Page 7 - TraceMetals_seagrass_algae

L. Campanella et al. / Environmental Pollution 111 (2001) 117±126 123
The mean metal concentrations in biota and seawater
are shown in Table 8, where only data from the four
clean sites are averaged. From these data it results that
accumulation ratios of the ®ve metals di€er both in
macrophytes and in molluscs. In P. oceanica metal
concentrations decrease in the following order:
Zn>Cu>Cd>Pb>Cr while in P. pavonica the
sequence is Zn>Cu>Pb>Cr>Cd. As for molluscs,
the following sequences were observed: Zn>Cu>Cd>
Cr5Pb in M. turbinata; Zn>Cd>Cu>Cr>Pb in P.
caerulea.
As far as net accumulation is concerned, P. caerulea
turned out to be the strongest accumulator for Cd, P.
pavonica for Cr and Pb, M. turbinata, P. oceanica and P.
pavonica for Cu, P. oceanica for Zn. Very high con-
centration factors (1.0Â104Ä 3.4Â104) can be calculated
in these cases with reference to soluble metal concentra-
tions in ambient seawater. Anyway it must be kept in
mind that in the studied organisms the actual uptake
mechanisms are probably rather complicated because
their exposure to trace elements is not limited to soluble dies are needed to fully elucidate this point.
metals in the aquatic medium. The metal uptake from
food, in the case of molluscs, and from sediments
(through the roots) in the case of seagrasses, cannot be
ignored. As both mollusc species here considered are
herbivorous, it can be supposed that the metals accumu-
lated in the algae on which they graze fairly a€ect their
overall uptake.
4. Discussion
The picture of bioavailable metal loads in the di€erent
sites of the experimental area provided by the four
selected organisms is rather univocal. An overall trend
of increased metal concentrations in station 5 is clear,
especially for Cd, Pb and Zn. Moreover, many correla-
tions between metal contents were observed in the four
species.
The results of the study on metal distribution in the
tissues of Posidonia oceanica are particularly mean-
ingful. In fact the di€erent behaviour of the metal cou-
ple Cd±Zn on the one hand, and Cr±Pb on the other,
suggests di€erent uptake and distribution routes for
these elements. The highest concentration of Cd and Zn
in leaves versus rhizomes leads to the hypothesis of a
preferential uptake of these elements from water to the
photosynthetic tissue. Warnau et al. (1996) reported
that Cd and Zn were linearly accumulated from ambient
water by seagrass shoots during a 15-day experiment.
However, in a ®eld study on a contaminated area,
Malea et al. (1994) described a signi®cant positive cor-
relation between Cd concentrations in leaves and those
in sediments. Therefore, an ecient distribution from
roots to green tissues cannot be excluded. Anyway,
the preferential accumulation of Cd and Zn in leaves
rather than in rhizomes is a well-established pattern,
our results matching those of many previous studies
(Sanchiz et al., 1990; Warnau et al., 1995; Schlacher-
Hoenlinger and Schlacher, 1998a).
In the present study, the opposite behaviour was
observed in the case of Cr and Pb. This ®nding matches
those of Warnau et al. (1995) and Schlacher-Hoenlinger
and Schlacher (1998a). However, Sanchiz et al. (1990)
described the sequence old leaves>rhizomes>young
leaves, while Catsiki and Panayotidis (1993) reported
somewhat erratic results, with inversions of the accu-
mulation ratio between leaves and rhizomes depending
on the sampling area.
The almost constant Cu content observed in all plant
tissues (as in Warnau et al., 1995), can be supposed to
arise from a certain regulation capability of the phaner-
ogam. Again, con¯icting results can be found in the lit-
erature (Catsiki and Panayotidis, 1993; Schlacher-
Hoenlinger and Schlacher, 1998a), so that further stu-
In leaf material Cd, Cr, Pb and Zn concentrations
decrease in the following order: leaf tip>whole leaf
>leaf basal tissue. This trend is consistent with the
observation that the seagrass leaves grow by formation
of new tissue at the base. Therefore, the tip becomes the
older part of the leaf, hence the tissue with a longer
exposure to metal loads in the ambient habitat.
As regards absolute metal contents, levels recorded in
this study at clean stations, generally fall in the range of
the lowest values available in the literature.
In detail, our data concerning the leaves of Posidonia
oceanica are well below those previously reported in the
case of Cr and Pb, while for Cu and Zn the results we
obtained are only slightly lower than the mean values
described elsewhere (Table 9). As for Cd, our mean
level is lower than those previously reported, with the
exception of the surprisingly small one observed by
Schlacher-Hoenlinger and Schlacher (1998a) in speci-
mens collected at Ischia (Bay of Naples, Italy). In Padina
pavonica the observed levels of Cd and Cr are generally
lower than those previously reported, while Pb, Cu and
Zn contents are higher (Catsiki et al., 1991; Tariq et al.,
1991; Schlacher-Hoenlinger and Schlacher, 1998b).
Very few data are currently available on trace metals
concentrations in the gastropod molluscs selected in the
present study. However, our values, except for Cd, are
lower than those reported by other authors (Ozretic et
al., 1990; Nicolaidou, 1994).
In the light of the foregoing remarks, the metal con-
centrations in the organisms collected at the `clean' sta-
tions may be considered as useful background levels to
which to refer for intraspeci®c comparison within the
Mediterranean area. It must be noted that in the pres-
ent context the notion of `background levels' has a
somewhat relative meaning, as local geochemical or
hydrodynamic conditions may always shift `natural'