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UNIVERSITY
OF
SOUTHAMPTON
THE
BIOGEOCHEMISTRY
AND
DISTRIBUTION
OF
DISSOLVED
TRACE
METALS IN THE
AEGEAN SEA
by
Virginie
Hart
Thesis submitted in
partial
fulfilment
of the
requirements
for
the
degree
of
Doctor of
Philosophy
SCHOOL OF OCEAN
AND EARTH
SCIENCE
FACULTY OF SCIENCE
ABSTRACT
Samples
for theanalysis
of dissolvedMn, Co, Fe,
Pb,
Cu,
Ni, Cd,
Zn,
nitrateplus
nitrite,
silicon andphosphate
were collected at 7 stations in the northern and 6 stations in the southernAegean
Sea,
easternMediterranean,
in both March andSeptember
1997. Thisrepresents
the first extensive dissolved trace metal data set for thisregion.
Additionalsamples
were collected nearhydrothermal
vents inMilos,
andacross the Thermaikos Gulf. Trace metal distributions and concentrations were
interpreted
withrespect
to theparticularly complex hydrography
and circulationpatterns
of theAegean
Sea.The results show elevated surface concentrations of
Mn,
Co,
Fe,
Pb, Cu,
Ni and Znwhichcanbe attributedtobothatmospheric deposition
in the southernAegean
andacombination of
atmospheric deposition,
the outflow of the Black Sea Water andpotential
coastalinputs
within the northernAegean.
Spatial
andtemporal
metalvariability
in surface waters wasobserved,
withhigher
concentrations for all metals inthe northern
Aegean
due to additional surface sources. InSeptember
1997 the overallhigher
concentrations ofMn,
Co, Pb,
Cu,
Ni andZn are attributed totheshallowing
of thethermocline locatedatapproximately
100 min Marchto around 50 minSeptember,
although
thepossibility
of enhanced aoliandeposition
isnotexcluded.Elevated
signatures
of dissolvedMn,
Co,
Fe and Pb are associated with BlackSea Water
and,
to a lesser extent with Levantine Intermediate Water in the northernAegean
for March 1997. In the morecomplex hydrography
of the southAegean
theelevated dissolved nitrate
plus
nitrite and silicon are associated with an intrusion ofTransitional Mediterranean Water. In
addition,
dissolved trace metal results from Thermaikos Gulf and Milos do notsuggest
asignificant input
of metals from thesesourcesinto the
Aegean
Seaduring
theperiod
ofsampling.
Atmospheric
metal fluxes for the central and north-western Mediterranean Seawere used to calculate the residence times in the
Aegean
surface waters. These fluxeswere
compared
with metal fluxes from sedimenttraps
andparticulate
metal data in thenorthern and southern
Aegean
during
theperiod
ofsampling.
Linearregression analysis
was
applied
to Black Sea influenced surface water waters in the northernAegean
inorder to estimate the concentrations of
Mn,
Co,
Fe,
Cu and Ni in the Straits of Dardanelles.For the metals
Mn,
Fe and Pb the calculated short residence times in southernAegean
surface waters of 0.3 to 6.0 years reflect theirhigh particle reactivity
andscavenging
ontoparticles
andtransport
todepth.
Co exhibits a similarcycling
pathway
toMn but with
longer
surface residencetimesof 4to40 years. The vertical distributions ofNi, Cu,
Cd andZn donotresemble those of nutrients.Ni,
Cu,
andZnexhibit surface elevation in the northernAegean,
andtoalesserextentin the southernAegean,
whereas Cddepth profiles
werefoundtobehomogenous.
Theseprofiles
arethe result ofsurfacemetal sources combined with the
oligotrophic
nature of theAegean
Sea. Thepresent
worksuggests
that theAegean
Sea,
as well asthe Mediterranean is notinasteady
statewith
respect
toNi, Cu,
Zn and Cd. Concentrations in the southernAegean
are overallcomparable
to recenthigh quality
observations in the open Mediterranean.However,
there appears to be an increase in dissolved Cu and Ni from the western to the eastern
ACKNOWLEDGEMENTS
There were moments when I
seriously
wondered whether this work would everbe
finished,
and itscompletion
is thanks to thesupport,
friendship
andguidance
ofnumerous
people. Firstly
I'd liketothank mysupervisor,
Dr. PeterStatham,
who has atall times been available with advice and has
inspired
me with his enthusiasm forscientific research. Also I thank Dr.
Doug
Connelly
andDr. NickMorley
for theirtracemetal
expertise,
support
andhumour,
and forshowing
me that there ishopefully
lifeafter trace metals. Dr. Miles Finch and Dr Paul
Wright
helped
immensely
with the dissolved nutrientanalysis
and I wish for Miles that all his futurehopes
will cometrue. Kate Davieskindly prepared
all themaps of theAegean.
Thanks toSophie Fielding
forbeing
'in theright place
at theright
time'. I have also reliedimmensely
on Dr MikisTsimplis,
whoseknowledge
on thephysical
oceanography
of theAegean
andMediterranean Sea has been essential for a broader
interpretation
ofmy data. Also Ithank Luca Centurioni for
helping
mekeep
mysanity
andDr. AndonisVelegrakis
forhis 'non-linear' attitudeto
science,
friendship
and advice. But tellmeAndonis,
I still donotknow what 'The
point
is...'?From
beyond
theSOC,
I'd like to thank Dr. Brian Price and FrancisLindsey,
Edinburgh University,
for verykindly providing
me with theirparticulate
metaldata,
sediment
trap
fluxes andreports.
Especially
thank you to Brian for his time andexpertise
ininterpreting
thetracemetal results.From
NCMR, Athens, Greece,
I'd like to thank Dr. Vassilis Zervakis and Dr. ManolisPapageorgiou
forproviding
CTD data for theperiod
ofsampling,
especially
Vassilis forproviding
additional information anddiscussing
future work on lateralfluxes for the northern
Aegean.
I'd also liketothank thecrewof theNCMR,
RVAegao
for their
help during
bothcruises,
the excellent food andespecially
theplate smashing
evening,
anda chance toexperience
thedelights
of theAegean
Sea and islands. I wasfortunate to meet Nadia
Papadopoulou
and Dr. Chris Smith fromIMBC, Crete,
and would like to thank them for theirfriendship
and theirhospitality
inCrete,
for both work and recreation.Finally,
close to my heart I'd like to thank myparents
for all theirsupport.
ToTracy,
Ithankforbeing
mysister and forbeing absolutely amazing.
Also Martin Esseen for hisfriendship,
encouragement
andinspiration.
To Sandra Freeh for herfriendship
andsupport
lastFebruary
andallowing
me into her world ofpassion,
colour and chaos.To
Manolis,
words are ofno use here other than to say thank youletting
me into yourLIST OF CONTENTS
CHAPTER1
INTRODUCTION TO TRACE METALSIN THE MARINE ENVIRONMENT
1.1 Introduction 1
1.2 Trace metals in rivers andestuaries 2
1.3 Trace metals in theocean 5
1.3.1. Conservative metals 5
1.3.2
Recycled
metals 61.3.3
Scavenged
metals 71.4Non-riverinetracemetal sourcestotheoceans 10
1.4.1
Atmospheric
sources 101.4.2
Hydrothermal activity
111.4.3 Benthicsources 12
1.5
Recycling
and removal oftracemetals 131.4.1
Absorption
and scavenging,-Particle interactions 131.4.2
Biological cycling
141.6 Recenttrace metal data for the Mediterranean and the Black Sea 15
1.7 Availabletracemetal data in the
Aegean
Sea 171.8
Objectives
19CHAPTER2
THEHYDROGRAPHIC ANDBIOGEOCHEMICAL BACKGROUND OF
THE AEGEAN SEA
2.1
Geomorphology
of theAegean
Sea 202.2 Water Masses of the
Aegean
Sea 222.2.1 Surface Waters 22
2.2.2 Intermediate Waters 23
2.2.3
Deep
waters 242.3 Generalcirculationof the
Aegean
Sea 262.3.1 Northern
Aegean
262.3.2 Southern
Aegean
262.4Freshwater
inputs
totheAegean
272.5 Climate of the
Aegean
282.5.1 Winds 28
2.5.2 Air
temperature
292.5.3
Evaporation
292.5.4.
Precipitation
29CHAPTER 3
SAMPLINGANDANALYTICAL PROCEDURE
3.1 The database for the
study
323.2
Sampling
andhandling
ofseawatersamples
for dissolvedtracemetals inseawater 36
3.2.1 CTD
Sampling
363.2.2 Pole surface
sampling
373.3.
Development
ofanalytical procedure
for total dissolvedtrace metalanalysis:
54Mn
additionsto seawatertocalculateefficiency
of chloroform extractionmethod 37
3.3.1 Generation of
reagents
andnecessaryapparatus
383.3.2 Method 1 39
3.3.3 Method2 39
3.3.4 Results 40
3.4
Adapted
method for total dissolvedtracemetalanalysis
inseawater 40 3.4.1 Generation ofreagents
and necessaryapparatus
413.4.2 Procedure 43
3.4.3 Measurement
by
atomicabsorption
spectrometry
44 3.4.4Calculation ofsample
concentration and methodperformance
463.5
Auxilarydata
463.5.1
Salinity
463.5.2. Nutrients 47
CHAPTER4
HYDROGRAPHIC AND CHEMICAL SETTING: RESULTS
4.1 Introduction 48
4.2 The north
Aegean
484.2.1.
Salinity,
temperature
anddensity:
Watermasses and circulation 484.2.2Nutrients 53
4.2.3
Chlorophyll
a. 564.3 Thermaikos Gulf. 57
4.3.1.
Salinity,
temperature
anddensity:
Watermassesand circulation 574.3.2 Nutrients and
Chlorophyll
a. 604.4 The south
Aegean
604.4.1.
Salinity,
temperature
anddensity:
Watermasses and circulation 604.4.2 Nutrients 68
4.5MĂśOS 71
CHAPTER5
TRACE METALSIN THEAEGEAN SEA: RESULTS
5.3
Mixing
series for metals between the Dardanelles and open northAegean
875.4 Thermaikos Gulf. 90
5.5 Trace metal results: The south
Aegean
(Cretan
Sea)
935.5.1
Manganese
and cobalt 935.5.2 Iron and lead 96
5.5.3
Copper,
nickel and cadmium 975.5.4 Zinc 101
5.6MĂśOS 101
5.7
Atmospheric inputs
tothe north and southAegean
104 5.7.1Atmospheric
tracemetal flux calculations 104 5.7.2Residence times calculations for surfacewaters 1055.8 Particulate data for the
Aegean
sea 1085.8.1 Particulate metal fluxes
(Fe,
Mn, Cu,
ZnandPb)
1085.8.2
Summary
ofparticulate
data IllCHAPTER 6
DISCUSSION
6.1 Introduction 113
6.2 Trace metal distributionand
biogeochemistry
in theAegean
1136.2.1
Manganese
and cobalt 1136.2.2Iron 117
6.2.3 Lead 120
6.2.4
Nickel,
copper and cadmium 1226.2.5 Zinc 125
6.3
Major
processes and/or sourcesinfluencing
the distribution oftracemetals inthe
Aegean
Sea 1276.3.1
Atmospheric
and coastal/BSWsources 1276.3.2 Circulation and
mixing
ofwatermasses 1306.3.3
Hydrothermal
activity
1326.3.4 Nutrients and the
oligotrophic
statusof theAegean
Sea 133 6.4Implications
of theAegean
seadata for the Mediterraneanseatracemetalbiogeochemistry
136CHAPTER 7
CONCLUSIONSAND SUGGESTIONS FOR FUTURE WORK 141
REFERENCES 144
APPENDIX I
Results of
54Mn
additionsto seawatertocalculateefficiency
of chloroformextraction method 156
APPENDIX n
LIST OF TABLES
Table 1.1 Concentrations of dissolvedtracemetals found in the North Atlantic
and Mediterranean Sea 16
Table 2.1
Major
riverineinputs
into theAegean
Sea 27 Table 2.2 Nutrient concentrations observed in the Eastern Mediterranean andAegean
Sea 30Table3.1
Summary
of CTD and surfacesample
stations in theAegean
Sea. Polesamples
are indicatedas 1 or + 1sample depth
33Table 3.2 The recovery efficiencies of
Manganese-54
inseawaterby
pre-concentration:
comparing
back extractionvs.evaporation
methods 40Table 3.3 Standard conditions for GFAAS 45
Table 3.4
Typical
blanks,
detectionlimits,
standard deviations andspike
recoveries 46
Table 3.5 Nutrient detection limits and
precision
47Table 4.1 Mainwatermasses of the northern and southern
Aegean
Sea with theircharacteristics 74
Table 5.1
Summary
oftrace metalsignature
concentrations forNAEGDW,
LIW and BSW in the northAegean
for March andSeptember
1997 76 Table 5.2 Linearregression
ofsalinity
metalrelationship
for BSWatstationsMNB5 and MNB6 in the north
Aegean
88Table 5.3 Linear
regression
calculation of metal concentrationatthe Straits ofDardanelles with the enrichment factor
(EF)
89Table 5.4
Summary
oftrace metal concentrations indeep
and surfacewaters inthe south
Aegean
for March andSeptember
1997 93Table 5.5
Comparison
between surface metal concentrations atMilos and theCretan Sea
(MSB)
and enrichment factor's(EF)
for March andSeptember
1997.. 104 Table 5.6Range
of total dissolvedatmospheric
tracemetal fluxesto surfacewaters
(calculated
fromwet anddry deposition) using
data from the central Mediterranean(1)
(Guerzoni,
etal,
1997)
and north-west Mediterranean(2)
(Guieu,etal, 1997)
105Table 5.7 Residence times of dissolvedtracemetals in surfacewatersin the north and south
Aegean
for March andSeptember
1997 and the Adriatic Sea(Tankere,
1998)
107LIST OF FIGURES
Figure
2.1 TheAegean
Sea 21Figure
3.1 MATERsampling
stations of theAegean
Sea 34Figure
3.2Sampling
stations for the northAegean
Sea 35Figure
3.3Sampling
stations for the southAegean
Sea 35Figure
4.1Salinity
andtemperature
depth
profiles
for the northAegean
Sea 49Figure
4.2Density depth profiles
for the northAegean
Sea 50Figure
4.3 6/Splots
for northernAegean
(MNB)
stations 51Figure
4.4Nitrateplus
nitrite and silicondepth profiles
for northernAegean
Seain March and
September
1997 54Figure
4.5Chlorophyll-
depth profiles
forthe northAegean
Sea 55Figure
4.6Salinity,
temperature
anddensity depth profiles
forThermaikosGulf. 58Figure
4.7Nitrateplus
nitrite,
silicon andchlorophyll-a depth profiles
forThermaikos Gulf. 59
Figure
4.8Salinity
andtemperature
depth
profiles
for the southAegean,
inMarch and
September
1997 62Figure
4.9Density depth profiles
for the southAegean
63Figure
4.10 0/Splots
for southAegean,
in March andSeptember
1997 64Figure
4.11 Contourplots
foraneast-west transectacrossthe CretanSea,
September
1997 65Figure
4.12 Nitrateplus
nitrite and silicondepth profiles
for the southAegean
Sea in March and
September
1997 69Figure
4.13 Nitrateplus
nitrite and siliconcontourplots along
an east-westtransectacrossthe Cretan Sea in
September
1997 70Figure
4.14Salinity,
temperature,
density,
nitrateplus
nitrite and silicondepth
profiles
forMilos 73Figure
5.1Manganese
depth profiles
fora)
March andb) September;
and Mnversus
salinity
forc)
March andd) September
1997 in the northAegean
78Figure
5.2 Cobaltdepth profiles
fora)
March andb)
September;
and Coversussalinity
forc)
March andd)
September
1997inthe northAegean
79Figure
5.3 Irondepth
profiles
fora)
March andb)
September;
and Feversussalinity
forc)
March andd) September
1997 in the northAegean
81Figure
5.4 Leaddepth profiles
fora)
March andb) September;
and Pbversussalinity
forc)
March andd) September
1997inthe northAegean
82Figure
5.5Copper
depth profiles
fora)
March andb)
September;
and CuversusFigure
5.6Nickeldepth profiles
fora)
March andb) September;
and Niversussalinity
forc)
March andd) September
1997 in the northAegean
5
Figure
5.7 Cadmium and Zincdepth
profiles
for March andSeptember
1997 in the northAegean
og
Figure
5.8Manganese, iron,
cobaltand leaddepth profiles
forThermaikos Gulf. ^Figure
5.9Copper,
nickel,
cadmium and zincdepth profiles
for Thermaikos Gulf92
Figure
5.10Manganese
and cobaltdepth
profiles
fora)
March andb)
September
1997inthe southAegean (Cretan Sea)
94
Figure
5.11Manganese
contourplot along
aneast-west transectacrosstheCretan
Sea,
September
199795
Figure
5.12 Iron and leaddepth profiles
fora)
March andb) September
1997 in the southAegean
Seagg
Figure
5.13Copper
andnickeldepth profiles
fora)
March andb)
September
1997 in the south
Aegean
Sea99
Figure
5.14 Cadmium and zincdepth profiles
fora)
March andb) September
1997 in the southAegean
SeaFigure
5.15Manganese,
iron,
cobalt and leaddepth profiles
atMilosABBREVIATIONS
(in
alphabetical order)
BSW Black SeaWater
CDW Cretan
Deep
WaterCIW CretanIntermediateWater CTD
Conductivity,
temperature,
depth
EMDW Eastern MediterraneanDeep
WaterLDW Levantine
Deep
WaterLIW Levantine Intermediate Water
LSW Levantine Surface Water MAW ModifiedAtlantic Water MSW Mediterranean SurfaceWater
NAEGDW North
Aegean Deep
WaterRT Residence time
SD Standard deviation
CHAPTER
1
INTRODUCTION
TO TRACE METALS IN
THEMARINE
ENVIRONMENT
1.1 INTRODUCTION
Since 1975 there have been
major
advances in theknowledge
oftrace metal concentrations and distributions in seawater(Brewer,
1975).
Prior to this little wasknown other than that trace metal concentrations are very low
(less
than micromolarlevels),
with observed distributionsexhibiting
littlerelationship
withbiological,
chemical and/orgeochemical
oceanographic
processes. Thisoceanographic
inconsistency
wasmostly
due to lack of instrumentalsensitivity
andproblems
of contamination(Bruland, 1983).
The'quantum
leap'
in theunderstanding
oftrace metal distributions isprimarily
due to the advent ofmethodologies
in the cleanhandling
andanalysis
ofsamples, along
withimprovements
inanalytical
methods. As aresultnearly
all trace metals were one to three orders of
magnitude
less than what had beenpreviously
estimated,
whereas metal distributions indicatedpatterns
consistent with oceanic processes, as demonstratedby
the collection of papersby
Wong
et al.(1983).
Thorough
reviews have also beenprepared by
Bruland(1983),
Whitfield & Turner(1987),
Burton & Statham(1990)
and Donat & Bruland(1995).
In the marine
environment,
wherephysical, biological
and chemical processesinter-relate,
a morecomplete understanding
of the effects of sedimentdynamics,
biological
activity,
transport
and formation of water masses,atmospheric
and freshwater interactions anddissolved/particulate inorganic/organic
marinechemistry,
are
required
tofully comprehend
the role oftracemetals in thedynamics
of theocean.1.2TRACEMETALSIN RIVERS AND ESTUARIES
Riverine runoff is a
major
sourceof dissolved andparticulate
trace metalsto the shelf seas. Each river can varyconsiderably
insize, altitude,
gradient,
flowregime,
turbulence,
temperature,
substratetype,
turbidity
and waterchemistry.
Within each catchment thegeographic
location,
rocktype,
vegetation,
climaticregime
and man's activities will influence the riverine flow andcomposition
(Hart
&Hines,
1995).
This results inlarge
variations of concentrations and fluxes of trace metals from the catchmentareatothe riverorstream.Natural sources of metals include the
weathering
ofsoils and baserocks,
a slowprocess which is influenced
by
rainwater madeslightly
acidicthrough
the solution ofCO2
from theatmosphere (Stumm
&Morgan,
1981);
and from theatmosphere,
in bothwet and
dry precipitation.
It has been observed in manyparts
of the world that anincrease in the
acidity
of rain can accelerate the 'natural'weathering
process in anumber of catchments
(Hart
&Hines,
1995).
Humanactivities,
such asagriculture
andanthropogenic
discharges
fromindustries,
are themajor
contributors oftrace metals torivers. For this reason, environmental
protection
authorities in most countries areplacing increasingly stringent requirements
onthetrace element concentrationsallowed in effluentsdischarged directly
into rivers.In the riverine
environment,
physical, biological
and chemical interactions control the behaviour oftrace metalsby
abiotic orgeochemical
processes(such
as forCu, Pb, Zn,
andCd)
orby
bothbiological
andgeochemical
processes, such as for FeandMn
(Hart
&Hines,
1995).
Trace metals can exist in anumber ofphysico-chemical
forms in natural waters: free
aquatic
ionicforms;
'dissolved'inorganic
ororganic
complexes; complexes
with colloidal orparticulate
matter; andcomplexes
associated with the biota(e.g.
phytoplankton
andbacteria).
Thespeciation
canhave amajor
effect onthebehavior,
bio-availability
andtoxicity
ofatrace metal. Forexample,
a free ionicform is more
likely
tobecome associated withsediments, suspended
particulate
matter(SPM)
orcolloids,
than when it isalready complexed
toorganic
matter.Most studies of trace metals inaquatic
systems
assume that chemicalequilibrium
exists(Salomons
&Förstner, 1984),
although
the kinetics will varydepending
on whether the reaction issurfaces.
Sorption
reactionsinvolving
colloidal matter, SPM and sediments are asignificant
influence ontrace metal behaviour andcanresult in the removal from riversby
flocculation(Sholkovitz,
1978;
Sholkovitz &Copeland, 1981).
Comparing
theatmospheric
and riverine contributions of trace metals to the western MediterraneanSea,
Guieu et al.(1997)
calculated fluxes ofCd, Co, Cu, Fe,
Mn,
Ni and Pb from the Rhone and Evros. Concentrations were observed to varygreatly,
with atendency
to decreaseduring
increased riverdischarge.
Theparticulate
fluxes calculated were found to begreater
than dissolved fluxes for allelements,
indicating
theimportance
ofparticles
as a source of metals for the shelfseas and theocean.
The behaviour of trace metals in
rivers,
as well as in the ocean, iswidely
accepted
to bepoorly
understood anddifficulttopredict
(Donat
&Bruland, 1995;
Hart &Hines,
1995).
Much of thepreviously
recorded data has been affectedby
contaminationduring sampling,
and the real concentrationsarerecently
shownto be far lower(Bruland,
1983;
Donat &Bruland,
1995).
Inaddition,
mostriver studies(Hart
&Hines,
1995)
still use asimple separation
method(based
on membrane filtrationusing
0.4 m
filters)
to separate the total element concentration into'particulate'
and 'dissolved'forms,
when in fact a considerable fraction of colloidal material passesthrough
the 0.4 m filter.Estuaries encompass the river-ocean
interface,
aregion
which is bothchemically
and
physically dynamic.
Pronouncedbiogeochemical reactivity, including sorption,
flocculation and redoxcycling
oftracemetals,
is inducedby sharp gradients
in the estuarine variable ofsalinity,
temperature,
dissolveO2,
pH,
andparticle
character and concentration that result from themixing
of fresh and saline end members(Martin
&Whitfield, 1983;
Millward &Turner,
1995).
These processesdrastically modify
the riverinecompositional signal
to the oceans and must befully
understood toaccurately
determinetracemetalfluxes.
salinity)
hasasignificant
role in estuarinetracemetalreactivity.
For thethermodynamic
equilibria
of in situ chemical reactions to takeplace,
theflushing
time of theestuary
mustbe belowacertain level asexplained by
Morris(1990).
The
particulate
phase plays
acomplex
yet
important
role in thecycle
oftrace metals inestuaries,
consisting
of various mineralphases
derived from the catchment and coastal erosion. Inaddition,
discrete andbiogenic phases
areproduced
in situthrough
flocculation andbiological activity respectively.
The increase insalinity, along
with increases in the concentration of themajor
sea-watercations,
can lead todesorption,
or flocculation and sedimentation ofelements,
such asFe,
which exist inriverwater
mainly
in acolloidal state.Transport
of theparticulate
phase
ishighly
non-conservative and
particles,
together
withparticle-reactive
tracemetals,
may besubjected
to manydeposition-resuspension cycles
in anestuary,
thereby residing
considerably longer
thanwater. Residence times ofsuspended
particles
of 18 years have beencalculated in the HumberEstuary
and in theturbidity
maximumzone of the Tamarestuary,
to be 1.4 years(Millward
&Turner, 1995).
This has the effect of'trapping'
tracemetals within theestuary
before finaltransport
tothe shelfedge
andoceans.Some dissolved trace metals are
simply
diluted uponmixing
of river and seawater
(conservative
behaviour),
whereasothersundergo dissolved-particulate
reactions,
which leadto their additionto, or removal from the dissolvedphase
(non-conservative
behaviour).
The behaviourpatterns
of each metal will behighly dependant
on localconditions and can vary between
(as
well aswithin)
eachestuary.
Sholkovitz &Copeland (1981)
determined thepercentage
oftrace metal removal from riverine waterby
flocculation,
finding significant
removal forFe and Cu(80%
and40%),
moderate for Ni and Cd(15%)
andnone for Mn and Co. Cu and Ni havecompetitive
affinities withsea water cations and will therefore be
removed,
whereas Cd and Co formstrong
chloro-complexes
in sea-water and are removedonly
to a small extent. In the Rhone and Evros estuaries in theMediterranean,
Co and Ni exhibit conservative behaviour(Zhang
&Wollast,
1993).
Millward & Turner(1995)
compared
the behaviour oftrace metals in severalestuariesand found that the axial distributions of each metal exhibited considerable inter-estuarinevariability,
with theexception
ofiron,
consistently
removedvariability
ofMn,
Co,
and Pb betweensummerand winter alsohighlights
changes
in theregimes
of solution and solidphase
interactions in estuaries(Morley
etah,
1990).
1.3 TRACE METALSIN THE OCEAN
There have been
major
advances in theknowledge
oftracemetal concentrations and distributions in seawater since the mid 70s(Bruland,
1983;
Whitfield &Turner,
1987;
Burton &Statham, 1990;
Donat &Bruland,
1995).
It is now known that in theopen ocean, dissolved trace metal concentrations are very low
(down
to nM and evenlower),
and such low concentrations haverequired
improvements
inanalytical
methods,
sampling
andsample
handling.
The classification oftracemetals
according
totheir behaviour in the ocean hasbeen well established. Bruland
(1983)
and Libes(1992) adopt
aclassification based on7
types,
whereas Whitfield & Turner(1987),
Burton & Statham(1988),
and Donat & Bruland(1995)
reduce thistothreemajor
categories
whichwill be discussedbelow.1.3.1 CONSERVATIVE METALS
These metals behave
conservatively,
thatis,
they
exhibit anessentially
constantconcentration with
regards
tosalinity
asaresult of their lowreactivity
inseawater. This lowgeochemical reactivity
isexpressed
in terms ofahigher
mean oceanicmixing
time(>105
years)
incomparison
to the time scales ofwatermixing
processes found in theocean. These metals behave
essentially
likemajor
constituents ofseawater, but at farlower concentrations due to the lower levels in source materials such as crustal rock.
The alkali trace metals
lithium,
rubidium and caesium fall into thiscategory
butthey
are not under review in this
work,
andthey
will not be discussed(see
Burton and1.3.2 RECYCLEDMETALS
The
recycled
metals(previously
referred to as 'nutrienttype')
exhibit a closecorrelation in behaviour with the micronutrients
phosphate,
silicate and nitrate as aresult of their involvement in the
biogeochemical
cycle.
Inaddition,
the concentrations ofrecycled
metals indeep
waters increasealong
the direction of the main advective flow of thedeep
water in the worlds oceans.Consequently,
their concentrations in thedeep
North Pacific arehigher
than indeep
North Atlantic waters(Bruland
&Franks,
1983).
This feature is due to the concentrationbuild-up
of elements which aretransported
intodeep
water, in association withbiogenous
particulates
thatundergo
decomposition
or dissolution processes that return material to the water column. The residence times of therecycled
elements are intermediate(lO3
to lO5years) compared
with conservative and
scavenged
elements(Donat
&Bruland,
1995). Recycled
metalsinclude
cadmium, zinc,
nickel and copper.Cadmium has been shown to have a nutrient
type
distribution in the ocean(Bruland
&Franks,
1983), exhibiting
a close correlation withphosphate.
Ithas,
however,
an unknownbiological
functionbut it shows astrong
affinity
forbiogenous
particulate
material(Burton
&Statham,
1988). Although
Cd has no biochemicalfunction of its own, there is now
strong
evidence for its utilisationby phytoplankton
inZn-depleted
surfacewaters,
where Cdreplaces
Zn in some enzymes without loss ofenzyme
activity
(Croot
&Hunter,
1998). Typical depth
profiles
such as in the western North Atlantic(Bruland
&Franks,
1983)
exhibit markeddepletion,
with values less than 0.05 nM in surface waters followedby
shallow water(<1000
m)
regeneration.
Maximum concentrations of 0.3 nM in the North Atlantic and 1.1 nM in the North Pacific have beenobserved
(Bruland
&Franks,
1983).
This distribution is characteristic ofphosphate
andnitrate whicharedepleted
atthe surface and released dueto oxidation and dissolution of soft tissue ofsinking biogenic particulates.
Thecadmiunrphosphate
ratio is notconstantbetween oceans,being significantly higher
in the olderdeep
waters. ofthePacific. In
addition,
the actual metal and nutrient concentrations differ as aresultof
deep
water circulationpatterns
of the oceans, with thehighest
values in the olderZinc also exhibits nutrient
type
behaviour,
showing
a close correlation withsilicon
(Bruland
&Franks, 1983),
andalong
residence time of50,000
years(Ruiz-Pino,
et
ah,
1991).
Zinc isstrongly depleted
atthesurface,
withdeep
waterregeneration
andfairly
constantconcentrationsbelow2000m. Thisprofile
indicates that themajor
carrierphase
is skeletal material.Deep
water values are 1.7 nM and 9 nM for the North Atlantic and North Pacificrespectively.
The observed difference in the Zn:Si ratio in these two oceans indicate that carrierphases
other thanbiogenous
silica contribute tothe down column
transport
ofthe metal(Burton
&Statham,
1988).
Nickel in the ocean shows a considerable
degree
of nutrient-likebehaviour,
but with much less marked surfacedepletions
than cadmium and zinc and lessregeneration
below 1000 m. Bruland & Franks(1983)
observed surface concentrations of 2-5 nM in the North Atlantic and NorthPacific,
with concentrations of 5.7 nM and 10.4 nM between 1000 - 3000 mrespectively.
The nickel:nutrient ratio ishighest
in the North Atlantic.Saager
et al.(1997)
observed a closerrelationship
of Ni withphosphate
and nitrate in theAtlantic,
concluding
that thebiogeochemical
cycles
of Ni and Si are notrelated.
Copper
has a shorter residence time(lO3
years)
than nickel and zinc and therange of factors
influencing
its distribution isgreater
than the other metals considered here(Burton
&Statham,
1988). Depth profiles
show nutrienttype
behaviorwithsurfacedepletion
andgradual
regeneration
withdepth, thought
to be due tobiological uptake
within the surfacemixed
layer
andsignificant diagenetic
remobilizationatthe sedimentwater interface. The
regeneration
fromsinking
primary products
is maskedby
scavenging
onto fineparticles.
Forscavenged metals,
absorption
ontoparticles
causethe older
deep
waters ofPacific ocean to have lower concentrations than the youngerAtlantic waters. However this is not the case for copper due to the
significant
sourcefrom the sediment water interface. The
magnitude
of the dissolved copper flux fromdepth depends
onthe sedimenttype
i.e. redclays
can contributegreater
fluxes of Cuscavenging
(Whitfield
& Turner1987).
Concentrations will decreasesignificantly
away from the sourceand, thus,
North Pacific concentrations are lower than those in theNorth Atlantic.
Examples
ofscavenged
metalsaremanganese,cobalt,
iron and lead.Manganese
distributions in sea water reflect theirhigh particle reactivity
and,
hence,
short residence times. Atypical depth profile
is characteristic ofasingle
source to the surfacelayer coupled
with oxidativescavenging
ontoparticulate
matter. Mn concentrations decrease with the age ofdeep
water due to continuousscavenging
(Landing
&Bruland,
1987).
The dissolvedMn(II)
oxidises to insoluble Mn(in, IV)
oxyhydroxide
coatings,
oftenquite amorphous
and mixed with solid Fe(III)
oxides(Landing
&Bruland, 1987;
Lewis &Landing,
1991; Saager
etal,
1997).
Thesefreshly
formedferro-manganese
oxidecoatings
are deemed to be very efficient absorbers for additional Mn andFe,
as well as other metals from seawater. Given such efficient removal ofMn from the watercolumn,
local sources and sinksstrongly
influence thedistribution of Mn
(Saager,
1997).
Deviations can occur associated with oxygenminimumzones andwaters advected from active
spreading
centres(Burton
&Statham,
1988). Atmospheric
sourcesof manganese enhance the surface concentrations. Guieuetal.
(1997) compared
dissolvedinput
fromatmospheric
and riverine sources formanganese, and observed that between 63 to 100% ofdissolved Mnwas derived from
atmospheric origin.
Information on cobalt in the ocean is
inadequate
to enableinterpretation
of itsgeochemical behaviour,
with concentrationsgenerally
below 100pM.
Astrong
correlation between Co and Mn has been found in the western Mediterranean Sea(Morley
etal,
1990)
andEnglish
Channel(James
etal,
1993), indicating
their similarsources to surface waters and
geochemical
behaviour. Moffett & Ho(1996) suggested
that both elements are co-oxidised via the same microbialcatalytic pathway
andthis isan
important
mechanismfor theincorporation
of Co into marineMnoxides. Theuptake
mechanism of dissolved Co ontosuspended
particles
isgoverned by
the abundance andactivity
ofMnoxidising
bacteria and thebiological
demand of Co as a micronutrient(Moffett
&Ho,
1996).
Due to the
sensitivity
of iron to contaminationduring sampling
andanalysis
decade
(i.e.
Landing
andBruland, 1987;
Burton etal,
1993;
Witter & LutherIII,
1998;
Nolting
etal, 1998;
Croot &Hunter, 1998;
de Baaretal,
1999).
Iron distributionjare effectedby
bothrecycling
andscavenging
processes with a behaviour similar tomanganese,
except
thatFe(+II)
is morerapidly
oxidised. Studies in the open oceanshow that the vertical distributions of dissolved iron are
closely
related tomicronutrients
(Croot
&Hunter,
1998),
withalgal
uptake
in surface waters,settling
of intactplankton
andbiogenic particles (e.g.
faecalpellets)
and bacterial remineralization of thesettling
particles
atdepth.
However,
deep
waterconcentrations of ironaresimilarbetween the Atlantic and the 'older' waters of the
Pacific,
insharp
contrasttoN andP. Fe accumulates indeep
oceanwaters, abehavior which is uncharacteristic forahighly
reactive metal
(Sunda, 1997).
To solve theseparadoxes,
Johnsonetal.(1997) proposed
amodel in which iron is taken upby phytoplankton
atthe surface andremineralizedatdepth,
likemajor
nutrients,
and where it is held in solution in subsurface ocean watersby
asolubilizing
mechanism;
mostlikely by
thestrong
chelation of Feby
one or moreorganic
ligands
now knownto bepresent
in ocean waters at concentrations of 0.4-0.6 nM(Sunda,
1997).
Thehigh reactivity
ofdissolved iron towardsparticles
is shownby
the residence times withrespect
to removal in the upper 500 m in the Pacific ocean,ranging
from 1 to 20 years(Landing
&Bruland,
1987).
Iron(+11)
occurs inhigh
concentrations in
hydrothermal
fluids,
occurs in suboxic and anoxic environments andmay be formed
by
reduction in the upperocean. Since iron is the mostimportant
metal essentialtophytoplankton,
itmaylimitprimary production
in nutrientrich,
oligotrophic
oceans where
atmospheric inputs
are low(Donat
&Bruland,
1995;
de Baar etal,
1999).
Ocean
profiles
of lead are dominatedby
the influence on the upper ocean ofanthropogenic
Pbthrough
theatmosphere
and thepenetration
of this material into subsurface waters. Oceanic concentrations canvarysignificantly
due to the interaction ofanatmospheric
source, advection anddifferencesinscavenging efficiency
(Burton
&Statham, 1988).
Surface concentrations in the North Atlantic Ocean arehigher
than1.4NON-RIVERINE TRACEMETALSOURCES TO THE OCEANS
Trace metals have two
major
external sources:i)
atmospheric
or riverineinputs
of
weathering products
of theexposed
continents;
andii) inputs resulting
from the interactions of seawater withnewly
formed oceanic crustal basalt atridge
crestspreading
centres andhydrothermal
vents. Inaddition,
benthicinputs
of certain metalscanbea
major
source.1.4.1 ATMOSPHERIC SOURCES
Trace metals in the
atmosphere,
enter the oceansby precipitation
(wet
deposition),
cloudorfog impaction (cloud deposition),
orby impaction
onto a surfacein the absence of
precipitation
and clouds(dry deposition). Deposition
overthe worlds oceans are seen to varyregionally
and can be ranked in thefollowing
order from thehighest:
North Atlantic>North Indian> South Atlantic >South Indian> South Pacific. Thisranking highlights
the elevated concentrations found in the northern. Rural and urbanconcentrations are also shownto range between 10 - 25 times and 35 -150 timeshigher respectively,
than forremote sites(Ross
&Vermette, 1995).
This demonstrates theimportance
ofanthropogenic
aerosolsoriginating
from industrialisedregions
forthe tracemetalinput
totheoceans.Chester &
Murphy (1990) compiled
reports
showing
that theatmospheric
flux ofsome metals
(especially
Pb, Zn,
andCd)
to surface coastalwaters canequal
orexceedtheir riverine flux. Dust
input
tothe nutrient-limited Mediterranean basin is one of thegreatest
in thecontemporary
ocean(Guerzoni
etal,
1999).
Mean annualatmospheric
mass fluxes were observed at Crete
(21
gm'2)
and Israel(50
gm'2),
andwerefound to be thehighest
in the Mediterranean. Incomparison
to riverinedischarge,
70% of insolubleparticles
in the Eastern Mediterranean were from theatmosphere, compared
to 19% in the Western Mediterranean
(Guerzoni
etal,
1999).
Most metals have arelatively
short residence time in thetroposphere
(between
a fewdays
and a fewweeks).
They
are not well mixedand, thus,
aerosols and rainwater areexpected
to exhibitstrong
spatial
andtemporal
variability (Guerzoni
etal,
1999).
In the north¬western
Mediterranean,
Guieu et al.(1997)
indicate that more than 50% of thedissolved metal
input (Cd,
Co, Cu, Fe, Mn, Ni,
Pb andZn)
originates
from thesuchasPb and
Cd,
whicharelargely
of man-madeorigin,
even in 'Saharan' dominatedaerosols. Ironwas foundatthe
highest
concentrations,
followedby
Zn,
Pb,
Mn, Cu,
Ni,
and
finally
Cd and Co. ForPb andCd,
atmospheric entirely
dominates the total externalinput
ofpollution-derived
elements. The fate ofanatmospherically-transported
tracemetal,
oncedeposited
at the seasurface,
depends
on itsthermodynamic
speciation
in seawater, itsspeciation
in theparent
aerosol and the kineticscontrolling
anyspeciation
change (Guerzoni
etal,
1999).
Significant
atmospheric
input
ofheavy
metal contaminants which aretransported
in theprevailing westerly
winds fromsourcesinEurope,
issuggested
in theMEDPOL
Programme
(Windom
etal,
1988)
and from the results of recent research conductedasapart
of the 1988 Black Sea EnvironmentalProgramme (Hacisalihoglu
etal,
1991).
1.4.2 HYDROTHERMAL ACTIVITY
Hydrothermal activity
had been shown to exert amajor
influence on thechemistry
oftracemetals in seawater. It hasbeenestimated that the entire ocean mixesthrough
thesehydrothermal
ventsystems,
undergoing high
temperature
interactionwith fresh basalt every 8-10Ma,
leading
to theproduction
ofhigh
temperature
(350
C),
acidic,
reducing, hydrothermal
fluids(Bruland, 1983).
Seawater and sediment enrichment of metals
(Fe,
Mn,
Cu andZn)
has been observed athydrothermal
ventsalong
the Hellenic Volcanic Arc(Varnavas
&Cronan,
1991;
Varnavas etal,
1993). Significant
fluxes ofFe,
Mn,
Cu and Zn at the seafloor result from seawater-rock interactionprocesses similar tothose described atmid-oceanridges,
were noted at Santorini and Milos(Varnavas
&Cronan,
1991;
Varnavas etal.,
1993).
Variations in time and space in thecomposition
of thehydrothermal
solutionsare due to
changes
in chemicalcomposition
of the rocks leachedby
seawater;temperature
and pressure and thedegree
ofmixing
ofhydrothermal
solutions withseawater. Iron was observed to have the
highest
concentrations(800
to 22040g/l)
in1.4.3BENTHIC SOURCES
Regeneration
on, orbeneath,
the sea floor is one of thekey
processes in thecycling
of elementsthrough
theoceans(Bruland,
1983).
In the openocean, thegreater
part
of trace metalcycling
takesplace
in the watercolumn,
although
asignificant
diffusive contribution from
pelagic
sediments can exist. Within interstitial waters ofsediments,
micro-organisms
use oxygen,thereby changing
the redoxpotential
of the sediment. This may result in trace metal mobilisation from sediments andpermit
diffusivetransport
to interstitial waters, and eventual release into theoverlying
water.Solubility
andmobility
ofparticulate-bound
tracemetals canbechanged by
fivemajor
factors in solid/interstitialwater
systems:
lowering pH,
increasing
occurrence of naturaland
synthetic
complexing
agents,
increasing
saltconcentrations,
changing
redoxconditions,
anddecomposing
organic
mattercontaining
trace metals(Hong
etal.,
1995).
Enrichments in
porewaters
of coastal sediments ofCu,
Cd and Ni have beeninvestigated (Westerlund
etal,
1986).
It was concluded that trace metal fluxes weredependant,
inpart,
on oxygen concentration. Burton and Statham(1982)
reviewedprofiles
from the Pacificocean, allexhibiting significant
increases in copper for bottom waters. Theseresults indicate aninput
of copper duetothediagenic
release from settledparticles,
which hadpreviously
taken up the element from the water column. Benthic fluxes will varysignificantly
with sedimenttype
fordifferent elements. Inpelagic
clays
and siliceous oozeof the Pacific ocean, 88% ofdeposited
Cuwas returnedtooverlying
waters,
whereas less than 5% of Mn wasregenerated (Hong
etal,
1995).
This difference is attributedto theassociation ofMn withterrigenous particles,
whereas theinput
of Cu waspredominantly regulated by biogenic
phases,
such asorganic
matter,
skeletal calcium
carbonate,
or silica. Cadmium also exhibits similar fluxes withbiogenic phases
(of
about80%)
in LaurentianTrough
sediments,
duetothedegradation
of freshorganic
matter.Migration
of Mn underreducing
sediments has also been documented(Hong
etal.,
1995).
IntheMississippi
Delta,
Mn2+ fluxes weregreatest
inthe nearshore and inner delta
(840
g cm'2yr."1),
providing
apotential
source ofMn to1.5RECYCLING AND REMOVAL OF TRACE METALS
The ultimate fate of most trace metals is to be removed from the oceans to
marine sediments.
However,
before thisthey
mayundergo
variousdegrees
ofrecycling
withparticulate
orbiological phases
withsubsequent
regeneration
into the water column.1.4.1 ABSORPTIONANDSCAVENGING-PARTICLE INTERACTIONS
The term
'scavenging'
wasgiven
by
Golberg (1954),
ondiscovering
that lowtracemetalconcentrationswerethe result of
absorption
ofpositively charged
metal ionsonto
negatively charged
marineparticles
ormanganese nodules. Asparticles
sinkdownthrough
the watercolumn,
the associated trace metals areeffectively being
removedfromtheocean. Thiswastermed 'The Great Particle
Conspiracy' by
Turekian(1977).
Rapid
removal oftracemetalsontoparticles
occursviaadsorption
ontosurfaces,
precipitation
andincorporation
intobiogenic
particles. Suspended particulates play
amajor
role ininfluencing
dissolved trace metal concentration and distribution. In situprimary
production
in the surfacelayers
of the ocean is the main source ofparticles
which have an
important
role in thetransport
andcycles
oftrace metals in the water column(Tappin,
1988).
Thisplant
material isgrazed
uponby phytoplankton
andZooplankton
and theundigested
residues arepackaged
into faecalpellets
withsinking
rates
varying
fromafewmeters to afew thousandmeters perday
(Bruland,
1983).
Theabsorptive properties
of marineparticles
are controlledby
organic coatings.
These humic substances are absorbed onto theparticle
surface in seawater(Murray, 1988).
Hunter(1980) suggested
thatcarboxylic
acid andphenolic
functional groups appear to bethemajor
ionizable groups in suchcoatings.
It is thelarge
particles
that sinkrapidly
(and
not the slower smallparticles)
that areresponsible
for the bulk oftransport
of materials to thedeep
sea(Bruland, 1983),
whereas smallparticles
areimportant
foradsorption/desorption
processes. Thisorganic
'rain' to the sediment surface is thewhilst others are not. These latter metals are
probably incorporated
into thecrystalline
lattice of mineral
phases,
such asamorphous polymetallic oxyhydroxides.
Theseprecipitates
formspontaneously
in seawater as themajority
of metal form insolubleoxides
(Libes, 1992).
Priceetal.
(1999a)
observedseasonal andspatial
variations inparticulate
Fe and Mn in the Cretan Sea. Iron isclosely
related toAl,
as most of its concentration isintimately
bound within the structure of aluminosilicates. The ratio of Fe/Al was foundtobe
higher
inSeptember
than March1994,
especially
in the upper 200 mof the watercolumn,
attributed to increasedatmospheric inputs. Manganese
concentrations in thephotic
zone are lowcompared
todeeper
waters and this is considered due tophotoinhibition
ofMnOx
precipitation
fromMn(II).
1.4.2 BIOLOGICAL CYCLING
There are a number of ways in which the activities of
phytoplankton,
Zooplankton
and bacteria caninfluence thecycling
oftrace metals. The assimilationofsome trace elements is
dependent
ontheir function as micronutrients.Mn,
Fe,
Co, Ni,
Cu andZnare
examples
of elements thatplay
animportant
role in metalrequiring
andmetal activatedenzyme
systems
whichcatalyse major
steps
inglycolosis,
thetriboxylic
acidcycle,
photosynthesis
andprotein
metabolism(Bruland, 1983).
Ironhasreportedly
thehighest
enrichment factor(Libes, 1992).
Thebiological
driven flux of an elementthrough
the ocean is determined notonly by
the concentration factor inphytoplankton
but alsoby grazing
and excretion of thephytoplankton
cropby Zooplankton.
The presence of metal
complexing ligands
inphytoplankton
is discussedby
Hudson and Morel(1993)
and Donat & Bruland(1995).
Formetals such asCu, Zn,
Fe,
Cd,
Co andNi,
organic complexation
may aid tokeep
the metal insolution, thus,
retarding
itsadsorption
ontoparticle
and/or assist the assimilationby
somespecies
ofphytoplankton (Donat
&Bruland,
1995).
Inupwelling
regions,
Cu-complexing ligands
may reflectstrong
selective pressure todetoxify
Cuby
lowering
its free ion concentration. Inaddition,
trace metals may actsynergistically
orantagonistically
toinfluence
growth
limitation ortoxicity. Organisms living
inregions
ofhigh
metalconcentration,
forexample
zinc in surfacewaters of the AlboranSea,
do notincorporate
&
Boyle,
1988).
This is consistent withauto-regulation
ofplanktonic
elementalcompositions,
eitherby
metaboliccontrolsorby simple
chemicalmeans.As wellas active
uptake,
much of the metaluptake
isby binding
tohigh
surfaceligands;
thisbinding
ispassive and, therefore,
will occurequally
on deadorganisms
orbe
incorporated
into faecalpellets (Murray, 1988). Finally
trace elements are alsoassociatedwith
phases
ofplankton
suchascalciumcarbonate andopal.
1.6RECENT TRACE METAL DATAFORTHEMEDITERRANEAN ANDTHE
BLACK SEA
Sincethe 198O's there has been a
significant
increase in trace metal research in the Mediterranean Sea. SeveralEuropean
Commission programmes have focused ondifferent areas of the Mediterranean. These include 'PHYCEMED'
(Laumond
etal,
1984;
Copin-Montegut
etal,
1986;
Ruiz-Pino etal,
1991);
'EROS2000', ('EROS
2000',
Water Pollution ResearchReport, European
Community,
1989; 1990;
1992;
Morley
etal,
1997;
Guieuetal,
1997);
'EUROMARGE' in the Adriatic Sea(Tankere
&Statham,
1996)
and morerecently
'PELAGOS' in the Cretan Sea(Varnavas
&Voutsinou-Taliadouri,
1986).
Additional trace metal research include Statham et al.(1985),
Van Geen et al.(1988),
Sherrell &Boyle
(1988), Saager
et al.(1993),
Saager
(1994)
and Yoon etal,
(1999).
A brief summary ofsome of thekey findings
on tracemetaldistributions and
profiles
will begiven
below.Table 1.1 summarises the range of trace metal concentrations found in the Mediterranean Sea and North Atlantic Ocean. The first
significant
observation is that surface waters of the Mediterranean Sea havehigher
trace metal concentrations than open Atlantic waters(Statham
etal,
1985;
Van Geen etal, 1988;
Ruiz-Pino etal,
1991;
Morley
&Burton,
1993).
The inflow ofAtlantic water, via itsexchange
withdeep
Mediterranean water, is amajor
influence to the trace metalbudget
in theTable 1.1 Concentrations of dissolved trace
Mediterranean Sea.
(1)
Statham etal.(1985),
al.
(1997), (4) Morley
&Burton(1993).
metals found in the North Atlantic and
(2)
Van Geen etal.(1988),
(3)
Morley
etCd
(pM)
Co(nM)
Cu(nM)
Fe(nM)
Mn(nM)
Ni(nM)
Pb(nM)
Zn(nM)
Atlantic Surface 20.40 (D(2) 23-29 0.037-0.057 1.30.85-1.13
0.81-0.93
1.3-1.6w0.5-1.03
1.8
1.65-2.01
0.044-0.0710.8
Deep
300-350140
1.3
0.1-0.5
2.7 1.5 WesternMediterranean Surface 50-800.084-0.139
1.37-2.271.33-4.45
2.5-4.51.9-3.9
0.078-0.321 2-18(4)Deep
60-110(1)
55-70 0.039-0.046 21.45-1.65
1.0-2.7
0.15-0.24
4.2
3.0-3.96
0.064-0.121
5 (2)The
mixing
ofa number ofwater masses in the Mediterranean is animportant
key
incontrolling
the distribution and concentrationoftracemetals in both surface and subsurface waters:1)
the inflow of North Atlantic Surface Water(NASW), typically
low in dissolvedmetals;
additional contributions to the Straits from the trace-metalenriched sources of
2)
Gulf of Cadiz Water(GCW)
and3)
subsurface Atlanticwaters,
identifiedasNorth Atlantic Central Waters(NACW) by
VanGeen etal.(1991);
and4)
from the eastern Mediterranean the Sicilian Strait Surface Water
(SSSW)
associated with low metal concentrations. This mixture forms the Mediterranean Surface Waters(MSW)
below which the Levantine Intermediate Water(LIW)
entersthrough
the Straits ofSicily, forming
alayer
at 150-600 m, and contributes to elevated subsurface concentrationsespecially
for Zn(Morley
&Burton,
1993).
LIW forms themajor
component
of the Mediterranean Outflow water(MOW) (Morley
etal,
1997).
Mediterranean
deep
waters arefairly homogenous
inconcentration(see
Table1.1)
and exhibitsprofiles
similar to Atlantic waters. Inaddition,
rivertransport,
mobilisation from shelfsediments,
human activities andatmospheric
inputs
arepotentially
significant
sourcesoftracemetalstothe Mediterranean(Statham
etal,
1985).
Asaconsequence of these varioussourcestothewestern
Mediterranean,
surface concentrations will vary in space(see
Table1.1).
Zinc exhibits a surface maximathe central and eastern basins
(although
concentrations willvary withatmospheric
and terrestrial sources, as well asseasonally).
From the Straits of Gibraltarto the Straits ofSicily
Ruiz-Pino et al(1991)
observed a subsurfaceminimum,
corresponding
to the lowsalinity
of the NACW. A Zn intermediate maximum at about 200 m appearsthroughout
the westernMediterranean,
coinciding
with the LIWlayer.
Tankere & Statham(1996)
foundCu, Ni,
Zn and Cdprofiles
in the Adriatic Sea to be similar toopen Mediterranean
profiles,
withhighest
concentrations near the Po river andconcluded that river
inputs
werenotasignificant
sourcetothe Adriatic Sea.The Black Sea is the worlds
largest
stable anoxic marine basin with astrong
oxic/anoxic interface which influencesthe
quantities
and distribution of dissolvedtrace metals. There is apotential
risk ofanthropogenic
sources from poorpolution
control measures in EasternEuropean
countries or those of the former SovietUnion,
whichcomprise
themajor portion
of the Black Sea watershed(Windom
etal,
1998).
Furthermore
, the
likelyhood
ofsignificant
atmospheric
input
ofheavy
metalcontaminants
transported
in theprevailing westerly
winds from sources inEurope,
issuggested
by
the results of the MEDPOLProgramme
andaspart
of the 1988 Black SeaExpedition (Hacisalihoglu
etal,
1991).
Recenttrace metals research includes the 1988 BlackSeaExpedition
(Lewis
&Landing,
1991; Tebo,
1991)
and EROS2000,
(Tankere,
1998).
Overall the metal concentrations observedaregreater
than in the MediterraneanSea,
although
there is muchmorevariability
within theBlackSea,
above andbelowthe chemocline.1.7 AVAILABLE TRACE METALDATA IN THEAEGEANSEA
Up
to 1983relatively
little research had been carried out on dissolved tracemetals in the
Aegean
Sea. Distributions ofCd,
Cu,
Zn, Ni,
Fe and Pb have beenreported
(Fukai
&Huynh-Ngoc,
1976; 1977;
Romanov etal,
1977;
Huynh-Ngoc
&Fukai, 1979;
Aubert etal,
1980; Fytianos
&Vasilikiotis, 1983).
They
found variable concentration ranges ofmostmetals,
withdepth, geographical
location and season, andthrough
the Dardanelles. Theproblems
associated with low trace metal concentrationswere observed
by
Fukai &Huynh-Ngoc (1977)
andHuynh-Ngoc
& Fukai(1979),
finding approximately
half the Cu and Cd datawerebelow the detection limit.The CEC/MAST
project
'PELAGOS'provided
the first extensive trace metal(Cu,
Cd, Pb, Ni,
Co, Mn,
Cr andFe)
database for the southernAegean
Sea(Varnavas
&Voutsinou-Taliadouri,
1986).
Concentrations were found to be of a similar range toMediterraneanwaters
(see
Table1.1),
asreported
by
Martin etal.(1993)
and Tankereet al.
(1994).
Distinctspatial
andtemporal
variations were observed to be linked toclimatological
conditions,
circulation of water masses andparticulate/dissolved
interactions. Data was collected in
June,
September
and December1994,
from theCretan Sea and Kirthira Straits in the east to the Rhodes Straits in the west. Maximum
concentrations were observed in the summer and in the eastern Straits
(except Cu);
whilst lowest concentrations were observed in winter and in the Cretan Sea. Verticalprofiles
werereported
to exhibit athree-fold maxima: atthesurface,
at 500-600 m andin
deeper
waters of around 1100m(related
to the creation of Cretan DenseWater,
CDW).
Elevated concentrations in dissolved andparticulate
metals were linked tosouthwards fluxes of Black Sea Water
(BSW),
Levantine water(LIW),
Transitional Mediterranean Water(TMW),
with its characteristic low oxygen content, increases inchlorophyll
and the presence ofananticyclone
in the Cretan Sea.When
considering
the distribution oftracemetals,
it isimportant
tonotethat theAegean
is one of the mostseismically
activeregions
on earth and has thehighest
seismic
activity
in Eurasia(Dando
etal,
1995).
Seawater and sediment enrichment of metals(Fe, Mn,
Cu andZn)
has been observedathydrothermal
ventsalong
the Hellenic Volcanic Arc atNisiros and Kos(Varnavas
&Cronan,
1991)
and Santorini and Milos(Varnavas
etal,
1993).
These seawater rock interaction processes are similarto those described from mid-oceanridges.
With the
growing
awareness of thepotential
pollution
to the marine environmentby anthropogenic
activities,
the rivers andbays
of theAegean
have also beenafocus of research for tracemetals. Elevated levels due topollution
of dissolved andparticulate
tracemetals have been observed in the Gulfs of Thessaloniki and KavalaDassenakiset
at,
1996);
the riversAxios,
Aliakmon(Samanidou
&Fytianos, 1987)
and Acheloos(Dassenakis
etah,
1997).
1.8 OBJECTIVES
The
primary
objectives
of this work fall within those of the MediterraneanTarget Programme
(MTPII)
MATER programmeduring
which allsamples
andanalysis
were carried out. That is to
'identify
the main sources of the different trace elements and theirspreading pathways
in the northernAegean
Sea. Also to understand the mechanisms whichareresponsible
for thetrophic
differentiationbetween the north and southecosystems
(oligotrophic
in the North andextremely oligotrophic
in theSouth),
combining
thetracemetal data with otherphysical
and chemicalparameters.'
Considering
the lack of informationondissolvedtracemetals in theAegean
Sea and thepredetermined
stationpositions,
the initialobjective
wasto obtain an overviewof the
biogeochemistry
and distribution of dissolved trace metals in theAegean
Sea. Based ontheexisting knowledge
oftrace metals in the Mediterranean and thegeneral
knowledge
of theAegean
Sea,
thekey
areas thepresent
work focuses on include: theinfluence of
atmospheric,
coastal and riverine andhydrothermal
sources on metalconcentrations and
distributions;
therelationship
of trace metals with thecomplex
circulation andhydrography
of theAegean;
thespatial
andtemporal
variability
of dissolvedtrace metals and their causes; theoligotrophic
nature of theAegean
and therelationship
between dissolved nutrients andmetals;
residenc