as a Historical Monitor Device of Lead Concentration in Marine

monitor the variability of environmental lead (Pb). We analyze lead concentration measured in the scales and rhizomes of Posidonia oceanica collected ...
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Environ. Sci. Technol. 2005, 39, 3006-3012

Posidonia oceanica as a Historical Monitor Device of Lead Concentration in Marine Environment L . T R A N C H I N A , * , † S . M I C C I C H EÅ , † , ‡ A . B A R T O L O T T A , †,§ M . B R A I , †,‡ A N D R . N . M A N T E G N A †,‡ Dipartimento di Fisica e Tecnologie Relative, Universita` degli Studi di Palermo, Viale delle Scienze, Edificio 18, 90128 Palermo, Italy, Istituto Nazionale per la Fisica della Materia, Unita` di Palermo, Viale delle Scienze, Edificio 18, 90128 Palermo, Italy, and Dipartimento Farmacochimico, Tossicologico e Biologico, Universita` degli Studi di Palermo, Via Archirafi 26, 90123 Palermo, Italy

We show that Posidonia oceanica is able to reliably monitor the variability of environmental lead (Pb). We analyze lead concentration measured in the scales and rhizomes of Posidonia oceanica collected in seven sites along the coasts of the Sicily island and subsequently fractioned them according to a lepidochronological analysis. We measure lead concentration in Posidonia oceanica tissues by using the flame atomic absorption spectrophotometry technique. We compare the measured lead concentration with the estimated lead emission in air due to the gasoline sold and used for combustion in car engines in Sicily. By computation of the Pearson crosscorrelation coefficient, we show that lead concentration, which is measured in the scales of Posidonia oceanica, is statistically correlated to lead emission in air and reflects the level of lead pollution in the coastal marine environment.

Introduction Posidonia oceanica (L.) Delile is a sea plant widely distributed all over the Mediterranean basin. Its longevity (it may live for more than 50 years), the fact that its growth persists over different seasonal cycles, and its characteristic of accumulating different types of pollutants, e.g., metals (1-4), radionuclides (5,6), and organochlorines, make this marine phanerogam an excellent candidate to monitor the coastal environmental conditions (7). It is known that Posidonia oceanica grows in dense meadows that live close to the coasts and extend down to 30-40 m deep, depending on water conditions such as turbidity, temperature, and salinity. Therefore Posidonia oceanica interacts with marine coastal biocenoses. In the Posidonia oceanica organism, three different types of tissues can be distinguished: (i) leaves that die within an annual cycle, (ii) rhizomes that sustain the leaves and have orthotropic directions, and (iii) scales that are at the bottom of the leaves. When the leaves of Posidonia * Corresponding author phone.: +39 091 6615047; fax: +39 091 6615063; e-mail: [email protected]. † Dipartimento di Fisica e Tecnologie Relative, Universita ` degli Studi di Palermo. ‡ INFM Unita ` di Palermo. § Dipartimento Farmacochimico, Tossicologico e Biologico, Universita` degli Studi di Palermo. 3006

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oceanica die, their blades detach from the bases, while the bases themselves stay on the rhizomes and form a scale. The dating of fragments of rhizomes and scales is usually performed by fractioning Posidonia oceanica according to its life cycle. In fact, the cyclical variation of the thickness of the scales remaining on the rhizome allows defining the start and the end of an annual life cycle. This methodology is called the lepidochronological technique (8). The fact that this sea plant is able to record the coastal concentration of pollutants has been recently shown. Several authors (9,10) pointed out that Posidonia oceanica is able to “take record” of the yearly concentration of mercury (Hg) in different tissues of the sea plant. In particular, they noticed that the Hg concentration of Posidonia oceanica was wellcorrelated to the change of environmental Hg concentration (10). Yearly concentration of metals can be measured after having selected the fragments of scales and rhizomes associated to the considered life year of Posidonia oceanica, according to the lepidochronological technique mentioned above (11-13). Further, by use of this technique, some researchers were able to prove that concentration of 137Cs found in the scales of Posidonia oceanica were correlated with 137Cs fallout (8). The aim of this paper is to show that Posidonia oceanica is also able to reliably monitor the variability of environmental lead (Pb) as much as mercury and radio-cesium. To this end, Pb concentration measured in the scales of Posidonia oceanica, which have been collected in seven different sites located along the coasts of the Sicily island and subsequently fractioned according to the lepidochronological methodology, is compared with data of gasoline consumption. We show that the lepidochronological lead content in the scales of Posidonia oceanica is statistically correlated to the historical pattern of the Pb emission in air, which is associated with the combustion of the gasoline sold and used in Sicily.

Theoretical Estimation of Lead Pollution in Air The main source of Pb pollution in the geographical area of our interest is gasoline combustion. In the last 20 years, scientists and ecologists indicated that lead is one of the major sources of environmental pollution and that it is harmful to human health. They have therefore suggested the phasing out of lead from gasoline as a possible remedy. These indications were indeed followed by the introduction of laws that progressively allowed sales of unleaded gasoline only. Anthropic Pb released in the environment is directly related to Pb contents in gasoline (14-16). Other sources of Pb are associated to industrial applications. However, the main anthropogenic input to the environment remains the combustion of gasoline containing lead alkyls as anti-knock additives. For instance, report 31 of the GKSS Forschungszentrum Geesthacht GmbH Institut fu ¨ r Ku ¨ stenforschung (17) estimates that the percentage of lead emission in air due to the transportation source in Europe was 76.1% in 1985, 72.1% in 1990, and 68.7% in 1995 with respect to the total lead emission in air. The U.S. Environmental Protection Agency (18) showed the existence of a positive correlation between Pb levels in consumed gasoline and Pb concentration in air. Moreover some authors found statistical correlation between the Pb concentration measured in some vegetables in Spain (19) with Pb emission in air. Other authors did not find any statistical correlation measuring Pb levels in the tree ring of plants in Scotland (20). The issue of statistically significant correlation between Pb emissions in air and Pb concentration in human blood (21,22) has also been considered in the literature. Finally, other authors 10.1021/es048870u CCC: $30.25

 2005 American Chemical Society Published on Web 04/02/2005

FIGURE 1. The picture shows the yearly Pb emission in air from year 1979 to year 2001. These data are obtained starting from the sales of gasoline in the geographical areas where sampling sites are selected and assuming that approximately 75% of Pb in gasoline is released in air after combustion. (15,23) outlined that atmosphere represents the largest source of estimated Pb deposition in marine environment, particularly in closed basins. It is therefore meaningful to consider the possibility of correlation between historical levels of Pb emission in air and lean concentration in a sea plant such as Posidonia oceanica. The phasing out of lead from leaded gasoline in Italy proceeded as follows. The process started in 1982 (22,24) with the lessening of the tetra-ethyl lead concentration Kl(Pb) from 0.6 to 0.4 g L-1 (Italian law DPR 485/82). This value was diminished to Kl(Pb)) 0.3 g L-1 in 1988 and finally fixed at the level of Kl(Pb) ) 0.15 g L-1 in 1992 (Italian law DM 214/88) until the total dismissal of leaded fuel at the end of 2001 (Italian law DPCM 434/2000). The tetra-ethyl Pb concentration in unleaded fuel has always been maintained at the level of Ku(Pb) ) 0.013 g L-1. The availability of the leaded and unleaded gasoline consumption data in Sicily (25) allows us to look for a possible statistical correlation between Pb emission in air due to gasoline combustion and the Pb concentration recorded in Posidonia oceanica tissues. Figure 1 shows the estimated values of yearly Pb emissions in air Eair(Pb) starting from year 1979 to year 2001. These values are calculated starting from the amount of gasoline sold in the geographical areas where the sampling sites were selected. The estimation is done by assuming that approximately 75% of Pb in gasoline is released in air after combustion (26,27) according to the equation

Eair(Pb)(t) ) 0.75(Kl(Pb)(t)Sl(t) + Ku(Pb)(t)Su(t))

(1)

where t labels the year considered, Sl(t) and Su(t) are the sales of leaded and unleaded gasoline (ton/year), and Kl(Pb)(t), Ku(Pb)(t) are the Pb concentration (Pb ton/gasoline ton) in gasoline given above. To check the validity of our procedure, by using the consumption data of gasoline in Italy, eq 1 was used to estimate the Pb emission in air at the aggregate national level. The estimated values were compatible with the data provided for the years 1990 and 1994 by the National Institute of Statistics (28).

Materials and Methods Sampling. To test the reliability of Posidonia oceanica as a biological monitor of lead pollution in the marine environment, the sites of survey have been chosen to be very similar with respect to their geochemical and lithological features but different with respect to their pollution conditions. In fact, the choice of sites was made by considering the condition

FIGURE 2. Sites of survey along the coasts of the Sicily island. One site is located in north Sicily, three sites are located in west Sicily, and four sites are located in southeast Sicily. PA labels the Province of Palermo, TP labels the Province of Trapani, and SR labels the Province of Siracusa. of automotive traffic, degree of industrialization, and urbanization of the neighboring of the sites. Posidonia oceanica samples were collected in the seven geographical sites shown in Figure 2. The Carini site is located in Northern Sicily, the Trapani and Marsala sites are located in Western Sicily, and the other four sites are located in southeastern Sicily. They are Cape Passero, Marzamemi, Cape Negro, and Ognina Bay. In each sampling site, only one location, which is representative of the average conditions of the Posidonia oceanica meadows, is selected. This location is called transect. Twenty shoots of the plant were collected for each sampling point. The choice of the transect and sampling depths, the collection of sea-plants and the collection of the associated superficial sediments together with the plant fractioning according to the lepidochronological technique have been carried out by another research group within the CIPE-MIUR MIR project (29,30). Further details can be found in ref 29. Metal Measurements. Epibiota and sediments were removed from Posidonia oceanica samples by a gentle scraping with a glass slide followed by a very quick rinsing with distilled water. This is done in order to minimize the known losses of elements as discussed in refs 2 and 31. Posidonia oceanica samples were dried at 105 °C for 48 h and then milled by using a Fritsch Pulverizette 2. Samples of a size of approximately 100 µm were obtained. Equal weight of the milled samples belonging to two consecutive lepidochronological years were mixed up to obtain a biennial sample when considering scales from the sites of Carini, Trapani, and Marsala. In the remaining cases, samples belonging to a single lepidochronological year were treated separately. We perform a flame atomic absorption spectrophotometry (FAAS) analysis. Specifically, a quantity of 500 mg is digested in an open cavity microwave system (C.E.M. Star system 2) using the following procedure: 5 mL of HNO3 65% and 5 mL of distilled and deionized water were first added to the 500mg sample aliquot and heated for 3 min at 50 °C. The temperature was then raised to 70 °C and kept to that value for 10 min. Afterward, 5 mL of HClO4 70% were added and the temperature was raised to 98 °C for 7 min. The mixture was then kept at 106 °C for 5 min, at 120 °C for 4 min, at 140 °C for 2 min, and 180 °C for 2 min. Digested samples, filtered through 0.45-µm pores, were cooled and diluted to 25 mL with distilled and deionized water. All the glassware was cleaned by soaking in a 10% HNO3 solution for 24 h and then rinsed three times with deionized water to prevent any contamination. All the chemical reagents were of Merck supra-pure grade. VOL. 39, NO. 9, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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composed of about 13 records covering the lepidochronological years from 1989 to 2001 with an annual time step. In one case (Cape Negro), we have measurements covering a longer lepidochronological time interval ranging from 1981 to 2001 with an annual time step. The degree of co-movement between two time series can be quantified by computing the linear (Pearson) crosscorrelation coefficient. Each of these correlation coefficients has associated a significance threshold, which is depending on the number of points Nµ in the time series Mµ considered. The significance threshold is customary quantified in terms of a p value. The p value is defined as the cumulative probability PN (|r| > |r0|) of finding a correlation coefficient r greater than the observed value r0, under the assumption that the two sets of data are random variables, which are Gaussian and uncorrelated. Under this assumption, the probability density function that two time series of any random sample of uncorrelated experimental data points would yield an experimental linear correlation coefficient equal to r is given by ref (32)

TABLE 1. Correlation Coefficients between Pb Concentration Measured in the Scales of Posidonia oceanica and Estimated Pb Emissions in Air for the Seven Sites Considered m

sitea

Nm

correlation coefficients

p value

1 2 3 4 5 6 7

Carini Trapani Marsala Cape Passero Marzamemi Cape Negro Ognina Bay

9 10 8 13 13 21 14

0.85 0.77 0.83 0.32 0.14 0.80 0.83

0.0037 0.0092 0.0108 0.2864 0.6483 0.00001 0.00024

a Sites are reported anticlockwise, following their location along the Sicilian coasts.

Pb concentration was measured by a Varian AA20 Plus FAAS. The spectrophotometer was equipped with a deuterium background corrector and an atom concentrator tube (ACT80) which was positioned in the air/acetylene flame to enhance the sensitivity by 2-3 times when compared to that of standard FAAS. The detection limit of Pb was 0.9 µg g-1. The Community Bureau of Reference (European Union) RM 60 (Lagarosiphon major) was used as a certified reference material for the measurements. Data and Statistical Analysis. Hereafter we label each investigated geographical site with a numerical label µ ranging from 1 to 7. The correspondence between number and location is given in Table 1. Data of different sites are slightly dishomogeneous with respect to the number of transects investigated in each site and with respect to the number of obtained lepidochronological records. Specifically, for the sites of Carini, Trapani, and Marsala, we obtain one single time series representative of the site by averaging the data of the sampled transects according to the formula

Mµ )

1

PN(r) )

The p value PN (|r| > |r0|) represents the area described by the definite integral of the above probability density function from the value r0 to 1. For example, having set a confidence level at p ) 0.05, for a cross-correlation coefficient computed over a pair of time series with 10 records, the lowest correlation coefficient which is statistically significant is about 0.63, while for a time series of 13 points the significance threshold is about 0.55. Equivalently, a correlation coefficient r0 ) 0.63 obtained from time series with 10 points is said to be statistically significant at a 5% confidence level. To directly assess the statistical reliability of each measured correlation coefficient, in Tables 1 and 3 we report the correlation

Tµ

∑M

Tµ a)1

µ,a

µ ) 1, ..., 3

2 Γ((N - 1)/2) (1 - r2)(N-4)/2 xπ Γ((N - 2)/2)

TABLE 3. Correlation Coefficients between Pb Concentration Measured in the Scales of Posidonia oceanica and Estimated Pb Emissions in Air for the Four Macrosites Considered

(2)

where Mµ indicates the time series of Pb concentration in site µ composed of Nµ records. The number Tµ indicates the effective number of transects over which the mean value Mµ was computed. Mµ,a indicates the time series relative to the metal Pb in site µ and in transect a, with a ranging from 1 to Tµ. In most cases, Tµ ) 3 and each time series Mµ is composed of about 10 records covering the lepidochronological years from 1981 to year 2001 with a biennial time step. However, in some cases (e.g., Marsala), measurements were not performed for some specific lepidochronological year, and therefore either Tµ < 3 or the resulting time series shows a smaller number of points. For the four sites of Cape Passero, Marzamemi, Cape Negro, and Ognina Bay, each time series Mµ (µ ) 4, ..., 7) is

a

macrosite

Na

correlation

p value

N W SE1 SE2

9 8 13 14

0.85 0.81 0.25 0.76

0.0037 0.0148 0.4100 0.0017

The second column indicates the number of records considered.

coefficient together with its associated p value.

Results and Discussion The data considered in this paper are the Pb concentration measured by FAAS in scales and rhizomes of Posidonia

TABLE 2. Concentration of Lead in Superficial Sediment, Scales, and Rhizomes (Last Lepidochronological Year) of P. oceanica and Concentration of Oxides of the Two Major Elements Si and Ca Obtained by XRF Analysis Measured in Samples of Superficial Sediments Collected at the Same Sites as for the Posidonia Oceanica site

lithology

SiO2a

CaOa

Pbb sediment

Pbb,c scales

Pbb,c rhizomes

Carini Trapani Marsala Cape Passero Marzamemi Cape Negro Ognina Bay

calcite calcite calcite magnesian calcite magnesian calcite calcite calcite

5.0 5.0 9.3 3.1 1.9 1.6 1.8

45 47 45 47 48 50 50

8.4 ( 0.9 7.3 ( 0.8 8.3 ( 0.9 1.0 ( 0.3 1.5 ( 0.2 4.0 ( 0.5 4.5 ( 0.5

10.3 ( 1.3 7.8 ( 0.8 6.5 ( 0.7 4.5 ( 0.6 4.5 ( 0.6 5.5 ( 0.6 8.5 ( 0.9

9.3 ( 1.0 4.2 ( 0.6 4.8 ( 0.6 5.9 ( 0.7 4.4 ( 0.6 4.0 ( 0.7 6.1 ( 0.8

a SiO2 and CaO are expressed as a mass percentage in the hydrated samples. b Pb concentration is expressed in µg g-1. c Pb concentration for the last lepidochronological year.

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correlation of these two sites is probably due to the fact that Cape Passero is a natural reserve and Marzamemi is very close to it. Therefore the two sites experience levels of urbanization, traffic conditions, and in general the anthropogenic level, which are characterized by a low level of pollution. The values obtained for the other five sites show a clear and statistically significant correlation between Pb in scales and lead emission in air. Among them, the Carini site and the Ognina Bay site show a high level of correlation, reasonably explained by the fact that these areas are highly populated, with heavy traffic conditions and with relatively high levels of industrialization. Figure 5 shows time series Mµ (µ ) 1, ..., 3) of Pb

FIGURE 3. Changes over time in Pb concentration measured in the scales of Posidonia oceanica in the sites of Carini, Trapani, and Marsala. The time series relative to Trapani and Marsala have been shifted in order to improve the readability of the figure. oceanica. Figure 3 shows the time series Mµ (µ ) 1, ..., 3) of Pb concentration measured in the scales of plants sampled in the geographical sites of Carini, Trapani, and Marsala of Northern and Western Sicily. The highest levels of Pb concentration, ranging from 18 µg g-1 in 1983 to 10.25 µg g-1 in 2000, are obtained in the Carini site, which is located approximately 10 km from the city of Palermo. This site shows the highest level of concentration also for other metal ions (33). For different sites located in the Mediterranean Sea, other authors (1) report values of Pb concentration ranging from 1.7 µg g-1 to 7 µg g-1. In Figure 4 we show time series Mµ (µ ) 4, ..., 7) of Pb concentration measured in the scales of Posidonia oceanica in geographical sites of Cape Passero, Marzamemi, Cape Negro, and Ognina Bay, which are located in southeastern Sicily. For this group of sites, the highest levels of Pb concentration, ranging from 18.5 µg g-1 in 1991 to 8.5 µg g-1 in 2001, were measured in the Ognina Bay site, which is very close to Augusta, an industrial site located nearby the historical city of Siracusa. In most cases, Pb concentration measured in the scales of Posidonia oceanica is statistically correlated with estimated Pb emissions in air. In fact, in Table 1, we show all the correlation coefficients for each of the seven investigated sites. Specifically, in five of the seven sites, the correlation coefficient assumes values that are statistically significant, with p values well below the confidence level of 5%. Only for the sites of Cape Passero and Marzamemi are the correlation coefficients not statistically significant. The low level of

FIGURE 5. Changes over time in Pb concentration measured in the rhizomes of Posidonia oceanica in the three sites of Carini, Trapani, and Marsala. No evident temporal trend is observable. The time series relative to Trapani and Marsala have been shifted in order to improve the readability of the figure. concentration measured in the rhizomes of Posidonia oceanica in the geographical sites of Carini, Trapani, and Marsala, which are located in Northern and Western Sicily. In Figure 6 we show all time series Mµ (µ ) 4, ..., 7) of Pb concentration measured in the rhizomes of Posidonia oceanica in the geographical sites of Cape Passero, Marzamemi, Cape Negro, and Ognina Bay, which are located in southeastern Sicily. Different from the case of Pb concentration in scales, the Pb concentration measured in the rhizomes of Posidonia oceanica is approximately constant in the entire lepidochronological period considered and no dependence on the site pollution can be detected. Consequently, Pb concentra-

FIGURE 4. Changes over time in Pb concentration measured in the scales of Posidonia oceanica in the four sites of Cape Passero, Marzamemi, Cape Negro, and Ognina Bay. VOL. 39, NO. 9, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Changes over time in Pb concentration measured in the rhizomes of Posidonia oceanica in the four sites of Cape Passero, Marzamemi, Cape Negro, and Ognina Bay. No evident temporal trend is observable. tion measured in the rhizomes does not show any statistically significant correlation with the estimated Pb emissions in air. It is known that rhizomes show lower values of Pb concentration with respect to the scales because their exchange phenomena with sediment are less relevant than in the case of scales (1). To support this argument with empirical observations, we report in Table 2 the concentration of two major oxides in superficial sediments together with the Pb concentration measured in sediment, scales, and rhizomes belonging to the last lepidochronological year. The comparison between Pb concentration in sediments and the Pb concentration in the scales for the last lepidochronological year considered confirms the existence of a statistically significant correlation. The correlation coefficient is r ) 0.77 with a p value of p ) 0.044. On the contrary, the data in Table 2 confirm the absence of a significant level of correlation between Pb concentration in sediment and in rhizomes. In fact, the correlation coefficient between Pb concentration in sediments and the Pb concentration in the rhizomes for the last lepidochronological year considered is equal to 0.32 with a p value of p ) 0.488. Data shown in Table 2 also indicate that the value of ratio between the Pb values measured in scales and sediments is approximately 1 for the northwest sites and greater than 1 in the southeast sites. This discrepancy can be interpreted by considering that sediments in the sites of northwest Sicily are richer of Si compounds which derive from detrital materials and act as a trap for metal ions which cannot be easily remobilized (34). Consequently metal ions, in these sites rich of Si compounds, are not fully bio-available. In other words, the difference in the ratio between Pb in scales and Pb in sediments across the sites might be explained by the fact that P. oceanica reflects the bio-available lead concentration in the environment rather than total lead concentration. It is worth recalling that the sensitiveness to bio-available pollutants is an important requirement for a biomonitor (35). The above analysis shows that the putative role of Posidonia oceanica as a biological monitor device able to capture the variability of environmental Pb must be associated only to the scales of the sea-plant. To further test the reliability of Posidonia oceanica as a bio-indicator of Pb pollution, all data shown in Figures 3 and 4 have been grouped according to different features, such as automotive traffic, industrialization, and urbanization conditions of the sites and lithology. We have therefore obtained four macrosites, each characterized by homogeneity of the above features. The north (N) macrosite includes the Carini site only, which is located very close to the city of Palermo and next to an heavily used motorway connecting Palermo and Trapani. The west (W) macrosite includes the Trapani and Marsala sites, characterized by intermediate levels of 3010

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FIGURE 7. Changes over time in Pb concentration measured in the scales of Posidonia oceanica in the macrosites of northern Sicily (a), western Sicily (b), and southeastern Sicily (c). In all panels is also shown the estimated Pb emission due to gasoline combustion in the Provinces where the macrosites are located, see Figure 2. urbanization and automotive traffic conditions. The southeast 1 (SE1) macrosite includes the Marzamemi and Cape Passero sites, characterized by poor levels of urbanization and automotive traffic conditions. Finally, the southeast 2 (SE2) macrosite includes the highly urbanized site of Ognina Bay and Cape Negro. Indeed, as mentioned above, Ognina Bay is located next to the chemical industrial site of Augusta, see Figure 2, while Cape Negro is located a few miles south of Ognina Bay and shows lower levels of pollution with respect to it. Despite these differences, these two sites were aggregated together due to the very similar lithological features (see Table 2). The aggregated time series for these four macrosites are shown in Figure 7. In the same figure, the Pb emissions in air calculated for the four macrosites are reported. Interestingly, the Pb concentrations for the W (Figure 7, panel b) and SE2 macrosites (Figure 7, panel c) are very close to each other, and the Pb concentration in the N macrosite (Figure 7, panel a) is much higher than the Pb concentration of the others. Moreover, the SE1 macrosite (Figure 7, panel c) shows the lowest levels of Pb.

For the four macrosites considered, the correlation coefficients between Pb concentration in the scales and Pb emissions in air are shown in Table 3. All macrosites except SE1 show a statistically significant level of correlation. The N and W macrosites show very high levels of correlation, possibly explained with the fact that these areas are highly populated, with heavy traffic conditions and with relatively high levels of industrialization. This is also partly true for the SE2 macrosite that indeed shows a high level of correlation. In fact, the SE2 macrosite includes the highly polluted site of Ognina Bay and the Cape Negro site, characterized by lower anthropogenic inputs. The existence of these two levels of pollution in SE2 implies that in the temporal period considered (1988-2001) the level of correlation is smaller than the one found for Ognina Bay only. The SE1 macrosite shows a level of correlation that is not statistically significant, according to the fact that such macrosite is characterized by low levels of urbanization and of automotive traffic conditions. In summary, different from the investigation of the Hg accumulation in Posidonia oceanica (10), we find that in each site Pb accumulates in scales rather than rhizomes. Such behavior is independent from the levels of pollution of the sites. Moreover, we notice that the Pb concentration measured in the rhizomes of Posidonia oceanica are approximately constant in the entire lepidochronological period considered (see Figure 5 and Figure 6) and no time dependence on the pollution level of the site can be estimated. On the contrary, Pb concentration in scales reflects the different level of contamination of the sites. In fact, in all the sites where the process of phasing out of lead from gasoline was expected to be effective, as in sites near cities and industrial zones, the Pb concentration in the scales shows a marked temporal trend correlated with lead emission in air. All these findings and the endemic diffusion of Posidonia oceanica in Mediterranean basin make the scales of this marine phanerogam a good biological monitoring device for the study of Pb pollution in Mediterranean coastal marine environment. The fact that Pb concentrations are retained in the scales for a long time, up to at least 20 years, makes it possible to evaluate the variations in Pb concentration over a long time period with only one sampling.

Acknowledgments The authors are grateful to Prof. G. Rizzo and Dr. F. D’Agostino for assistance with AAS. The work was supported by the CIPEMIUR Grant, Cluster 10, Ambiente Marino, Italy.

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Received for review July 20, 2004. Revised manuscript received December 23, 2004. Accepted January 12, 2005. ES048870U