Heavy metals in sediments and halophytes of saltmarshes in the Marano and Grado Lagoon (Northern Adriatic Sea)

Results and Discussion Saltmarshes are important constituents of transitional environments, where they provide several services to the ecosystem and are essential to the sedimentary budget (Pethick, 1984). On the other hand, they are subject to the effects of increasing human pressures, especially where anthropogenic activities occur. As a consequence, the accumulation of contaminated sediments (e.g., heavy metals) may affect these fragile environments (Cacador et al., 1996). Plants living in saltmarshes often show their ability to accumulate heavy metals both in roots and shoots, thus providing a natural metal-extraction pathway useful for remediation activities.


Introduction
Saltmarshes are important constituents of transitional environments, where they provide several services to the ecosystem and are essential to the sedimentary budget (Pethick, 1984).On the other hand, these peculiar environments are also subject to the effects of increasing human pressure, especially where anthropogenic activities occur.As a consequence, sediments trapped by the saltmarsh vegetation may be affected by the accumulation of several contaminants, heavy metals included (Cacador et al., 1996).
Plants living in saltmarshes show specific adaptations to these hypersaline environments (Breckle, 2002).A common characteristic is their ability to accumulate heavy metals both in roots and shoots (Cacador et al, 2009;Canario et al., 2010).In this manner, they act as hyper accumulators, thus providing a natural metal-extraction pathway for remediation activities.
The Marano and Grado Lagoon (Northern Adriatic Sea) is the second largest lagoon area of the Adriatic after Venice.The Lagoon is generally considered a well-preserved area where conservation of the natural environment must coexist with several human activities such as fishing, shipping and industries.
The main source of contamination in the Lagoon is represented by the inflow from the Aussa-Corno river system which is impacted by industrial discharges from several industries and is connected to the open sea by a navigable channel.Another source of contamination is the Isonzo River, the largest contributor of mercury into the northern Adriatic Sea since the 16th century, due to its transport of cinnabar (HgS) rich tailings from the Idrija (Slovenia) mining district (Covelli et al., 2001).The aim of this study was to evaluate the content of several heavy metals (As, Cd, Cr, Cu, Hg, Mn, Ni, Pb, Zn) in sediments and in plants from two selected saltmarshes located in the Lagoon.Enrichment Factors (EF) and metal translocation from the belowground to the aboveground biomass were also considered.

Materials and Methods
Sampling operations took place in July 2011.Two saltmarshes were investigated: M5B in the Marano Lagoon and BARB in the Grado Lagoon (Fig. 1).The saltmarshes differ for morphology and heavy metal contents and are representative of a highly impacted area (Marano Lagoon) and a moderately (except for high Hg contents) contaminated area (Grado Lagoon).
Fig. 2. The hand-corer used for sediment and belowground biomass sampling.
In both saltmarshes, individuals of Sarcocornia Fruticosa and Limonium Vulgare, two of the most abundant halophytes in this environment, were considered.The aboveground biomass (shoots and leaves) was collected and sealed in plastic bags.In correspondance to the severed shoots, the belowground biomass and the attached rhizo-sediment were sampled using a 5.5 by 50 cm hand-corer (Fig. 2).
The obtained sediment cores (including roots) were sectioned on field (Fig. 3) into 9 depth levels: every 2 cm in the first 10 cm and then into 5 cm slices to a maximum depth of 30 cm.In the laboratory, roots were carefully separated from the rhizo-sediment, washed with distilled water and thoroughfully rinsed with Milli-Q water.Sediment, roots, leaves and shoots were freeze-dried, finely ground and homogenized.The obtained samples were totally decomposed (Loring and Rantala, 1992) using a mixture of mineral acids in a closed microwave system (Milestone MLS 1200), as required by the different procedures.The obtained solutions were analyzed for heavy metal content by ICP-AES.Total Hg content in the solid-phase was determined by .

Results and Discussion
The heavy metal content in the sediments was quite high at both saltmarshes.Compared to local background levels, M5B was the most contaminated site, with higher values especially for As, Cd, and Pb.On the other hand, Cu and Hg were highest in BARB.Such results reflect the different contamination history of the two areas: M5B is directly influenced by the industrial area that insists on the Marano basin, while BARB is subject to 06006-p.2 contamination by the Hg-rich particulate matter inflowing from the Isonzo River.
To evaluate metal accumulation in the plants, the Enrichment Factors (EF = [metal] roots /[metal] rhizo-sediment ) were calculated.In both saltmarshes, the halophyte vegetation showed accumulation (EF>1) of As and Cd, but not of Ni (Fig. 4).S .fruticosashowed the greater bioaccumulation while M5B seemed to be a more favorable site for such processes, probably due to redox conditions in the sediment that enhance metal mobility.The translocation of metals from the halophytes' roots to their shoots and leaves was evident at both saltmarshes.Notably, at BARB, Cr in the shoot of S. fruticosa was particularly high (181 µg g -1 ) compared to its content in the roots (69 µg g -1 ) and rhizo-sediment (86 µg g -1 ).In L. vulgare, the Zn content in the leaves was almost double of that in the shoots and about a half of that in the roots.Zn is an essential element for plant physiology, playing a key role as enzymatic co-factor and, thus, its presence in the leaves is not surprising.No translocation was observed for Cd and Hg, and they appeared to be immobilized in the halophytes' roots

Conclusions
The present study is the first attempt to investigate heavy metal content in the sediments and vegetation of the Marano and Grado Lagoon saltmarshes.The results showed that these areas can be considered as a sink (and a possible source) for several heavy metals.Halophytes showed the ability to absorb metals from the saltmarsh sediment and to accumulate them in the roots and/or translocate them to the aboveground biomass.Bioaccumulation varied with the species and the different environmental conditions, showing the highest efficiencies in S. fruticosa and in the industrially-impacted Marano basin.

Fig. 4 .
Fig. 4. Vertical concentration profiles for As, Ni, Zn, and Pb in sediments (continuous line) and roots (dashed line) of the halophytes S. fruticosa and L. vulgare in the Marano and Grado Lagoon saltmarshes (sites M5B and BARB).