Pearce and Mann 2006

Trace metal variations in the shells of Ensis siliqua record pollution
and environmental conditions in the sea to the west
of mainland Britain
Nicholas J.G. Pearce *, Victoria L. Mann
Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, SY23 3DB, Wales, UK
Abstract
Shells of the pod razor shell (Ensis siliqua) from 13 locations around the west coast of mainland Britain have been analysed by laser
ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for a range of trace metals including Zn, Cd, Pb, U, Ba, Sr and
Mg. The trace metal record in these shells is a proxy record for changes in seawater chemistry during the 1990s. Regional variations exist
in the median concentrations of the analysed metals. Barium concentrations are related to increased productivity from sewage sludge
dumping at sea. Strontium shows a local relationship to salinity, but there is no clear relationship over the study area, instead high
Sr is often associated with high Ba, and may reflect ontogenetic factors such as growth rate. Magnesium shows a seasonal variation
within individual shells and can be used to calculate sea surface temperatures from groups of shells. Contaminant metals show a clear
regional relationship with known sources, thus high Pb and Zn are typically associated with former metal mining areas (e.g. Cardigan
Bay, Anglesey), and high Pb, Zn, Cd and U are associated with industrial activity in Liverpool Bay. Anomalies such as the high U in
shells from northern Scotland cannot at present be explained. A seasonal variation of Pb is also seen in Cardigan Bay and Liverpool Bay,
relating to increased winter fluxes of these metals to the marine environment. The regional distribution of these metals is consistent with
known sources of contamination and patterns of seawater migration around the coast of Britain.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Ensis siliqua; Shell chemistry; Trace elements; Environmental change; Laser ablation; ICP-MS; Contamination
1. Introduction
The hard parts of aquatic organisms have the potential
to record in their shells changes in the environmental con-
ditions in which the organism lived. In bivalves, the hard,
carbonate shell may survive for many years after the death
of the organism, and thus, unlike the soft parts which will
decay shortly after death, has the potential to be used as a
long-term record of aquatic conditions (Dodd, 1965; Lutz
and Rhoads, 1980). Bulk shell analysis provides a record
for the lifetime of the organism, but the advent of analyti-
cal methods with high spatial resolution and low limits of
detection, such as laser ablation inductively coupled
plasma mass spectrometry (LA-ICP-MS), has meant that
the temporal record stored within the shells can be deter-
mined. This methodology has been applied to determine
a range of contaminant metals (e.g. Zn, Cd, Pb, U) and
environmental indicator elements (Mg, Sr and Ba reflecting
temperature, salinity and productivity respectively) in a
range of bivalve species by many authors including Fuge
et al. (1993a), Raith et al. (1996), Stecher et al. (1996),
Schettler and Pearce (1996), Price and Pearce (1998), Van-
der Putten et al. (1999, 2000) and Toland et al. (2000).
Ensis siliqua, the pod razor shell, is widespread around
the coast of the British Isles and northern Atlantic area,
where in places it is exploited commercially (Fahy and
Gaffney, 2001). E. siliqua lives at or below the Spring tide
low water mark and can burrow rapidly into the fine sands
0025-326X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.marpolbul.2005.11.003
*
Corresponding author.
E-mail address: Nick.Pearce@aber.ac.uk (N.J.G. Pearce).
www.elsevier.com/locate/marpolbul
Marine Pollution Bulletin 52 (2006) 739–755

in which it lives, a feature that makes live individuals diffi-
cult to collect. E. siliqua grows to a maximum length of
about 20 cm, living for between 10 and 19 years (Alexan-
der, 1979; Fahy and Gaffney, 2001; Fish and Fish, 1996;
Henderson and Richardson, 1994). There is only a small
amount of literature on E. siliqua, with the ecology of
E. siliqua described by Holme (1954), and the most recent
research concentrating on growth rates of the shell, which
can be up to about 90 lm per day (Henderson, 1993; Hen-
derson and Richardson, 1994), damage to shells during
dredging (Gaspar et al., 1994b) and population/reproduc-
tive dynamics (Gaspar and Monteiro, 1998; Gaspar et al.,
1994a) or its immunity to toxic events (Wootton et al.,
2003).
The relatively rapid growth of E. siliqua (which provides
a good temporal resolution), and the clear growth structure
that it displays, make E. siliqua a potentially useful bio-
monitor. Here the analysis of shells of E. siliqua for both
contaminant and environmental indicator metals from a
range of sites around the west coast of the British Isles is
described.
2. Sampling and analytical methods
E. siliqua is difficult to find live, living close to or below
the spring low water mark (Alexander, 1979). Despite sev-
eral attempts, no live samples were recovered, and instead
at each site, intact, articulated shells, with a complete peri-
ostracum were collected. In all cases sample sites were
broad sandy beaches. These relatively fragile shell are unli-
kely to be transported great distances without suffering
damage, and can be assumed to be derived locally, and
to have died within the previous year or two. Samples were
collected during the summer of 1999 from 12 sites around
the western coast of the Britain (see Fig. 2). Sample sites
were selected to encompass areas of known contamination
(e.g. the disused Pb–Zn mines of Mid Wales, the Mersey
Estuary) and areas suspected to be relatively free from
anthropogenic effects (e.g. northwest Scotland).
One valve from each sample was cut using a diamond
saw into sections $4 cm long, showing a series of well
spaced growth checks (see Figs. 1 and 3). These growth
checks represent the annual (winter) slowing in growth of
the organism which results in a thickening of the shell
(Gaspar et al., 1994a) and the section of shell chosen for
analysis typically included 3 or 4 growth checks (thus 3
or 4 years) during the middle of the life cycle of the organ-
ism, i.e. from its 3rd or 4th year to its 7th or 8th year of
growth. Typically, then analyses represent growth which
occurred in the shell during the mid-1990s, from about
1992/3 to 1996/7. In some cases, in larger shells, material
from later in the life of the organism was analysed, but this
still represented growth during the mid-1990s. This
approach gave the best compromise between the time
required for analysis and temporal resolution on the shell.
Fig. 3 also shows the internal structure of one valve
sketched from an acetate peel, which shows clear growth
bands that are asymptotic to the inner wall of the shell,
and that intersect the outer surface at a steep angle.
Prior to analysis samples were immersed in 50 volumes
H2O2 and left until all visible traces of the periostracum
had been removed. The periostracum has been shown to
contain elevated levels of trace metals compared to theFig. 1. Morphology of Ensis siliqua, the pod razor shell.
N o r t h
S e a
IrishSea E n g l i s h C h a n n e l
250 km
Harrapool
Lower
Breakish
Ravenglass
Wallasey
Borth
Benllech
Dulas Bay
Porthmadog
Mochras
Barmouth
Freathy
Pendower
Liverpool
Bay
Bristol Channel
Cardigan
Bay
Cornwall
Wales
Scotland
AtlanticOcean
Ireland
NorthChannel
StGeorge’s
Channel
Fig. 2. Location map of samples collected for this study. The following
locations are local names, unrecorded in many atlases: Freathy is a small
hamlet on Whitesands Bay, 5 km southwest of Plymouth and the Tamar
estuary. Pendower is a beach 1.5 km north of the town of Portscatho,
Cornwall and 8 km northeast of the Fal estuary. Harrapool is a small
village 8 km west south west of Tarbert on South Harris. Lower Breakish
is a small village 4 km east of Broadford on the Isle of Skye. Dulas Bay is
an estuary receiving drainage from Parys Mountain, and drains to the sea
between Amlwch and Moelfre on the northeast coast of Anglesey.
Mochras is a small tidal ‘‘island’’, also known as Shell Island, 4 km south
south east of Harlech.
740 N.J.G. Pearce, V.L. Mann / Marine Pollution Bulletin 52 (2006) 739–755

adjacent carbonate (Dermott and Lum, 1986) and thus, if
not removed, may have affected the analyses of the shell.
Analyses were performed using a VG PlasmaQuad
II + ICP-MS with a frequency quadrupled Nd:YAG Spec-
tron Systems LaserLab laser ablation accessory, modified
to operate at 266 nm (ultra violet). The laser was operated
at an incident power of approximately 0.75 mJ at a repeti-
tion rate of 5 Hz. This gave ablation craters of about 25 lm
in diameter and about 25 lm in depth. Routinely, analyses
were taken approximately every 400 lm along a traverse,
covering 3 or 4 years growth (see Fig. 3) giving between
40 and 60 analyses per shell. The shallow nature of the
ablation craters means that material of predominantly
the same age as the material at the surface will be sampled,
and the contribution to the signal of considerably younger
material will be minimal. Spectra were acquired for 25 s in
‘‘peak jumping’’ mode, with the spectrometer being
stepped from one analyte peak to the next. The following
elements (with the analyte isotope) were determined:
25
Mg, 44
Ca, 64
Zn, 88
Sr, 114
Cd, 138
Ba, 208
Pb and 238
U.
55
Mn and 65
Cu were included in the analytical scans, but
high blanks which drifted during the analyses at these
masses make the data for these elements unreliable. Cali-
bration of analyses was achieved using the National Insti-
tute of Standards and Technology (NIST) Standard
Reference Material (SRM) 610, a synthetic soda-lime sili-
cate glass, spiked with approximately 60 trace elements,
using concentrations given in Pearce et al. (1997). All anal-
yses are blank subtracted (using the instrumental gas blank
to remove atmospheric interferences and polyatomic spe-
cies) and drift corrected based on repeat analyses of the cal-
ibration standards at the start and end of each run, which
would last no longer than 2.5 h. 44
Ca was used as the inter-
nal standard for the analyses, assuming a concentration in
the shells of 55.5% CaO (see Fuge et al., 1993a). The ele-
mental concentrations are thus, in essence, a ratio to the
fixed CaO concentration (55.5 wt.%) in the shell. Instru-
mentation and methodology is described fully in Perkins
and Pearce (1995) and Price and Pearce (1998). Analytical
accuracy and precision are both around ±10% for elements
at concentrations of $1 mg/kg and above (Perkins et al.,
1997). Typical detection limits are in the range 0.02–
0.01 mg/kg, although these vary with instrumental condi-
tions on a day-to-day basis.
3. Data presentation
Analyses along the axis of growth of shells from E. sili-
qua represent time series analyses that have the potential to
record changes in the environment in which the shell lived.
Time series such as these may include a short-term varia-
tion (noise) superimposed on longer-term change, this rep-
resenting the wider, environmental change (Davis, 1986;
Swan and Sandilands, 1995). In addition, analytical factors
(e.g. the ±10% precision typical of LA-ICP-MS) will also
introduce random noise to the data. To reduce the effects
of noise in time series analyses, and to enable the underly-
ing signal to be seen more clearly it is common to
‘‘smooth’’ the data to provide an estimate of the value of
a centre point, by taking a weighted average of that point
and points either side. The methodology for this is
described by Davis (1986) and by Swan and Sandilands
(1995) and was derived by Savitzky and Golay (1964) by
fitting polynomial curves to short sections of time series
data to derive an estimate for the central point. Thus, the
‘‘smoothed’’ value (Xsm) for any point can be calculated
by weighting the average for the central points and the 2
points either side (‘‘5-point smoothing’’) using the follow-
ing equation:
Xsm ¼ ½17Xi þ 12ðXiþ1 þ XiÀ1Þ À 3ðXiþ2 þ XiÀ2ÞŠ=35
(Davis, 1986; Swan and Sandilands, 1995) where Xi is the
central point, XiÀn and Xi+n are the analyses n places before
or after the central point. The effects of this process are
Fig. 3. Upper diagram shows the typical position of an analytical traverse
along the outer surface of a cut section of one valve. Lower diagram shows
the internal structure of the shell sketched from an acetate peel of a
longitudinal section at the posterior of one valve.
Mg,mg/kg
Analysis number along traverse
Raw data
Smoothed data (5-point)
Fig. 4. Magnesium concentration in mg/kg in a shell from Borth. Raw
data (dashed) shows a spiky, noisy profile, which reveals a clearer cyclicity
when the data is smoothed (solid). See text for methodology.
N.J.G. Pearce, V.L. Mann / Marine Pollution Bulletin 52 (2006) 739–755 741

illustrated in Fig. 4, where Mg analyses are presented raw
and 5-point smoothed for a shell from Borth. The smooth-
ing clearly reduces the noise in the signal and makes the
longer wavelength cyclicity within the data clearer, In addi-
tion, this 5-point smoothing for most of the peaks, with the
exception of a few clearly very high spikes, does not reduce
the maximum values by more than a few percent (which is
less than analytical precision). Smoothing to progressively
more points causes a gradual reduction in the magnitude
of the cycles, although the patterns remain similar. 5-point
smoothing is thus the most appropriate to reveal cyclicity
at the spatial resolutions used in this study without causing
significant modification to the smoothed, central concen-
tration (Xsm). Care however must be exercised in interpret-
ing these smoothed patterns, a series of what may appear
to be random ‘‘spikes’’ in a signal (for example increased
concentrations due to a short-lived pollution event repre-
sented in only one analysis) will become spread out when
smoothed, and the smoothed data may give a false impres-
sion of cyclicity.
4. Results and discussion
The following section presents and discusses the concen-
trations of those elements which may reflect environmental
factors including temperature, salinity and productivity
(i.e. Mg, Sr and Ba) and those which may be associated
with anthropogenic activity such as metal mining or indus-
trial processes (i.e. Zn, Cd, Pb and U) from 23 individual
samples of E. siliqua collected from the sites in Fig. 2.
The discussion is divided into the regional variation in
median metal concentrations, and the seasonal signal
recorded within individual shells. Table 1 summarises the
mean summer and winter salinity and temperature data
for each sample site based on data presented by Lee and
Ramster (1981) and Dickson (1987). Fig. 6 shows the win-
ter and summer salinity in the coastal waters to the west of
the United Kingdom.
Table 1
Mean winter and summer salinity and temperature based on data compiled by Lee and Ramster (1981), and temperature data for sites monitored by
CEFAS (Norris, 2001)
Salinity (&) Mean sea surface
temperature, °C,
interpolated from Lee and
Ramster (1981)
Mean sea surface
temperature at sites near
sampling sites, °C, from
Norris (2001)
Summer Winter Summer Winter Summer Winter
Harrapool 34.75 34.5 13.25 7
Lower Breakish 34.5 34.25 13.25 6.5
Ravenglass 32 31 15.75 4.75 Heysham 16.8 3.9
Wallasey 31 30 16.25 4.75
Benllech 32 33 16.25 6.5 Moelfre 15.8 5.8
Dulas Bay 33 33 16 6.75 Amlwch 15.3 6.6
Porthmadog 34.25 34 15 7.25
Mochras 34.25 34 15 7.25
Barmouth 34.25 34 15 7.25
Borth 34.25 34 15 7.25 Angle 16.2 7.7
Pendower 34.75 34.75 16 8.75 Newlyn 14.5 8.1
Freathy 34.75 34.75 16 8.75 Plymouth 16.0 7.9
Salinity is given as the value of the next contour concentration below the point at which the samples were collected, i.e. the summer salinity for Harrapool
is between 34.75& and 35&, and thus the lower value of 34.75& has been tabulated. Temperature has been estimated by interpolation between the 0.5 °C
isotherms given.
Sr Mg Ba
1435 123 9.94
Sr Mg Ba
2187 141 7.65
1811 72.2 5.34
Sr Mg Ba
2315 73.8 8.91
2922 37.9 7.72
2454 84.2 7.64
Sr Mg Ba
1809 59.9 2.44
1563 60.0 4.10
2314 16.1 3.95
1955 77.4 3.40
Sr Mg Ba
1239 67.7 2.87
Sr Mg Ba
1329 55.1 5.06Sr Mg Ba
1611 68.8 1.94
1998 72.7 3.36
Sr Mg Ba
1724 21.8 2.37
1973 69.2 2.96
Sr Mg Ba
1437 64.7 2.36
1496 61.2 1.69Sr Mg Ba
1999 56.6 3.12
2280 79.1 4.40
Sr Mg Ba
2773 34.6 8.11
Fig. 5. Median concentration of Sr, Mg and Ba (mg/kg) in the shells of
Ensis siliqua.

4.1. Regional variations
Fig. 5 shows the median concentrations of Sr, Mg and
Ba in mg/kg from each shell analysed. Median Sr concen-
trations are lowest in samples from along the North Wales
coast (at around 1300 mg/kg) and highest in samples from
Lower Breakish and Ravenglass (up to about 2900 mg/kg).
Strontium concentrations from Harrapool, Cardigan Bay
and the south west coast are generally similar ranging
between about 1500 and 2100 mg/kg, and thus there is no
clear, large scale, regional pattern to the median Sr concen-
trations in these shells. However, within Cardigan Bay
there is a gradual decrease in the median Sr concentration
northwards, alongside a decrease in salinity in the same
direction (Bowden, 1980) and a similar relationship is
observed between the Pendower and Freathy samples (see
Fig. 6). The decrease in salinity between Pendower and
Freathy, and between Borth and Porthmadog is small
(<0.25&) but in both cases there is a change in the median
Sr concentration of about 25%. It has been suggested that
variations in the Sr concentrations in bivalves reflect a
range of factors, including variations in salinity, tempera-
ture and metabolic controls within the organism including
a relationship between Sr incorporation and growth rate
(Brand and Morrison, 1987; Klein et al., 1996b; Rosenberg
and Hughes, 1991; Stecher et al., 1996).While salinity may
thus affect the Sr content of the shells in areas such as
Cardigan Bay, the relationship appears to be on a local
scale and other factors may be more important in control-
ling Sr uptake.
Magnesium concentrations show a similar order of mag-
nitude variation to Sr, with median Mg concentrations
ranging from 16.1 to 141 mg/kg across the study area.
The lowest Mg is recorded in shells from Harrapool, and
individual shells from Ravenglass (median Mg = 37.9 mg/
kg), Barmouth (median Mg = 21.8 mg/kg) and one shell
from Borth (median Mg = 16.1 mg/kg). These low Mg
shells are, in general, the larger shells, although similar
sized shells from other sites show typical median Mg con-
centrations. With the exception of the Barmouth shell, this
lower Mg is typically accompanied by higher Sr, and may
reflect ontogenetic changes related to growth rate reflected
in Sr content (see below) and the age of the organism. Mag-
nesium concentrations in bivalve shells show a strong asso-
ciation with temperature (Brand and Morrison, 1987; Fuge
et al., 1993a; Klein et al., 1996a; Vander Putten et al., 2000)
although other factors (salinity, growth rate etc) may con-
found this (Zolotarev, 1974). In the E. siliqua samples ana-
lysed here, there is no clear relationship between sea surface
temperature (SST) and median Mg concentration in the
shells, and on a local scale, no clear relationships can be
discerned.
Median Ba concentrations in the shells show a similar
pattern of distribution to Sr, although at much lower
35
34.7534.534.25
34
34.25
34.5
34.75 35
35
35.25
35
34.75
34.5
34.5
35
33
34
33
32
34.25
34.5
34.75
35
35.25
35
35.25
34
35
34.75
34.534.25
35
34.75
34.25
34.5
34
33
32
31
34
Summer salinity Winter salinity
Fig. 6. Summer and winter sea water salinity to the west of the United Kingdom. Contours of salinity in &, based on data from Lee and Ramster (1981).

concentrations, and on a regional scale there is a good cor-
relation between Ba and Sr. Relatively high median Ba
concentrations (>5 mg/kg) are found in shells from Scot-
land and Ravenglass, in Wallasey, near the estuary of the
River Mersey, which drains the large industrialised areas
around Manchester and Liverpool, and at Dulas Bay on
Anglesey, at the mouth of the Afon Goch which drains
the disused metal mine of Parys Mountain (Whiteley and
Pearce, 2003). The highest Ba concentration, from a shell
from Dulas Bay, is probably related to contamination from
the mining activity. Barium concentrations in samples from
Ravenglass, Wallasey and Scotland show a strong linear
relationship with Sr (r = 0.862) with a Sr/Ba ratio of about
300 (see Fig. 7), lower than the average ratio in seawater of
$400 (Martin and Whitfield, 1983).
Lower concentrations of Ba, between 1.5 and 4 mg/kg
are recorded for samples from Cardigan Bay and the coast
of Cornwall, and as with Sr these decrease in the direction
of decreasing salinity. In these locations there is also good
correlation between Ba and Sr (r = 0.737) with a Sr/Ba
ratio of about 650 (see Fig. 7), greater than typical
seawater.
Barium concentrations in ocean waters have been
related to primary productivity. Barium in molluscs has
been correlated with higher production and it has been sug-
gested that high Ba/Ca ratios may reflect influxes of baryte
to the sea floor from phytoplankton blooms at the surface
(Bishop, 1988; Chan et al., 1977; Stecher et al., 1996;
Stroobants et al., 1991; Vander Putten et al., 2000; Zwols-
man and van Eck, 1999). In turn, along the west coast of
mainland Britain this may be related to sewage inputs
which would have added nutrients to coastal waters. Much
of Ireland’s sewage is discharged into the Irish Sea, and sig-
nificant inputs also occur at St David’s, (SW Wales), the
River Mersey and at Haverigg, just to the south of Raven-
glass. In addition, prior to 1999, dumping of sewage sludge
occurred in the Irish Sea with the largest input at Liverpool
Bay, and smaller amounts from the Bristol Channel and
Belfast Lough (N Ireland). These inputs would have had
the largest effect on the northern Irish Sea (Irish Sea Study
Group, 1990; Lee and Ramster, 1981). In addition, the
Mersey estuary provides a substantial quantity of the nutri-
ents N and P to the northern Irish Sea (Foster, 1984b).
Added to these nutrient fluxes, as Irish Sea water passed
through the North Channel, would have been sewage
inputs from the Firth of Clyde. This included a substantial
tonnage of sewage sludge dumped at Garroch Head up to
the end of 1999, when dumping was stopped in UK waters,
although the amount of this which escaped the dumping
site may only have been relatively small (Moore, 2003).
Thus sewage inputs, particularly where there was a high
proportion of raw sewage in the effluent, and sewage sludge
dumping provided a significant nutrient source into the
northern half of the Irish Sea. The relationship observed
between Ba and Sr may thus reflect increasing productivity
(high Ba) which in turn leads to more rapid growth of Ensis
and thus a greater incorporation of Sr into the shell (Chan
et al., 1977; Stecher et al., 1996) and this compares with the
behaviour of these elements in other molluscs (cf. Iglesias
and Navarro, 1991).
Circulation in the Irish Sea is important in the dispersal
of pollution inputs but is not straightforward (see Fig. 8)
(Irish Sea Study Group, 1990; Lee and Ramster, 1981;
Ramster and Hill, 1969). The general, long-term flow of
water along the western coast of mainland Britain is north-
wards, with Atlantic water entering the Irish Sea through
St George’s Channel and exiting through the North Chan-
nel where it forms the foundation of the Scottish Coastal
Current (Irish Sea Study Group, 1990). Marine inputs from
the Bristol Channel and the south/mid Wales coast will be
the only likely contaminants in the southern Irish Sea, but
material from Ireland, south western Scotland and Liver-
pool Bay will pass generally north through the Irish Sea
and eventually flow out along the western Scottish coast.
Circulation along the north western coast of England is
particularly complex, with material being effectively
trapped by slow moving currents which make the eastern
Irish Sea and Liverpool Bay a backwater with long resi-
dence times. It thus seems probable that sewage inputs into
the northern half of the Irish Sea caused increased produc-
tivity, this in turn leading to elevated Ba concentrations in
shells from this area. These nutrient-rich waters will even-
tually pass out through the North Channel and along the
Scottish coast, again increasing productivity and poten-
tially leading to high Ba in the shells of E. siliqua along
the Scottish coast. This is analogous to the migration of
the radionuclide 137
Cs, discharged into the Irish Sea at Sel-
lafield, Cumbria, and which is transported around the
Scottish coast into the North Sea (Hunt, 1979; Irish Sea
Study Group, 1990). The strong correlation of Ba and Sr
suggests that the Sr concentration of the shells is also in
part related to nutrient availability, and not directly to
salinity, although generally good, inverse correlations are
described for salinity/nutrient relationships (Foster,
Coast of Cornwall
Cardigan Bay
Liverpool Bay, Ravenglass
and Scotland
Sr mg/kg
Bamg/kg
0
1
2
3
4
5
6
7
8
9
10
1000 1500 2000 2500 3000
Fig. 7. Median concentrations of Sr v. Ba in shells of Ensis siliqua from
different geographical regions. ‘‘Liverpool Bay’’ includes Wallasey and
Benllech. Dotted line marks the average seawater Sr/Ba ratio of 400
(Martin and Whitfield, 1983).

1984a,b), and thus several interrelated factors may be con-
trolling both the uptake of Sr and Ba.
Fig. 9 presents the median concentrations of Zn, Cd, Pb
and U from each analysed shell. The highest concentra-
tions recorded for each of these elements are presented in
Table 2.
Cadmium is present at concentrations close to or below
the lower limit of detection in most shells, with the excep-
tion of those from Anglesey (Dulas Bay and Benllech) and
Wallasey. The presence of a median Cd concentration of
50 mg/kg in the shells from Dulas Bay can be directly
attributed to the drainage from the Parys Mountain mine.
Zinc in the Dulas Bay sample is also the highest median
concentration at 69.5 mg/kg, but is only slightly higher
than Cd, and gives a median Zn/Cd ratio of $1.4. This is
considerably lower than the Zn/Cd ratio of Irish Sea water
of $50 (Lee and Ramster, 1981) or of waters draining into
the Irish Sea from former mining areas with Zn/Cd
between 350 and 900 (Abdullah and Royle, 1972; Fuge
et al., 1993b,c; White, 2000) or from industrialised areas
and sewage sludge with Zn/Cd in the range 100–300 (Dick-
son, 1987). In those shells which grew in an area where Cd
influxes, and thus marine concentrations, are high (viz. Liv-
erpool Bay), the Zn/Cd ratios in shells is low (<3), this
despite the seawater Zn concentrations also being high in
these areas (Irish Sea Study Group, 1990; Lee and Ram-
ster, 1981). Clearly, in these high Cd environments, E. sili-
qua preferentially concentrates and partitions Cd into the
shell over Zn, and this must reflect a metabolic process in
the shell. The high organic matter content of the waters
in Liverpool Bay, resulting from sewage dumping etc.,
may also be a factor in the uptake of Cd (cf. Ba).
Median Zn concentrations show a general increase
northwards from Cornwall through Cardigan Bay, to gen-
erally high concentrations in Liverpool Bay, the highest
being recorded from a shell from Dulas Bay, at the mouth
of the river draining Parys Mountain. Concentrations
decrease in the samples from Scotland to levels somewhere
between the Cornish and Cardigan Bay samples, and this
may reflect transport of contaminant Zn from the Irish
Sea northwards around the coast of the British Isles (cf.
Ba). There is a gradual increase in median Zn contents of
Ensis shells through Cardigan Bay, and this correlates with
river inputs draining the mid-Wales and north Wales ore
fields which discharge at Borth, Barmouth and Porthma-
dog (Abdullah and Royle, 1972; Fuge et al., 1993c; Pearce,
1992). A similar pattern of metal distribution in Mytilus
edulis and Patella sp. shells was recorded by Fuge et al.
(1993a).
Median Pb concentrations in Ensis shells, with the
exception of some shells from Cardigan Bay and all exam-
ples from Liverpool Bay, are in the 1–2 mg/kg range. In
Cardigan Bay, shells from Borth have 3.2–6.5 mg/kg Pb
and this reflects Pb-rich mine water inputs draining the
Mid Wales ore field and entering Cardigan Bay via the
Rheidol, Ystwyth and Dyfi rivers (Davies, 1987). Slightly
higher Pb concentrations are recorded from Mochras and
Porthmadog, again sites influenced by metal mine drain-
age, and the drift of material northwards through Cardigan
Bay (Abdullah et al., 1972). Once again, the metal mine
drainage from Parys Mountain produces the highest
median Pb concentration at Dulas Bay, and industrial
and sewage discharges from the Mersey and in Liverpool
Bay cause elevated Pb concentrations in the shells from
Wallasey, Benllech and Ravenglass. Unlike Zn and Cd,
which remain relatively soluble in sea water as Zn2+
and
Cd2+
, dissolved Pb forms insoluble carbonates and sul-
phates on entering the marine environment and will not
be transported great distances except as a particulate mate-
rial. Thus elevated Pb concentrations in the shells of filter
feeders such as E. siliqua may be expected proximal to
sources of contamination, but dissolved Pb attenuates rap-
idly, and thus samples from Scotland record essentially a
background Pb concentration of $2 mg/kg, similar to
shells from Cornwall.
Median U concentrations in Ensis shells are generally
low (<0.1 mg/kg) from Cardigan Bay and Cornwall, and
indicate no sources of U, other than background sea-water,
in these areas. In Liverpool Bay, shell U concentrations
Summer
Winter
Tidal circulation from
Irish Sea Study
Group (1990)
Tidal circulation from
Lee and Ramster
(1981)
Fig. 8. Tidal circulation along the western margin of mainland Britain
compiled from published sources (Irish Sea Study Group, 1990; Lee and
Ramster, 1981; Ramster and Hill, 1969).

range 0.2–0.5 mg/kg, except for the Dulas Bay samples
which has the highest recorded median U concentration
at 2.23 mg/kg, and is probably a result of acid mine waters
leaching U from the black shales of the Parys Mountain
area (Fuge et al., 1993b; White, 2000). Elsewhere in Liver-
pool Bay, high U probably results from industrial
discharges circulating around the Bay, including a phos-
phate processing plant at Whitehaven, on the Lake District
coast (Dickson, 1987; Price and Pearce, 1998), high U in
drainage from near Sellaﬁeld (British Geological Survey,
1984), remobilisation of sediment hosted U released from
Sellaﬁeld nuclear reprocessing plant (MacKenzie and
Scott, 1993) and drainage from the industrial northwest
of England. The high U concentrations in Ensis shells from
Ravenglass are similar to high U concentrations recorded
in shells of Cerastoderma edule from Haverigg (just south
of Ravenglass) by Price and Pearce (1998). The high U con-
centrations from Ensis shells from northwest Scotland are
unusual, and cannot readily be explained. Uranium in sea-
water, with an average concentration of 3.2 lg/L (Taylor
and McLennan, 1985), forms a soluble UCO2À
3 ion, and
minor concentrations of U in shells are to be expected, with
the U ion readily accommodated in carbonate minerals.
Uranium from Irish Sea sources would be transported
along the northwest coast of Scotland, but would be grad-
ually attenuated (cf. Zn), and thus slightly elevated concen-
trations in Ensis shells would be unsurprising. However the
high concentrations from Harrapool and Lower Breakish
must be derived from another, as yet unrecognised, source.
A similar pattern is seen in the maximum concentrations of
U recorded in each shell also, with the exception of one of
Pb U
7.37 2.23
Pb U
2.24 1.11
1.84 0.35
Pb U
3.43 0.19
5.20 0.57
3.66 0.35
Pb U
3.19 0.01
4.40 0.06
6.48 0.10
4.70 0.02
Pb U
3.22 0.21
Pb U
4.60 0.19
Pb U
1.30 0.00
2.69 0.02
Pb U
1.49 0.00
2.82 0.00
Pb U
1.12 0.18
1.57 0.04
Pb U
1.61 0.00
1.47 0.00
Pb U
2.09 0.03
1.77 0.05
Pb U
1.58 2.20
Zn Cd
69.5 50.7
Zn Cd
10.8 n.d.
7.40 n.d.
Zn Cd
14.1 0.10
20.3 0.12
8.07 0.01
Zn Cd
9.84 0.07
6.37 0.00
8.84 0.15
9.43 0.05
Zn Cd
13.2 11.6
Zn Cd
15.3 6.87Zn Cd
21.7 n.d.
6.83 0.01
Zn Cd
10.3 0.26
12.5 0.08
Zn Cd
2.55 0.00
6.23 0.01
Zn Cd
3.83 0.00
7.80 n.d.
Zn Cd
4.64 n.d.
Fig. 9. Median concentration of Zn, Cd, Pb and U (mg/kg) in the shells of Ensis siliqua.
Table 2
Highest concentration of Zn, Cd, Pb and U recorded in each analysed
shell of Ensis siliqua
Zn (mg/kg) Cd (mg/kg) Pb (mg/kg) U (mg/kg)
Harrapool 81.7 1.60 21.5 1.02
Harrapool 91.7 9.73 8.87 1.79
Lower Breakish 23.5 1.87 7.10 3.15
Ravenglass 48.2 0.94 22.5 0.99
Ravenglass 120 1.00 10.4 0.87
Ravenglass 70.1 0.36 12.6 0.79
Wallasey 83.2 25.7 10.6 1.66
Benllech 115 127 7.39 1.71
Dulas Bay 295 156 21.9 4.74
Porthmadog 88.9 0.45 16.8 0.06
Porthmadog 91.0 1.02 32.2 0.58
Mochras 93.2 0.26 11.3 0.12
Mochras 184 3.71 13.6 0.37
Barmouth 59.4 0.95 16.2 0.58
Barmouth 42.3 0.72 3.06 0.37
Borth 125 2.08 13.4 0.11
Borth 122 5.03 12.1 0.50
Borth 66.7 0.73 15.1 0.38
Borth 23.9 0.30 11.4 0.23
Pendower 32.8 2.81 11.9 0.13
Pendower 66.4 0.49 12.4 1.15
Freathy 23.9 0.24 12.9 0.08
Freathy 83.5 0.15 7.20 0.08

the Pendower samples, which probably results from a
single unusual pollution event (see Section 4.2).
4.2. The temporal record within individual shells
The rapid growth of E. siliqua, which deposits up to
$20 cm of aragonite shell over a maximum $15 year life-
span makes this a potentially high resolution biomonitor,
particularly during its early to middle life when shell depo-
sition and growth is fastest. This section deals with the
results of analytical traverses along the shell of E. siliqua,
describing the temporal record stored within these shells.
4.2.1. Sr, Mg and Ba
Samples from Borth show an extremely clear seasonal
variation in the concentrations of Mg and Sr, which corre-
spond with growth layers on the shells. Fig. 10 shows the
Mg and Sr concentrations in an analytical traverse run
across a shell from Borth. The highest Mg and Sr concen-
trations coincide with the surface growth checks on the
shell. The Sr peaks in this shell are sharp, marked typically
by a single high analysis ($1000–2000 mg/kg higher than
the surrounding points) occurring right at the growth
check. This is occasionally bordered by one slightly ele-
vated analysis on either side. In contrast the Mg peaks
are broad (covering 2 or 3 analyses) and drop gradually
from 100–120 mg/kg at the growth checks to about
40 mg/kg between checks. The Mg peaks occasionally lag
behind the Sr peaks, occurring slightly after the surface
growth check. Similar features are seen for Sr and Mg in
shells from elsewhere in Cardigan Bay (see Figs. 11 and
12, cf. Toland et al., 2000).
The relationship between growth checks, peaks in Sr
and particularly Mg is of interest. High Mg concentrations
in shells have been related to high temperatures by many
authors (Brand and Morrison, 1987; Fuge et al., 1993a;
Klein et al., 1996a; Vander Putten et al., 2000) and high
Sr has been linked to salinity (Brand and Morrison,
1987; Klein et al., 1996b; Rosenberg and Hughes, 1991;
Stecher et al., 1996). In Cardigan Bay, the sea surface tem-
peratures can be estimated from published data (Norris,
2001), showing the annual high to occur in August and
the low in January/February. Thus by ﬁxing the highest
Mg concentration of each seasonal cycle in August, and
Fig. 10. Variation in Mg and Sr concentrations in an analytical traverse of an Ensis siliqua shell from Borth, superimposed on a photograph of the shell.
Analyses were performed from just right of the right hand of the pair of tick marks, parallel and about 1 mm above the black line, to the right hand tick
mark. The length of the traverse is approximately 16.5 mm. Growth checks are clearly visible in the shell and coincide closely with the highest Mg and Sr
concentrations. Symbols and the light dotted line are the raw data; the heavy line is the smoothed data. Broken vertical lines mark the positions of surface
growth checks.

spreading the analyses between peaks evenly across each
year, allows (i) the timing of the growth checks to be esti-
mated and (ii) the relationship between sea surface temper-
ature (SST) and metal concentrations to be investigated.
Adopting this approach, the growth checks in the shell
occur typically in June or July. This is probably a response
to reproductive activity which occurs in Ensis around July,
as sea temperatures increase (Henderson, 1993). After
spawning, a decrease in tissue mass in the organism is likely
to be reﬂected in less shell growth, and this will give rise to
0
500
1000
1500
2000
2500
3000
3500
4000
4500
1 4 7 10 13 16 19 22 25 28 31 34 37 40
Barmouth
Srmg/kg
Ravenglass
0
500
1000
1500
2000
2500
3000
1 4 7 10 13 16 19 22 25 28 31
Srmg/kg
0
200
400
600
800
1000
1200
1400
1600
1800
2000
1 5 9 13 17 21 25 29 33 37 41
Wallasey
Srmg/kg
0
200
400
600
800
1000
1200
1400
1600
1800
2000
1 4 7 10 13 16 19 22 25 28 31 34 37 40
Freathy
Srmg/kg
0
500
1000
1500
2000
2500
3000
3500
1 4 7 10 13 16 19 22 25 28 31 34 37
Porthmadog
Srmg/kg
Mochras
0
500
1000
1500
2000
2500
3000
3500
1 5 9 13 17 21 25 29 33 37 41 45
Srmg/kg
Fig. 11. Variation in Sr concentrations in analytical traverses across selected Ensis siliqua shells from a selection if sites. Symbols and the light dotted line
are the raw data; the heavy line is the smoothed data. Symbols and the light dotted line are the raw data; the heavy line is the smoothed data. Broken
vertical lines mark the positions of surface growth checks.
Mochras
0
50
100
150
200
250
1 5 9 13 17 21 25 29 33 37 41 45
Mgmg/kg
0
20
40
60
80
100
120
1 4 7 10 13 16 19 22 25 28 31
Ravenglass
Mgmg/kg
Freathy
0
10
20
30
40
50
60
70
80
90
1 4 7 10 13 16 19 22 25 28 31 34 37 40
Mgmg/kg
0
20
40
60
80
100
120
140
1 5 9 13 17 21 25 29 33 37 41
Wallasey
Mgmg/kg
0
20
40
60
80
100
120
1 4 7 10 13 16 19 22 25 28 31 34 37
Porthmadog
Mgmg/kg
Barmouth
0
20
40
60
80
100
120
140
160
180
1 4 7 10 13 16 19 22 25 28 31 34 37 40
Mgmg/kg
Fig. 12. Variation in Mg concentrations in analytical traverses across selected Ensis siliqua shells from a selection if sites. Symbols and the light dotted line
are the raw data; the heavy line is the smoothed data. Broken vertical lines mark the positions of surface growth checks.

the marked growth check, where the shell thins. As waters
cool towards the autumn and winter, growth will slow,
depositing a thinner shell.
Strontium concentrations in shells from several other
localities (Ravenglass, Wallasey and Freathy, see Fig. 11)
show a gradual rise throughout the year after each growth
check, followed by a drop at the next growth check. This
drop is much more pronounced in the samples from Rav-
englass and Wallasey, which have a 1& diﬀerence between
winter and summer salinity, than at Freathy, where sum-
mer and winter salinities cannot be distinguished (see Table
2). Once again, the Mg concentration peaks close to, or
slightly later than these growth checks (see Fig. 12).
Barium concentrations generally do not show a clear
seasonal cyclicity, although the concentration varies across
the year, with Ba tending to increase by a factor of 2–3
above a local ‘‘baseline’’ for short periods (weeks to a
few months). In Cardigan Bay, high Ba concentrations
usually associated with high Sr and Mg and the growth
checks (see Fig. 14) representing increased productivity in
late summer, although occasionally high Ba concentrations
occur in the winter/spring months (e.g. Barmouth). From
Wallasey, elevated Ba concentrations occur between
growth checks, probably representing a spring algal bloom
as waters warm, and at Freathy each year shows a double
peak in Ba, again occurring between growth checks in
spring and early summer. In both these cases, elevated Ba
concentrations last for periods of a few months during
the growth of the shell, and in some cases these periods
are associated with an increase in the shell Sr concentra-
tions which may or may not persist. Thus in the shell from
Wallasey, spikes in Ba cause a ramping increase in Sr which
does not drop, whilst in shells from Barmouth and Borth,
high Ba is associated with a spike in the Sr concentration.
This may attest to periods of increased productivity caus-
ing increased growth in Ensis shells and is consistent with
the Ba–Sr relationships described above (Iglesias and
Navarro, 1991; Stecher et al., 1996; Stroobants et al.,
1991; Vander Putten et al., 2000).
4.2.2. Relationship of Mg and Sr to SST
Both Mg and Sr concentrations from shells at Borth
show the clearest temporal variation of all shells analysed.
In Fig. 13 the smoothed analyses of Mg and Sr from the
shell in Fig. 10, assumed to have died in 1998, are plotted
alongside sea surface temperature (SST) interpolated from
Norris (2001). The analyses between each growth check
have been spaced evenly across a year to give a time series,
0
20
40
60
80
100
120
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999
0
2
4
6
8
10
12
14
16
18
1000
1200
1400
1600
1800
2000
2200
2400
2600
2800
3000
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999
0
2
4
6
8
10
12
14
16
18
Mg,mg/kg
Sr,mg/kg
Temp,CTemp,C
Year
Fig. 13. Variation Mg and Sr concentrations (mg/kg) compared to sea surface temperature in a shell of Ensis siliqua from Borth, plotted against the time
at which the shell was deposited. Broken lines with symbols show smoothed metal concentrations, solid line shows sea surface temperature variation from
Norris (2001).

thus removing the effects of the slowing growth as the
organism ages. Magnesium shows a clear temporal cyclicity
with variations of the same order as temperature. Stron-
tium also shows a clear seasonal cyclicity, but is clearly
not as closely coupled to SST as Mg, suggesting Sr concen-
trations are not directly linked to SST. Taking the
smoothed maximum and minimum Mg concentrations
from this Borth shell between 1991 and 1997, and compar-
ing these with SST gives a best fit linear relationship
SSTð

CÞ ¼ 0:147 Â Mgðmg=kgÞ þ 0:369 r ¼ 0:908 ð1Þ
and using the raw, (i.e. not smoothed) data gives
SSTð
CÞ ¼ 0:115 Â Mgðmg=kgÞ þ 2:146 r ¼ 0:934 ð2Þ
From this relationship it is possible to calculate SST from
the Mg concentration of Ensis shells from other areas.
These are listed in Table 3 for both the raw and smoothed
Mg concentration data. Many shells give a SST range which
is reasonable (cf. Table 2) with the raw Mg data giving tem-
perature ranges which are marginally greater than the
smoothed Mg data. Harrapool and Dulas Bay give anoma-
lously high SSTs, a result of both having a relatively high
median Mg concentration. The shell from Lower Breakish,
and individual shells from Borth and Barmouth all give low
SSTs because of low median Mg. These may simply reflect
ontogenetic factors during the growth of individual shells
(cf. Vander Putten et al., 2000), and the variations in the
ages of material analysed. However when looked at region-
ally, a group of shells reflect the local SST conditions much
more closely. Thus shells from Borth give an average SST
range of 15.9–5.3 °C (smoothed Mg concentrations); from
Cornwall (Pendower and Freathy) 15.6–7.5 °C; from north
Cardigan Bay (Mochras, Barmouth, Porthmadog) 15.9–
5.7 °C; and from Liverpool Bay (Benllech, Wallasey and
Ravenglass) 16.7–6.8 °C. Too few samples from Scotland
were collected to give an accurate picture, although the
one sample from Harrapool, with a median Mg concentra-
tion in the normal range, gives a SST range of 15.3–8.7 °C.
The SSTs and ranges compare extremely well with the SST
data presented in Table 2, and thus the Mg concentrations
of groups of shells appear to represent SST well.
4.2.3. Pb, Zn, Cd and U
In most shells the contaminant metals Pb, Zn, Cd and U
show an irregular variation as the shell grows, with no clear
relationship to growth checks. This is exemplified by the
shell from Dulas Bay which has the highest concentrations
of these metals (see Fig. 15). Here peaks in metal concentra-
tions show no systematic seasonal behaviour, and form in
response to relatively short-lived changes (weeks to months)
in metal concentrations in the overlying waters. This may be
related to increased fluxes of metals from the drainage of
Parys Mountain, as well as potential sources from Liver-
pool Bay. It is notable that Zn and Cd, which show very
Table 3
Calculated sea surface temperatures based on Mg concentrations of Ensis siliqua shells
Raw Mg
concentration data
(mg/kg)
Smoothed Mg
concentration data
(mg/kg)
Temperature (°C)
calculated from raw
Mg data
Temperature (°C)
calculated from smoothed
Mg data
Max Min Max Min Max Min Max Min
Harrapool 306 58.8 257 62.0 37.3 8.9 38.2b
9.5
Harrapool 119 48.5 102 56.8 15.8 7.7 15.3 8.7
Lower Breakish 66.0 26.3 50.0 28.0 9.7 5.2 7.7 4.5
Ravenglass 105 46.9 91.8 47.3 14.2 7.5 13.9 7.3
Ravenglass 87.4 15.6 82.7 10.6 12.2 3.9 12.5 1.9
Ravenglass 169 65.8 149 65.1 21.6 9.7 22.3 9.9
Wallasey 123 38.2 113 44.1 16.3 6.5 17.0 6.9
Benllech 190 47.8 178 51.0 17.3 7.6 17.7 7.9
Dulas Bay 266 45.5 222 82.5 32.7 7.4 33.0c
12.5
Porthmadog 145 42.8 108 39.3 18.8 7.1 16.3 6.2
Porthmadog 110 14.0 83.0 17.9 14.8 3.8 12.6 3.0
Mochras 121 50.7 102 51.5 16.1 8.0 15.3 7.9
Mochras 211 50.7 161 47.5 26.4 8.0 24.0 7.3
Barmouth 110 13.2 64.1 14.7 14.8 3.7 9.8 2.5
Barmouth 164 48.0 118 48.7 21.0 7.7 17.7 7.5
Bortha
150 22.8 114 20.0 19.4 4.8 17.1 3.3
Borth 178 43.9 120 45.6 22.6 7.2 18.0 7.1
Borth 45.9 7.69 40.1 7.78 7.4 3.0 6.3 1.5
Borth 193 52.3 149 60.2 24.3 8.2 22.3 9.2
Pendower 102 42.5 94.0 43.2 13.8 7.0 14.2 6.7
Pendower 173 59.8 141 60.7 22.0 9.0 21.1 9.3
Freathy 129 38.9 105 41.6 17.0 6.6 15.8 6.5
Freathy 77.3 45.1 75.7 47.8 11.0 7.3 11.5 7.4
a
Shell from Borth used to generate the relationship between SST and Mg concentration.
b
Many analyses 200 mg/kg, shell has a high median Mg concentration.
c
A few analyses 200 mg/kg. The typical high Mg concentration is in the range 150–160 mg/kg.

similar geochemical behaviour (Fuge et al., 1993c) are
decoupled, with high Cd occurring where there is no
marked change in Zn. Zn/Cd ratios in this shell vary from
0.12 to 10.3, with a median value of 1.49, far less than typ-
ical Zn/Cd ratios of $300 for inputs to this area (see above).
This range is too great to result from variation in the Zn/Cd
ratio of the inputs to this area, and thus must be a result of
different biological processing of Cd and Zn, with Zn
retained in the soft tissues and Cd partitioned into the shell.
Fig. 16 shows Pb and Zn concentrations in Ensis shells
from Borth and Wallasey. In both cases, Pb concentrations
are low at the annual growth checks, and peak between
these. This implies that seawater Pb concentrations are
lowest at the end of the summer and peak in the winter.
At Borth, this is consistent with increased fluxes of Pb
being washed from the Mid-Wales ore field during the wet-
ter winter months (Davies, 1987; Fancourt, 2004; Pearce,
1992). A similar high winter flux will also be responsible
for the high winter Pb in the shell from Wallasey, with
increased runoff from the urban and industrialised north-
west of England being the most likely source. In both cases,
Zn shows no clear seasonality. This is particularly surpris-
ing at Borth, where the river drainage also contains high
dissolved Zn, and seasonal variation similar to Pb might
be expected. Again this implies an ontogenetic control on
Zn partitioning.
Fig. 17 shows Pb and Zn variations in shells from Frea-
thy and Pendower in Cornwall. In the shell from Freathy
there is no systematic variation in Pb or Zn, showing a series
of short lived increases (2 weeks) in metal concentration.
These spikes will be responses to short-lived variations in
seawater metal concentrations, most likely the result of
inputs from the River Tamar which drains the Cu–Pb–Zn
mineralised area to the west of Dartmoor and the city of
Plymouth (National Rivers Authority, 1994). In contrast,
one Ensis shell analysed from Pendower, Cornwall, shows
distinct peaks in Zn, Cd and U (see Fig. 17) which coincides
with a very pronounced cleft in the shell. Such a pro-
nounced cleft in the shell marks a period of disturbance to
growth, and such clefts have been reported to be caused
by abnormal environmental conditions (Henderson and
Richardson, 1994) or by dredging (Robinson and Richardson,
0
1
2
3
4
5
6
7
8
9
10
1 4 7 10 13 16 19 22 25 28 31 34 37 40
Freathy
Bamg/kg
0
5
10
15
20
25
1 5 9 13 17 21 25 29 33 37 41
Wallasey
Bamg/kg
0
5
10
15
20
25
30
35
40
1 4 7 10 13 16 19 22 25 28 31
Bamg/kg
0
5
10
15
20
25
30
35
1 5 9 13 17 21 25 29 33 37 41 45
Mochras
Bamg/kg
0
1
2
3
4
5
6
7
8
9
1 4 7 10 13 16 19 22 25 28 31 34 37 40
Barmouth
Bamg/kg
0
2
4
6
8
10
12
14
16
18
1 4 7 10 13 16 19 22 25 28 31 34 37
Bamg/kg
Borth
Bamg/kg
Fig. 14. Variation in Ba concentrations in analytical traverses across selected Ensis siliqua shells from a selection if sites. Symbols and the light dotted line
are the raw data; the heavy line is the smoothed data. Broken vertical lines mark the positions of surface growth checks.

1997). The cleft in the Pendower shell dates from between
1991 and 1993 based on later growth clefts, and coincides
with an outpouring from the Wheal Jane metal mine into
the Fal estuary in January 1992. In this single, extreme
0
50
100
150
200
250
300
350
1 5 9 13 17 21 25 29 33 37 41 45 49
Znmg/kg
0
5
10
15
20
25
1 5 9 13 17 21 25 29 33 37 41 45 49
Pbmg/kg
0
20
40
60
80
100
120
140
160
180
1 5 9 13 17 21 25 29 33 37 41 45 49
Cdmg/kg
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
1 5 9 13 17 21 25 29 33 37 41 45 49
Umg/kg
Fig. 15. Variation in Zn, Cd, Pb and U concentrations in an analytical traverse across an Ensis siliqua shell from Dulas Bay. Symbols and the light dotted
line are the raw data; the heavy line is the smoothed data. Broken vertical lines mark the positions of surface growth checks.
0
2
4
6
8
10
12
14
16
1 4 7 10 13 16 19 22 25 28 31 34 37
Borth
Pbmg/kg
0
2
4
6
8
10
12
1 5 9 13 17 21 25 29 33 37 41
Wallasey
Pbmg/kg
Borth
0
10
20
30
40
50
60
70
80
1 4 7 10 13 16 19 22 25 28 31 34 37
Znmg/kg
0
10
20
30
40
50
60
70
80
90
1 5 9 13 17 21 25 29 33 37 41
Wallasey
Znmg/kg
Fig. 16. Variation in Pb and Zn concentrations in analytical traverses across Ensis siliqua shells from Borth and Wallasey. Symbols and the light dotted
line are the raw data; the heavy line is the smoothed data. Broken vertical lines mark the positions of surface growth checks.

event, some 50,000 m3
of highly contaminated water carried
approximately 100 tonnes of oxidised heavy metals into
Restronguet Creek and on into the Fal estuary, causing a
plume of ochreous water to migrate north eastward along
the Cornish coast (Bowen et al., 1997; Grant, 1999). Such
an extreme short-lived event could cause severe damage to
marine biota, and seems a likely cause of the increased
metal concentrations in the Pendower shell.
5. Conclusions
Shells of E. siliqua from the coastal waters to the west of
mainland Britain record a range of environmental informa-
tion. Their rapid growth makes them useful biomonitors
with a high temporal resolution, with the potential to record
events in the marine environment lasting only a matter of
days. Strontium, Mg and Ba have been used as records of
the physicochemical environment in which molluscs grow
(Fuge et al., 1993a; Klein et al., 1996a,b; Stecher et al.,
1996; Vander Putten et al., 2000). In the coastal waters of
western Britain, E. siliqua shows no clear relationship
between Sr and salinity, but on a local scale, e.g. within Car-
digan Bay or on the Cornish coast, median Sr concentra-
tions within shells decrease as salinity decreases. Barium
concentrations in Ensis shells vary considerably, and are
higher in areas where sewage sludge dumping was operative
during the growth of the shells, such as Liverpool Bay.
Barium uptake in molluscs has been related to marine pro-
ductivity (Bishop, 1988; Chan et al., 1977; Stecher et al.,
1996; Stroobants et al., 1991; Vander Putten et al., 2000),
and this is consistent with nutrient inputs around the British
coast, with shells growing in cleaner Atlantic water having
lower Ba. High Ba in shells from the Scottish coast suggests
long distance transport of nutrients from the northern Irish
Freathy
Znmg/kg
Pendower
0
10
20
30
40
50
60
70
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49
Znmg/kg
0
1
2
3
4
5
6
7
8
1 4 7 10 13 16 19 22 25 28 31 34 37 40
Freathy
Pbmg/kg
Pendower
0
2
4
6
8
10
12
14
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49
Pbmg/kg
Pendower
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49
Umg/kg
Fig. 17. Variation in Pb and Zn concentrations in an analytical traverse across Ensis siliqua shell from Freathy, and Pb, Zn and U from a shell from
Pendower. Symbols and the light dotted line are the raw data; the heavy line is the smoothed data. Broken vertical lines mark the positions of surface
growth checks.

Sea around the Scottish coast, analogous to the migration
of radionuclides from Sellafield (Hunt, 1979). In addition,
on a regional scale there is a strong positive correlation
between Sr and Ba, and this may be related to increased
Sr uptake during periods of greater productivity and thus
more rapid growth. This compares with published relation-
ships between these elements (cf. Chan et al., 1977; Iglesias
and Navarro, 1991; Stecher et al., 1996). The temporal
record of Ba and Sr within some shells also shows a ramping
of the Sr concentration as Ba increases, again consistent
with a greater incorporation of Sr at times of higher produc-
tivity. The relationship between Sr, Ba and environmental
conditions, such as salinity, is thus complex (Vander Putten
et al., 2000; Zolotarev, 1974).
In some shells, Mg shows a seasonal cyclicity, with the
highest Mg concentrations being deposited just after
annual growth checks. High Mg in carbonate shells has
been linked with temperature by many authors (Brand
and Morrison, 1987; Klein et al., 1996a; Vander Putten
et al., 2000), and this suggests that the growth checks in
these shells are related to slowing of growth after spawning,
and not to slow winter growth (Henderson, 1993). Sr also
often shows a marked peak at these checks. Comparison
of Mg in a shell from Borth with sea surface temperatures
give a relationship which can be used to calculate SST from
other sites. Many individual shells give a SST range close to
reported values (Lee and Ramster, 1981; Norris, 2001),
although some shells produce SST ranges which are too
cold or too hot. This may reflect ontogenetic processes in
the shell (cf. Vander Putten et al., 2000). Regionally how-
ever, groups of shells give SST ranges which are close to
reported values (e.g. Cardigan Bay, Cornwall) and the
potential exists to use this to reconstruct SST.
Contaminant metals in Ensis shells record both regional
variations in marine chemistry, such as high Zn and Pb
associated with metal mining in Cardigan Bay and Angle-
sey, or high U and Zn in Liverpool Bay from industrial
activity in this area. Where Cd is particularly high (Dulas
Bay, Wallasey) this becomes preferentially incorporated
into the shells over Zn. Cyclic seasonal changes in marine
chemistry are also recorded in Ensis shells, such as winter
rains washing higher concentrations of Pb from the Mid-
Wales ore field into Cardigan Bay. Zn, also high in these
waters however, does not show the same seasonality in
these shells, implying either excretion of Zn or retention
in the organism. Similar high winter Pb is also recorded
from shells near Wallasey. Mostly however, the contami-
nant metal record in Ensis shells records single events or
short periods (days to months) where Pb, Zn, Cd and U
were higher than the normal background concentration.
These will reflect short-lived incidents such as increased
runoff, periods of sewage dumping and ‘‘normal’’ pollution
incidents/discharges etc. (cf. Fuge et al., 1993a). In some
cases, a single extreme occurrence is recorded, such as the
Ensis sample from Pendower, which shows a major influx
of metals, most probably the breach of the Wheal Jane
mine in January 1992. Ensis shells thus hold a potentially
high resolution record of changes in coastal waters, and
regionally they paint a picture of relatively clean Atlantic
water becoming increasingly polluted on its passage north-
wards along the coast of the British Isles.
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