts to regulate and manage the environmental impact of industrial discharges, past contamination still represents a significant ecological risk. Specifically, in coastal areas, dredging may be a serious hazard for aquatic ecosystems, as several studies demonstrate that dredged sediments may be responsible for increased toxicity and adverse ecological effects (see references in Han et al., 2011 and in Onorati et al., 2013 and references therein).

This study analyses the changes in heavy metal, carbon and nutrient levels in the surficial sediments of a coastal transitional ecosystem that has been affected for long time by significant contaminant inputs due to urban, agricultural and industrial discharges. Since the mid 20th century, the ecosystem of Venice Lagoon (North-western Adriatic Sea) has been exposed to several substantial anthropogenic pressures causing sediment pollution (Bellucci et al., 2000, Bellucci et al., 2002, Bernardello et al., 2006 and references therein; Guerzoni et al., 2007, Secco et al., 2005, Zonta et al., 2007), water contamination (Micheletti et al., 2011), eutrophication (Sfriso et al., 2003), overexploitation of biological resources (Pranovi et al., 2004), degradation of biota due to bioaccumulation of pollutants through the food chain (Raccanelli et al., 2004, Turetta et al., 2005, Sfriso et al., 2008), sediment erosion (Sfriso et al., 2005, Sarretta et al., 2010, Rapaglia et al., 2011) and consequent salt marsh losses (Molinaroli et al., 2009). On May 2003, the Italian National Authorities formally started the construction of a system to regulate tidal floods at the three inlets of the Lagoon by means of mobile barriers, called MOSE. The system is supposed to be in operation in 2016, after the realization of 78 floodgates 20 m wide, 18.5–29.6 m high and 3.6–5 m thick, depending on the depth of the inlet (http://www.salve.it/uk). Beyond the realization of the barriers, several supporting interventions were made to adapt the inlet beds.

In the present paper, particular attention is paid to describe the contaminant concentrations just at the beginning of the MOSE project (June 2003) to have a benchmark of lagoon conditions before a so important anthropogenic intervention that is expected to deeply change lagoon’s dynamics. Moreover, multivariate statistical approaches were applied to simplify and summarise the information contained in a dataset of 22 variables (sediment texture and density: 12 elements including heavy metals; inorganic and organic carbon; nitrogen; inorganic and organic phosphorus) and 100 observations and find common patterns in pollutant inter-annual variations, focusing on bio-physical processes affecting contaminant concentrations and distribution during 4 separate phases:

  • June 1987: eutrophication due to massive Ulva proliferation was the main driver of biogeochemical cycles (Sfriso and Facca, 2007); dystrophic–anoxic crises periodically occurred during algal degradation.

  • June 1993: macroalgal biomass almost disappeared (Sfriso and Facca, 2007) mainly due to climate change and the synergic combinations of many factors (Sfriso and Marcomini, 1996).

  • June 1998: uncontrolled clam harvesting peaked (40,000 tons y−1, Pellizzato et al., 2011); hydraulic and mechanical dredging systems seriously affected the benthic habitat, by increasing sediment resuspension (about one order of magnitude, Sfriso et al., 2005), changing sediment texture and amplifying erosion processes (Molinaroli et al., 2009).

  • June 2003: at the beginning of the 2000s clam harvesting was regulated by licensing ∼35 km2 for clam farming (Pellizzato et al., 2011). The environmental impact significantly decreased and seagrasses recolonized extensive areas of the lagoon (Sfriso and Facca, 2007).

Metal concentrations were interpreted in relation to sediment grain-size and bathymetric modifications in the same period in order to assess the link between changes in contamination with erosion or deposition processes. The relationship between contaminants and sediments is a key to understanding the fate of heavy metals in the ecosystem.

Section snippets

Study area

The Lagoon of Venice (Fig. 1a) is one of the few wetlands still present on the northwestern coast of the Adriatic Sea (45° 24 N, 12° 20 E) among those originating from the Flandrian transgression ca. 6000 years ago (Gatto and Carbognin, 1981, Brambati et al., 2003). It has a surface area of ∼550 km2 and is connected to the Adriatic Sea through the inlets of Lido (∼900 m wide, 14 m deep), Malamocco (∼600 m wide and 14 m deep with a depression of ∼53 m, the deepest in the northern Adriatic Sea) and

Sediment characteristics and contaminant concentrations (2003 data)

Table 1 summarises the analytical results (all expressed as dry weight, dw) of the surficial sediments sampled in 2003. Maps showing the spatial distribution of mud content and heavy metals, exceeding law quality standards and background values are shown in Fig. 1. The maps of the distributions of all elements are available as Supplementary material (Fig. SI1).

The distribution of fine sediments indicated a rather variable grain-size distribution (Table 1) which is comparable with those derived

Conclusions

The contamination of Venice Lagoon was assessed by analyzing heavy metals, carbon and nutrient concentrations in surficial sediments collected at 25 sites in 2003. Most heavy metals (Fe, Mn, Co, Cu, Zn, As, Cd, Pb), organic carbon and total nitrogen showed similar spatial distributions, having higher values in the inner part of the lagoon, probably due to both discharges from the industrial zone and drainage from urban areas. Mercury has its own distinctive spatial distribution, with high

Acknowledgments

The authors are grateful to Consorzio Venezia Nuova (Dr. B. Bertani and Arch. G. Biotto) for providing bathymetric data and to Dr. M. Marchiori (Regional Environmental Protection Agency, ARPAV) for mercury analyses. The authors are grateful to Mr. George Metcalf for the English editing.

References (34)

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