Hydrobiologia 462: 221–232, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.

221

Ulva and Enteromorpha as indicators of heavy metal pollution

R. Villares1, X. Puente1 & A. Carballeira2

1Ecologıa, Escuela Politecnica Superior, Universidad de Santiago de Compostela, 27002 Lugo, SpainTel: +982-252-303. Fax: +982-241-835. E-mail: bfrubenv@usc.es2Ecologıa, Facultad de Biologıa, Universidad de Santiago de Compostela, 15706 Santiago de Compostela, Spain

Received 28 December 2000; in revised form 10 July 2001; accepted 5 August 2001

Key words: heavy metals, macroalgae, Ulva, Enteromorpha, sediments, NW Spain

Abstract

We studied the use of two genera of green macroalgae, Ulva and Enteromorpha, as indicators of heavy metalcontamination on the northwest coast of Spain. We evaluated the extent of external contamination by fine particlesadhering to the algal thallus and found that although not a problem in Ulva, it may be in Enteromorpha samples.The mean levels of metals in both seaweeds were in accordance with previously reported levels in different speciesof the genera in clean areas. A large number of significant correlations between levels of metals in macroalgaeand in sediment (for both the total and labile fractions) were found. However, almost all of the coefficients ofcorrelation decreased after geochemical normalization of sediment metal concentrations.

Introduction

Ulva and Enteromorpha as indicators of heavy metalcontamination

Evaluation of the degree of contamination in an areaby the conventional method of water analysis has anumber of disadvantages. The low concentrations ofheavy metals in seawater increases the risk of con-tamination during handling of samples and makesnecessary the use of complex analytical techniques.There is also a high degree of variability in the levelsof heavy metal in coastal areas, and therefore a largenumber of samples must be collected to give reliableresults. Furthermore, the concentration of an elementin water does not supply us with information about itsbioavailability (Bryan et al., 1985).

Analysis of sediments also suffers from limita-tions; the concentration of a metal in sediment de-pends, amongst other factors, on rates of depositionand the nature of particles and as before does notreflect bioavailability (Phillips, 1977).

These problems can be resolved by using organ-isms, which by definition reflect the bioavailability ofcontaminants. According to Phillips (1990), the pre-requisites for a species to be considered as suitable for

use as a bioindicator are that it is: sessile or sedentary;tolerant to high levels of contaminants and wide rangesof salinity, permitting laboratory studies of the kinet-ics of contaminants; abundant within the study area,easy to identify and collect and should provide suffi-cient amounts of tissue for analysis; and there shouldbe a simple correlation between the concentration ofthe contaminant in the tissues of the organism and theaverage concentration bioavailable in the environment.

Macroalgae appear to be the most suitable indic-ators of dissolved metals, which are taken up bothactively and passively so that the metal concentrationsin the thallus can become many times those found inthe surrounding water. One drawback with the use ofthese organisms is that any factor that tends to alterthe growth rate, such as light intensity, temperature,etc., can thus alter their effectiveness as bioindicat-ors. Another problem is that of contamination by fineparticles adhering to the surface of the algae (Phillips,1990). However, despite these problems, the use ofmacroalgae and other organisms as bioindicators isthe best option available when information about thebioavailability of contaminants is required.

The different species of the genera Ulva and En-teromorpha are considered to be good biondicators ofmetal contamination. The laminar structure of Ulva

222

lactuca leads to a relatively high surface area: volumeratio and the thallus cells are structurally uniform andphysiologically active. In addition, the cosmopolitannature of the alga and its capacity to grow in heavilycontaminated areas increases its potential as a usefulbioindicator (Ho, 1990). The study carried out by thisauthor in the waters of Hong Kong demonstrates thecapacity of this macroalga as an indicator of contam-inantion by Mn, Fe, Cu, Zn and Pb in tropical andsubtropical regions. Talbot & Chegwidden (1982) con-sider that Ulva lactuca may be a good indicator ofCd, Fe, Mn and Pb contamination, but not of Cu andZn contamination, and Haritonidis & Malea (1999)identify Ulva rigida as an indicator species for Pb,Zn and Cd. Barraza & Carballeira (1997) and Barraza(1999) carried out bioassasys of metal kinetics accu-mulation and stress using Ulva rigida and concludedthat the potential capacity for bioconcentration and thephysiological sensitivity of this alga endorse its useful-ness as a bioindicator of metals in marine ecosystems.A similar conclusion was reached by Wang & Dei(1999) in a study of kinetic measurements of metalaccumulation in Ulva lactuca.

Bryan et al. (1985) consider Enteromorpha intest-inalis to be a versatile indicator of dissolved metals,but highlight potential problems with contaminationby fine particles adhering to the thallus (which canbe evaluated if sediment analysis is carried out). Thesame authors mention the advantage of Enteromorphaover Fucus vesiculosus in that it reflects the changesin metal levels in the environment more quickly. Sayet al. (1990) also recommend the use of Enteromorphaas an indicator of metal contamination in estuaries. Inan experiment on the kinetics of metal accumulationin Enteromorpha clathrata, E. linza and E. flexuosa,Seeliger & Wallner (1988) found a linear relation-ship between concentrations of Cd, Cu, Pb and Znin solution and in the algae, and concluded that itwas possible to extrapolate the metal contents in theEnteromorpha tissue in order to evaluate the concen-trations of dissolved metals in the water. A similarsituation was found in a study by Seeliger & Edwards(1977) in which they found good correlations betweendissolved and particulate Cu and Pb levels in waterand those in Enteromorpha linza and other seaweeds.Another advantage of Enteromorpha is its resistance tohigh levels of contamination (Castilla, 1996; Marsden& DeWreede, 2000).

Heavy metals in sediments

Despite the previously mentioned drawbacks, the ana-lysis of contaminants in abiotic compartments hassome advantages. The knowledge of the levels ofheavy metals in sediments is important because theycan act as a source of contaminants to organisms. It isalso possible that an organism could be affected by aharmful compound, but because it is quickly excretedby the organism, it will not be detected. It is, there-fore, necessary to carry out combined monitoring ofbiotic and abiotic compartments (Kramer, 1986). Sed-iment analysis offers several advantages over wateranalysis: the sediment concentrations give an integ-rated and stable picture of contamination over timeand the analytical procedures give better results at lesscost because of the higher concentrations in sedimentsamples.

On the other hand, numerous factors that affectthe levels of metals in sediments (apart from inputs)must be taken into account. These include: texture,iron oxide content, organic matter, etc., which shouldbe considered before reaching conclusions about theconcentrations of metals found.

Investigation of metal concentrations in sedimentsoften involves study of a labile fraction. This frac-tion is obtained by weak extraction, which releases themetal most weakly bound to particles, and is thereforethe most readily bioavailable. As a result, it has greaterbiological significance than the total fraction (Puenteet al., 1996), which includes metal that is stronglybound to the sediment (e.g. forming part of the crys-talline structure of the mineral particles) and which isunlikely to be assimilated by living organisms.

Study area

The study was carried out on the northwest coast ofSpain, in four rias (a type of estuary) known as theRías Baixas, which are of great economical and eco-logical significance in the area. They are characterizedby having a very high level of primary productivity,due mainly to the periodic upwelling that takes placebetween April and October (Álvarez-Salgado et al.,1993). The high level of primary productivity main-tains sizeable populations of consumers, for example,the mussel production in the Ría de Arousa constitutesone of the highest productions of protein per unit ofarea in the world (Tenore et al., 1982). The surround-ing areas are densely populated (more than 250 inhab-itants per km2), except around the Ría de Muros where

223

there are less inhabitants. The main centres of pop-ulation are Vigo and Pontevedra with approximately280 000 and 75 000 inhabitants, respectively.

The objectives of this study were: to measure theconcentrations of certain heavy metals in two generaof macroalgae widely used as bioindicators of metalcontamination and to compare the data obtained withthat of other studies; to investigate the existence ofcontamination of samples by fine particles of sed-iment adhering the thallus and to analyze possiblerelations between metal concentrations in sedimentand in algae.

Materials and methods

Sampling

Sampling was carried out from the shore at low tide.The algae were collected monthly at 22 selected sites(Fig. 1) except at sites A2, A8, P2, P5 and V4,where sampling was fortnightly. The study was car-ried out between March 1994 and February 1995, butEnteromorpha sampling was suspended in September1994 because this alga ceased growing at many sites.The fortnightly sampling of selected sites took placebetween April and November. Sediment samples werecollected, at the same sites as the algae, on two oc-casions, in April and in August; they were collectedwith a plastic spatula, taking only the surface layer (0–1 cm). Samples were transported to the laboratory at 4◦C.

Most of the fronds were tentatively identified asUlva rigida and Enteromorpha intestinalis, but due tothe complex taxonomy of these genera, many of thespecimens were not identified to species level. Speci-mens of Ulva are often assigned to different specieswhen they are in fact different morphotypes of onespecies (Malta et al., 1999). Considering the simil-arities in the ecology and physiology of the differentspecies of Ulva (Littler & Littler, 1980) it is probablethat accumulation of metals in the different speciesfollows a similar pattern. Furthermore, Seeliger &Wallner (1988) did not find any significant differencesin the accumulation of metals in four different speciesof Enteromorpha in laboratory experiments, thus jus-tifying the use of field samples made up of variousspecies of this alga.

The algal samples were carefully washed with dis-tilled water, dried in a forced air oven at 50±1 ◦Cto constant weight, crushed in a mortar and stored in

plastic bags until analyzed. Sediment samples werethen also dried at 50±1 ◦C, sieved at <0.2 mmand stored in plastic bags. All glassware used in thefollowing procedures was acid washed (HNO3 15%).

Macroalgae

Metal extraction was carried out by digesting 0.5 gof alga in 10 ml of 65% HNO3 in Teflon� bombs ina microwave oven. The extracts were made up to 50ml with distilled water. Concentrations of Al, Cr, Cu,Fe, Mn, Ni and Zn were determined by flame atomicabsorption spectrophotometry (AAS) using a Perkin-Elmer 2100 spectrophotometer with background deu-terium correction. Concentrations of Cr in Ulva werebelow the limit of detection. Quality control was car-ried out by parallel analysis of certified reference ma-terial of the aquatic moss Platihypnidium riparioides(BCR-61).

Sediment

The labile fraction was extracted using 20 ml of 1 NHCl; samples were left overnight to remove the effer-vescence produced by carbonates, then maintained atroom temperature with mechanical shaking for 1 h, theextract was centrifuged at 5000 rpm for 2 min.

The total fraction was obtained by extraction inTeflon� bombs in a microwave oven with 10 ml ofaqua regia (37% HCl and 65% HNO3 in a propor-tion of 3:1). Samples were predigested for 6 h. Beforecompletion of digestion, 5 ml of 45% HF was added.

Both fractions were analyzed by AAS, as for themacroalgal samples. The concentrations of Ni werebelow the limit of detection. Quality control was car-ried out by parallel analysis of certified referencematerial (MESS-2, a marine sediment) supplied bythe National Research Council of Canada. The organicmatter content was calculated as loss on ignition at 400◦C for 6 h.

Results and discussion

Contamination by particulate material

One of the problems associated with the use of mac-roalgae as indicators of metal contamination is ex-ternal contamination by fine particles adhering to thethallus. Problems of this kind are likely to be en-countered in the analysis of Enteromorpha, becauseof its morphology. One indication that particulate

224

Figure 1. Location of sampling sites.

225

Table 1. Spearman’s rank coefficients of correl-ation, and level of significance, for sediment,Ulva and Enteromorpha collected at the sametime as the sediment. n, is shown in brackets

Cr Fe

Al sediment 0.354∗ 0.540∗∗∗(44) (44)

Al Enteromorpha 0.504∗∗ 0.653∗∗∗(40) (40)

Al Ulva −0.049 n.s.

(44)

Fe sediment 0.868∗∗∗(44)

Fe Enteromorpha 0.474∗∗(40)

∗Significant at p<0.05, ∗∗Significant atp<0.01. ∗∗∗Significant at p<0.001. n.s.=notsignificant.

contamination has occurred is the existence of signi-ficant correlations between the levels of certain metals,such as Fe, Al and Cr in the tissues, which willalso have similar magnitude to those in the sediment(Bryan et al., 1985). We carried out statistical analysesof the metal concentrations in algae sampled at thesame time as the sediments. In Enteromorpha, therewere significant correlations (Spearman rank correl-ation coefficients) between these elements (Table 1),similar to the correlations between Fe–Al and Cr–Al inthe sediment. In Ulva, the concentrations of Cr werebelow the limit of detection, the levels of Fe and Alwere not significantly correlated, and the coefficientof correlation was very low. These results may indicatethat there was contamination by particulate material inEnteromorpha but not in Ulva.

Another way of estimating the extent of this prob-lem is to find the ratios of these metals in sedi-ment and in macroalgae, which should be of similarmagnitude if there is particulate contamination. Theaverage ratios of Cr:Fe in sediment and in Entero-morpha sampled simultaneously were similar (1:575and 1:438, respectively), whereas those of Cr:Al werevery different (1:2818 and 1:333). The ratio of Fe:Alwas different in algae and sediment: 1:0.76 in Entero-morpha, 1:0.49 in Ulva, and 1:4.9 in the sediment.

In conclusion, we believe that contamination byparticulate material in algal samples is not import-ant in Ulva, but may be of some importance inEnteromorpha.

Table 2. Percentages of recovery of reference materialMESS-1 (marine sediment). Extractions carried out in pre-vious studies in our laboratory with HNO3 in Teflon�bombs in a microwave oven

Barreiro (1991) Carral (1992) Real (1994)

Al 24 – 26

Fe 77 75 77

Mn 65 61 60

Cr 40 37 35

Table 3. Descriptive statistics of metal concentrations (µg g−1)in Ulva and Enteromorpha in the present study

Metal Mean Range SD C.V.

Ulva (n=282)

Al 175 2.14–872 145 85.7

Cu 7.84 2.18–20.6 3.80 48.4

Fe 365 38.6–2623 292 80.2

Mn 37.5 4.59–512 63.1 168

Ni 2.14 0.00–16.2 1.61 75.4

Zn 23.6 6.96–66.8 9.31 39.5

Enteromorpha (n=166)

Al 531 61.7–4198 441 83.1

Cr 1.59 0.196–5.84 0.764 50.0

Cu 10.58 4.00–48.12 5.56 52.6

Fe 696 103–5046 498 71.6

Mn 35.6 9.42–302 37.1 104

Ni 3.23 1.16–18.05 1.81 56.0

Zn 41.3 16.21–147 17.8 43.0

It must be taken into account that the same extrac-tion procedure was not used for macroalgae and sedi-ment (HNO3 for the former and aqua regia with HF forthe latter) and therefore comparisons of concentrationsand of correlations in macroalgae and sediment shouldbe viewed with caution. The use of HNO3 to extractmetals from sediments underestimates the actual con-centrations of Fe, Mn, Al and Cr compared with othermore aggressive techniques using different mixtures ofacids with HF, which are capable of breaking the crys-talline structure of the minerals where these metals arebound (Carral, 1992). Table 2 shows the low recov-ery rates of these elements extracted from sediment(reference material MESS-1) using only HNO3, inprevious studies in our laboratory. Therefore, the ex-tractions from algae, in which only HNO3 was used,the recovery of these metals, especially Al and Cr,

226

from any contaminating sediment would therefore bepartial, thus minimizing the problem.

Mean concentrations of metals in Ulva andEnteromorpha

The descriptive statistics of the metal concentrationsfound in Ulva and Enteromorpha are shown in Table3. The level of Cr in Ulva was, as already mentioned,below the limit of detection.

The mean concentrations of all metals in bothkinds of macroalgae were, in general, of similarmagnitude to those found by various authors in differ-ent species of Ulva and Enteromorpha in supposedlyclean areas (Tables 4 and 5). In some instances, themaximum value registered for a metal was similar tothat found in contaminated areas, as with Cu in En-teromorpha; the highest concentration of Zn (147 µgg−1) in this seaweed was close to the level given bySay et al. (1990) as indicating high ambient contam-ination (≥150 µg g−1), whereas the mean value weobtained for this metal (41.3 µg g−1) was below therange considered by these authors to indicate mod-erate contamination (50–150 µg g−1). However, ourvalues for Cu and Zn (both means, minima and max-ima) were very similar to those obtained by Catsiki &Papathanassiou (1993) in U. lactuca, measured duringseasonal sampling in a contaminated area over a periodof several years. The only values of Al levels in Ulva,that we found reference to in the bibliography (Güvenet al., 1992), were much higher than our maximumvalues. Our mean values were always lower than thoseobtained by Puente (1992) in a study carried out overa wide geographical area (including our own studyarea), in estuarine regions in northwest Spain, partic-ularly those of Cu and Ni levels, although differencesin the levels of Zn were not so pronounced. In Entero-morpha, the values obtained by this author were allhigher than in our study, particularly those of Cr, Cuand Ni.

Tables 4 and 5 show that the data are highlyvariable, in some instances the levels of metals insupposedly contaminated areas were lower than thosefound in other studies in supposedly clean areas. Itmust be taken into account that the variation in data inthe literature, as well as being due to different amountsof contaminants in the environment, may also be re-lated to factors such as the different analytical methodsused, seasonal variations, salinity in the sampling area,etc. The higher values reported by Puente (1992) maybe due to the fact that the samples came from estu-

arine areas where levels of contamination are usuallygreater, and possibly to lower salinity, which canlead to an increase in the concentration of metals inmacroalgae (Munda, 1984; Wang & Dei, 1999).

In both algae, the most variable metal, according tothe coefficients of variation, was Mn, followed by Aland Fe. The least variable was Zn, followed by Cu inUlva and Cr in Enteromorpha. It was, therefore, thosemetals most susceptible of being affected by humanactivity that displayed the most stable levels.

The mean concentrations in Enteromorpha werealways significantly higher (Mann–Whitney U test)than those in Ulva collected at the same time. Thisseems to be the most commonly found situation;Güven et al. (1993) found higher concentrations, ofall the metals studied, in Enteromorpha compressathan in Ulva lactuca. Agadi et al. (1978) report higherlevels of Co, Cu, Fe, Mn and Zn in Enteromorphaclathrata than in Ulva reticulata, with only levels ofNi and Pb being lower. Haritonidis & Malea (1995)also found higher levels of Ni and Cr in E. linza than inU. rigida, whereas the concentration of Co was moreor less similar. Brown et al. (1999) report significantlyhigher levels of Cu and Zn in E. intestinalis than in U.lactuca. Studies in which higher levels of metals arefound in Ulva than in Enteromorpha are less frequent;Wahbeh et al. (1985) report higher concentrations ofZn, Mn, Cu and Mg in U. lactuca than in E. clathrataand lower levels of only Cd and Fe.

A bar chart of the mean concentrations of somemetals in Ulva at the different sampling sites is shownin Figure 2. In Ulva, maximum values of Cu werefound at sites A3, A7 and V4, with all three beingabove 10 µg g−1. Sites A3 and A2 had the highestlevels of Ni. The highest levels of Zn were found atA7 and, again, at A3. The lowest levels of Cu werefound at M2 and A8. The lowest level of Zn (16.3 µgg−1) occurred at site A8 and there were many siteswhere levels of around 19 and 20 µg g−1 were found.Minimum levels of Ni were found at sites V1 and P5.

In Enteromorpha (Fig. 3) we also have data for Cr,which was at a maximum at site A3, possibly origin-ated from a tannery situated at the upper region of theriver Tambre estuary (Real et al., 1993), which waslocated close to this sampling site. Minimum levelsof Cr in this alga (only 0.94 µg g−1) were found atsite P4. The highest levels of Cu were found at theinnermost sites in the Ría de Vigo (V4, V2 and V3),whereas the lowest levels were found at sites A6, M2and M3. Maximum levels of Ni were at sites M4, P1and M3, and minimum levels at V4, A5 and A6. Max-

227

Tabl

e4.

Prev

ious

lyre

port

edco

ncen

trat

ions

ofm

etal

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gg−

1)

indi

ffer

ent

spec

ies

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dard

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anda

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viat

ion.

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tes

the

supp

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51.0

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ring

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16±2

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sp.

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(6.1

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Gal

icia

(N.W

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in)

Sum

mer

19U

lva

lact

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5.50

±0.3

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rgen

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CA

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n20

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0(S

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Ref

eren

ces:

(1)

Talb

ot&

Che

gwid

den

(198

2).(

2)W

ahbe

het

al.(

1985

).(3

)H

o(1

990)

.(4)

Ram

ırez

etal

.(19

90).

(5)

Aga

diet

al.(

1978

).(6

)H

äger

häll

(197

3).(

7)St

enne

r&

Nic

kles

s(1

974)

.(8

)St

enne

r&

Nic

kles

s(1

975)

.(9)

Lac

erda

etal

.(19

92).

(10)

Mah

er(1

986)

.(11

)G

üven

etal

.(19

93).

(12)

Kar

ezet

al.(

1994

).(1

3)M

unda

&H

udni

k(1

991)

.(14

)C

atsi

ki&

Papa

than

assi

ou(1

993)

.(15

)L

acer

daet

al.(

1985

).(1

6)H

arito

nidi

s&

Mal

ea(1

995)

.(17

)H

arito

nidi

s&

Mal

ea(1

999)

.(18

)G

üven

etal

.(19

92).

(19)

Puen

te(1

992)

.(20

)M

use

etal

.(19

99).

228

Tabl

e5.

Prev

ious

lyre

port

edco

ncen

trat

ions

ofm

etal

s(µ

gg−

1)

inE

nter

omor

pha.

(S.E

.)=

stan

dard

erro

r(S

.D.)

=st

anda

rdde

viat

ion.

‘STA

TE

’ind

icat

esth

esu

ppos

edde

gree

ofco

ntam

inat

ion

inth

est

udy

area

:C=

clea

n,P=

pollu

ted,

I=in

term

edia

te

Cr

Cu

FeM

nN

iZ

nL

ocal

itySt

ate

Seas

onR

ef.

Ent

erom

orph

asp

.11

.035

.0N

.Bra

sil

Ann

ualc

ycle

1E

.int

esti

nali

s0.

88–1

.55

Indi

aA

utum

n2

Ent

erom

orph

asp

.1.

60–1

.71

E.c

ompr

essa

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229

Figure 2. Mean concentracions of metals in Ulva at the different sampling sites. Error bars represent the standard error.

Figure 3. Mean concentracions of metals in Enteromorpha at the different sampling sites. Error bars represent the standard error.

Table 6. Spearman’s rank coefficients of correlation and corresponding levels of significance between metal concentrations in sediment(<0.2 mm fraction) and metal concentrations in algae. Mean values of all samples at each site were used

Al Cr Cu Fe Mn Zn

Metal from total fraction of sedimentUlva 0.3890 0.7854 0.4658 0.4647 0.4353

0.074 0.000 0.029 0.029 0.043

n.s. ∗∗∗ ∗ ∗ ∗Enteromorpha 0.2163 0.5302 0.6025 0.1417 0.6589 0.1462

0.334 0.011 0.003 0.529 0.001 0.516

n.s. ∗ ∗∗ n.s. ∗∗ n.s.

Metal from labile fraction of sedimentUlva 0.6680 0.8125 0.6014 0.1880 0.5313

0.001 0.000 0.003 0.402 0.011∗∗ ∗∗∗ ∗∗ n.s. ∗

Enteromorpha 0.5200 0.1858 0.5246 0.1282 0.3563 0.1621

0.013 0.408 0.012 0.570 0.104 0.471∗ n.s. ∗ n.s. n.s. n.s.

∗Significant at p<0.05, ∗∗Significant at p<0.01, ∗∗∗Significant at p<0.001, n.s.=not significant, n=22.

230

Table 7. Spearman’s rank coefficients of correlation and level of significance between normalized metal concentrations in sediment (<0.2mm fraction), and metal concentrations in algae. Mean values of all samples at each site, were used

Al Cr/Fe Cu/OM Mn/Fe Zn/OM

Normalized metal concentration (total fraction) in sedimentUlva 0.710 0.0435 0.240

0.000 0.848 0.282∗∗∗ n.s. n.s.

Enteromorpha 0.6093 0.418 0.0920 −0.2770.003 0.053 0.684 0.212∗∗ n.s. n.s. n.s.

Normalized metal concentration (labile fraction) in sediment

Al/Fe Cr/Fe Cu/OM Mn/Fe Zn/Fe

Ulva 0.6126 0.721 −0.5370 0.39580.002 0.000 0.010 0.068∗∗ ∗∗∗ ∗ n.s.

Enteromorpha 0.4444 0.1654 0.292 −0.1158 0.2682

0.038 0.462 0.187 0.608 0.227∗ n.s. n.s. n.s. n.s.

∗Significant at p<0.05, ∗∗Significant at p<0.01, ∗∗∗Significant at p<0.001, n.s.=not significant, n=22.

imum levels of Zn were found at sites P1, V1 and V3,and minimum levels at A6, A4 and A5.

Relationship between metal in sediment and inmacroalgae

By comparing the means of the two sediment samplesand the annual mean of all the algal samples at eachsampling site, we found that there were significant cor-relations for many of the metals, particularly in Ulva(Table 6). This may appear unusual in the absenceof particulate contamination, as the uptake of metalsby seaweeds is mainly of metals in solution. How-ever, it should be considered that, to a certain extent,the concentrations of metals in sediments reflect thedegree of contamination in the water column, and itis, therefore, not surprising that there was a signific-ant correlation between certain metal concentrations insediment and in algae. Furthermore, it may be that partof the metal was scavenged from the sediment (Luomaet al., 1982).

As previously mentioned, Enteromorpha is moresusceptible to contamination by particulate material,and it might be expected that there would be highercorrelations between metals in sediments and in thisseaweed than in Ulva. However, stronger correlationswere almost always found in Ulva, with the strongestbeing for Cu in both fractions. In Enteromorpha, the

coefficients were almost always lower, confirming thatcontamination by particulate material was of little im-portance. It is possible that the lower correlations inEnteromorpha were due to the shorter and thus lessrepresentative sampling period than with Ulva.

Of the significant correlations obtained, strongerlevels were found, between Al in both algae, andCu, Fe and Zn in Ulva, and the labile fraction of thesediment. This is not surprising as the labile metalicfractions in sediments are considered to be the mostbioavailable (Lacerda et al., 1992). In contrast, Mnin both macroalgae, and Cr and Cu in Enteromorphawere better correlated with the total metal fraction insediment.

Metals are not distributed in a uniform way insediment and are generally associated with the finerfractions because of a combination of physical factors,such as surface area, and chemical factors, such asgeochemical substrates (Horowitz, 1991). It is, there-fore, necessary to normalize the metal concentrationsto be able to interpret the results correctly. We foundthat in these sediments, depending on the metal andmethod of extraction, Fe and organic matter are goodnormalizers, except for Al in the total fraction (Villareset al., submitted).

Contrary to our expectations, most of the coeffi-cients of correlation for the normalized data decreased(Table 7). There was an increase in the correlation

231

between Cr in sediment (total fraction) and Entero-morpha. The correlation between Mn in sediment(labile fraction) and in Ulva was significant but witha negative sign. These results are in contrast withthose of other authors, which explain the bioavailab-ility of metal levels in sediments better once correctedby a normalizing element (Luoma & Bryan, 1978;Langston, 1986; Puente et al., 1994).

Conclusions

Our results show that contamination by particulate ma-terial may be of some importance in Enteromorpha butnot in Ulva; this effect was lessened by the method ofextraction used in the macroalgae, which is less ag-gressive than that required to release all of the metalfrom the sediment. The mean concentrations of metalsin both algae were generally similar to those found inother studies with different species of these genera inclean areas. A large number of significant correlationsbetween concentrations of metals in sediment and inmacroalgae were found. In contrast to our expecta-tions, the corrrelations between normalized concentra-tions of metals in sediment and metal concentrationsin algae were generally lower than those obtained withthe non-normalized data.

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