UNIVERSITY OF LATVIA
FACULTY OF GEOGRAPHY AND EARTH SCIENCES
DEPARTMENT OF ENVIRONMENTAL SCIENCE
Rita Poikāne
THE ROLE OF SUSPENDED PARTICULATE
MATTER AND SEDIMENT IN THE DYNAMICS
OF METALS IN THE GULF OF RIGA
Summary of thesis for applying for the Doctoral degree in chemistry
(sub-branch: environmental chemistry and ecotoxicology)
Scientific supervisors:
docent, Dr. geogr., Juris Aigars
professor, Dr. chem., Arturs Vīksna
RIGA, 2008
The research was carried out in Latvian Institute of Aquatic Ecology in the time
period from April 1998 until May 2007. Some parts of research were performed in
Denmark at National Environmental Research Institute Centre of Marine Research
and Monitoring. European Social Fund supported the research.
Scientific supervisors:
Docent, Dr. geogr., Juris Aigars,
Professor, Dr. chem., Arturs Vīksna.
Opponents:
Professor, Dr. habil. chem Māris Kļaviņš,
Associate professor, Dr. chem. Pēteris Mekšs,
Dr. biol. Zinta Seisuma.
Doctoral committee:
Associate professor, Dr. biol. Viesturs Melecis - chairman of committee,
Associate professor, Dr. biol. Gunta Spriņģe - secretary of committee,
Professor, Dr. habil. chem. Māris Kļaviņš,
Professor, Dr. geogr. Oļģerts Nikodemus,
Professor, Dr. habil. chem. Andris Zicmanis,
Professor, Dr. chem. Arturs Vīksna,
Professor, Dr. eng. Viesturs Jansons.
The defence of doctoral thesis will be held at 17th of January, 2008 in public session of the Doctoral Committee at the Faculcy of Geography and Earth Sciences,
10 Alberta street, Riga.
The dissertation is available at Library of University of Latvia, 4 Kalpaka blvd. and
Latvian Academic Library, 4 Lielvārdes street.
References should be addressed to: Associate professor Dr. biol. Gunta Spriņģe,
University of Latvia, Faculcy of Geography and Earth Sciences, Department of Environmental Science, 19 Raiņa blvd., LV-1586, Riga. Fax: +371 67332704, e-mail:
gunta.springe@lu.lv.
© Rita Poikāne, 2008
© Latvijas Universitāte, 2008
ISBN 978-9984-802-98-5
www.lu.lv, www.lhei.lv
45
Introduction
The activities of mining and recycling industries as well as the usage of metals are
increasing year by year. The result of this is environmental pollution with toxic metals.
Metals can accumulate in the food chain; the high level of metal concentrations in
any link of the food chain can create damage in biological organisms and cause
death. The health of predators and human are threatened. According to HELCOM
(2003a), assessment metal concentrations in the Baltic Sea ecosystem are still essentially higher than natural background level despite procedures targeted to prevent, reduce and control pollution.
Metals in the Baltic Sea and the Gulf of Riga ecosystem enter mostly via atmospheric
deposition and river run-off. The Baltic Sea is the largest brackish water ecosystem
in the world. It is very sensitive to pollution because it is a relatively shallow, semienclosed water body with large drainage basin and high population around its shores.
Water exchange with the North Sea is limited (HELCOM, 2003a). The ecosystem of
the Gulf of Riga is among most sensitive ecosystems in the Baltic Sea area. Water
exchange with the Baltic Sea via narrow and shallow straits is limited. Three large rivers
Daugava, Lielupe, Gauja are situated in the South of the gulf and total approximately
80% of freshwater. The Daugava River alone brings 64% of freshwater entering the
Gulf (Yurkovskis et al., 1993;Lazniket al., 1999). Run-off from the Daugava River and
biogeochemical processes in the river strongly affect ecosystem of the Gulf of Riga.
The Gulf of Riga has its own character of physical, chemical and biological processes
due to the hydrology and geography of the Gulf of Riga.
Metals in the water can be found in dissolved and suspended form. Metal’s
physically chemical forms affect the chances of organisms absorbing metals from
their surroundings and affect how metals reach sediments. Sedimentation and biogeochemical processes in sediments play an important role in the marine ecosystem.
Metal accumulations in sediments are controlled by metal’s ability to attach to
suspended particulate matter and by sedimentation of these particles (Moore,
Ramamoorthy, 1984; Hallberg, 1992). Redox conditions in the water column
andsediments affect geochemical behavior of metals in aquatic system. Factors controlling redox conditions are the presence of dissolved oxygen in the water column
and its penetration in sediments, organic material content in sediments as well as
eutrophication of water body (HELCOM, 2003a, 2003b; Hallberg, 1992).
Contrary to the other Baltic Sea regions, distribution, geochemical behavior and
dynamics of metals in the Gulf of Riga were investigated poorly. In the previous
investigations different sampling methods, equipment, sample pre-treatment, storage, and analytical determination methods as well as sample partially digestions or
extraction methods were used. Therefore it is not possible to compare previously
obtained results with todays.
Previous investigations did not show clearly dynamics and transportation of
metals in the Gulf of Riga and the coastal zone of the Baltic Sea (territory of Latvia).
46 ______________________________________________________________
This determines the actuality of this work "The role of particulate matter and sediment in the dynamics of metal in the Gulf of Riga". It is focused on:
1) the role of suspended particulate matter in transportation of Al, Fe, Mn, Cd,
Cu, Zn, Cr, Ni and Pb from the Daugava River to the Gulf of Riga as well as in
these element dynamics in the Gulf of Riga,
2) the effect of biogeochemical processes on the dynamics of these metals in the
water,
3) the role of sediments and biogeoghemical processes in burial of these elements
or their secondary transport to the water column.
Investigation in the Ventspils Harbour was the first step in the complex research
that includes studies of metal dynamics and transport from the Venta River to the
Baltic Sea as well as how this investigation is comparable with scenario of metal dynamics and transport from the Daugava River to the Gulf of Riga.
The objective of this research is to describe mechanisms and factors controlling
metal (Al, Fe, Mn, Cu, Cd, Cr, Ni, Pb un Zn) distribution and dynamics in the water
column of the Gulf of Riga and the Daugava River as well as in the sediment of the
Gulf of Riga in the past and present, using previous studies, forecast biological,
chemical and physical processes that can affect metal concentrations in the sediment
and water column of the Gulf of Riga, as well as assess probable changes in the ecosystem of the Gulf of Riga and its processes due to metal concentration increases
or decreases.
Main tasks:
In order to fulfill aim of this work, the following steps were set:.
1. To collect samples and measure metal concentrations in the following:
• suspended particulate matter in the water column of the Gulf of Riga,
• sediment of the Gulf of Riga,
• suspended particulate matter of the water column of the Daugava River (from
the Vanšu Bridge to the mouth of the Daugava) and the Daugava River estuary,
• sediments of Ventspils Harbour.
2. To describe behavior and dynamics of particulate metals in the water column
of the Daugava River estuary and the Gulf of Riga.
3. To describe metal concentration dynamics in the sediments of the Gulf of
Riga and Ventspils Harbour.
______________________________________________________________ 47
4. To describe mechanisms of metal behavior on sediment - water interface.
5. To describe mechanisms which govern metal cycling in the Daugava River
and the Gulf of Riga in comparison with the Venta River estuary and other
regions of the Baltic Sea.
6. To describe history of anthropogenic pollution of the Gulf of Riga with metals
and metal burial mechanisms in the Gulf of Riga.
Supporting information includes data on dissolved and particulate nutrients,
dissolved oxygen, chlorophyll a, temperature, salinity, pH, total carbon and biologically available silica collected within scientific projects and grants coordinated by
Latvian Institute of Aquatic Ecology Centre of Marine Monitoring, Dr. A. Yurkovskis
(Yurkovskis, 2004; Yurkovskis, Poikāne, in press), Dr. A. Andrušaitis (Poikāne et al.,
2005), Dr. J. Aigars. All analyses were performed in the accredited laboratory (LVS
EN ISO/IEC 17025:2005 L) of the Latvian Institute of Aquatic Ecology Centre of
Marine Monitoring which participates successfully in QUASIMEME exercises.
This dissertation is supported by the European Social Fund.
For data storage and illustration the software program "Ocean Data View" (ODV)
was studied and used in cooperation with the EC expert network "SeaDataNet".
Scientific novelty and practical importance of the studies
Data collected in previous investigations about metals in the Gulf of Riga and
the Daugava River estuary were not correlated limiting its use in complex analyses.
This is the first complex study of metals in the Gulf of Riga, and includes analysis
of fluvial material and its distribution in the water and sediment of the Gulf of
Riga. This research gives an over-all picture of the biogeochemical processes in
the Gulf of Riga with a sequential for metal budget analysis and modeling. This
dissertation is the first step and includes total metal investigations of the suspended
particulate matter and sediment in the Daugava River and the Gulf of Riga. The data
describe biogeochemical processes and its controlling factors specific to the Gulf
of Riga and allow forecasting of the progress of processes on long-term industrial
development and intensive eutrophication and its impact on the ecosystem. The
data on the sediment cores and suspended particulate matter in the water column
can be used for anthropogenic pollution description of the past and forecasting
anthropogenic metal pollution level in the future as well as for assessment of the
quality of environment of the Gulf of Riga.
The main results of research were published in three articles and were presented
in seven international and one local scientific conferences; the draft of this work
was presented and discussed in international student scientific meeting.
48 ______________________________________________________________
The main results were presented in articles
Müller-Karulis, B., Poikāne, R., Segliņš, V., 2003. Heavy metals in the Ventspils Harbour: normalization based on a multi-parameter dataset. Environmental Geology 43(4), 445-456.
Poikāne, R., Carstensen, J., Dahllöf, I., Aigars, J., 2005. Distribution patterns of particulate trace metals in the water column and nepheloid layer of the Gulf of Riga.
Chemosphere 60,216 - 225.
Yurkovskis, A., Poikane, R. Biogeochemical, physical and anthropogenic transformation in the Daugava River estuary and plume, and the open Gulf of
Riga (Baltic Sea) indicated by major and trace elements. Accepted for publication in Journal of Marine System. In Online System since 06.04.2007. DOI:
10.1016/j.jmarsys.2007.03.003.
The main results were presented in scientific conferences
Aigars, J., Poikane, R., Ledaine, I. Seasonal pelagic-bentic interactions in shallow
temperate ecosystem of Baltic Sea, with emphasize to biogenic silica. ASLO summer meeting. Santiago de Compostella, Spain. June, 2005.
Andrušaitis, A., Aigars, J., Ledaine, I., Poikāne, R., Suse, I. Suspended organic matter
production un sedimentation in the Gulf of Riga. 60th scientific conference of the
University of Latvia. Riga, February, 2002.
Poikane, R., Aigars, J., Ledaine, I. Phytoplankton spring bloom impact to iron
and manganese geochemistry in the surface sediments (nepheloid layer) of the
Gulf of Riga (the Baltic Sea). 9th International Biogeochemistry Symposium,
Warnemuende, Germany. May, 2006.
Poikane, R., Jansons, M. The geochemistry of heavy metals in the Gulf of Riga
(the Baltic Sea) surface sediments. Metals in the Environment. II International
Conference. Vilnius, Lithuania. September, 2001.
Poikāne, R., Müller-Karulis, B. Heavy metals in the Ventspils Harbour. 16th Baltic
Marine Biologists Symposium, Klaipeda, Lithuania. June, 1999.
Yurkovskis, A., Poikane, R. Particulate trace metals in the Daugava estuary, the
Eastern Baltic Sea. SECOTOX World Congress and 6th European Conference
on Ecotoxicology and Environmental Safety. Krakow, Poland. August, 2001.
Yurkovskis, A., Poikane, R. Biogeochemical barriers in the Gulf of Riga: the Daugava
River estuary and bentic nepheloid layer. 5th Baltic Sea Science Congress. The
Baltic Sea changing ecosystem. Sopot, Poland. June, 2005.
Yurkovskis, A., Poikāne, R. Biogeochemical Restructuring the Suspended Phase in the
Daugava River Estuary (Baltic Sea). ECSA 41st International Conference. Measuring and Managing Changes in Estuaries and Lagoons Venice, Italy. October, 2006.
______________________________________________________________ 49
The draft of thesis was presented and discussed in the international student
scientific meeting within Marie Curie Research Training Networks at Danish National Environmental Research Institute Centre for Research and Monitoring of
Marine Environment in Roskilde, November, 2005.
50
Methods
Water and sediment samples were taken and treated by me and other qualified
specialists from Latvian Institute of Aquatic Ecology. Sampling, pretreatment and
analyses were performed according to methods recommended by HELCOM and
international standards which are accredited by LATAK (The Latvian National Accreditation Bureau).
Fig. 1. The map of the Gulf of Riga and the Daugava River lower reaches with
sampling stations.
Water and sediment samples were collected according to ISO standard (ISO
5667-9:1992 (E)) from February 2001 till October 2006 on a board of the research
vessel "Antonija" and National Naval Forces ships. Water samples were taken in the
Daugava River 15 km along the Daugava River lower reach as far as the mouth of
the Daugava River (Fig. 1) and in the pelagic and central part of the Gulf of Riga
in the territory of Republic of Latvia. Sediment samples and samples of trapped
suspended material were collected at station 119 in the central part of the Gulf
of Riga (Fig. 1), sediment samples in Ventspils Harbour were taken on 2 and
3 September 1998 (Fig. 2).
51
Fig. 2. Sampling stations in the Ventspils Harbour.
Water samples were collected with a 5 L Van Dorn TW - 3612 plastic water
sampler (Watanabe Keiki Mfg. Co. Ltd). Sinking suspended particulate material
was collected with a particles trap (KC Denmark A/S), and sediment and nepheloid samples were collected with a modified Kajak gravity corer (Blomqvist,
Abrahamsson, 1985). Water for suspended particulate matter and suspension of
nepheloid were immediately filtered by a vacuum filtration stand through preweighed "Millipore" nitrocellulose membrane filters with 0.45 µm pore size (Pohl,
1997; Pohletal., 2001).
Total digestion for metal analyses of suspended particulate matter, sediments
and nepheloid with HF and HNO3 acid mixture was performed using a modified
method (ICES by Hovind and Skei, 1992; Pokāne et al., 2005). Metal concentrations
were determined by atomic absorption spectrometry (VARIAN Spectra AA880
and Spectra AA880Z). For quality assurance QUASIMEME (Quality Assurance of
Information for Marine Environmental Monitoring in Europe) reference standards
with assigned values were used. The analyses of sediments of the Ventspils Harbour
were performed by XRAL laboratory in Canada (ISO 9002). Samples were digested
with HCl, HClO4 and HF acid mixture. Metals were determined by inductively
coupled plasma atomic emission spectrometry (Fisons ICP-ARL 3560).
52 ______________________________________________________________
Results and discussion
Hydrological, hydrochemical and hydrobiological condition in the Gulf of Riga
Hydrological conditions in the Gulf of Riga varied annually during time period
from 2001 and 2006. Within this study period, two saltwater inflows from the Baltic
Proper were observed in 2003 and 2006 (Fig. 3), and the most effective inflow was
in 2006. Inflows created well pronounced stratification and stagnancy in water layer
above sediments. The result of water stagnancy was oxygen deficiency. In the water
layer above sediments at station 119 in December 2003, oxygen concentration was
0.73 mL∙L-1, but in October 2006 oxygen concentration was 1.11 mL∙L-1 (Bērziņš V.,
pers. com., Aigars et al., 2007) (Fig. 4). Except for these occasions, oxygen concentration less than 3 mL∙L-1 was not observed during the experiment period.
Fig. 4. Oxygen (mL∙L-1) distribution pattern in the water column at station 119
from 2001 to 2006.
Commonly, freshwater runoff (run-full) is strongest in spring, with relatively
lower runoff (run-full) levels in summer - autumn. Strongest freshwater runoff
(run-full) of the Daugava River was observed in April 2004 (Fig. 5), while lowest
______________________________________________________________ 53
spring runoff (run-full) was observed in 2003. However, during 2003 relatively high
runoff (run-full) in was observed the autumn.
Fig. 5. Daily run-full (m3∙s-1) recorded at Pļaviņas electric plant reservoir from January
2001 to December 2006. Concentration of particulate matter (mg∙L-1) in the surface
water of the Daugava River at stations: DG-2 (Vanšu bridge); DG-1 (Commercial
port); DG (entrance to naval port in Bolderāja).
Salinity distribution patterns at station DG-2 in the Daugava River illustrate that
in June 2002, at the depth below 7.5 m, salinity was 1.5 - 2.5 PSU, and in September,
at the depth below 5 m, salinity was 3,5 PSU and on the surface - 0.77 PSU. The
salinity pattern is in agreement with characteristic salinity inflow amounts from the
Baltic Proper of this year, which was lowest during the experiment period.
The dynamics of phytoplankton exhibited pronounced annual variations between
2001 till 2003 (see p. 56 Fig. 6). Spring phytoplankton blooming in 2001 was unusual
in the Gulf of Riga because very intensive and massive dinoflagellate species
development was observed in the photic layer (0 -10 m) of the water column at station
119. In contrast, only minor diatom species development was observed (I. Jurgensone,
B. Kalveka, pers. com.; ULIAE, 2002). In the Gulf of Riga, the dominant phitocenosis usually is diatom species. In spring 2001, the dominant dinoflagellates species
were 77% of biomass, particularly Peridinella catenata, and the concentration of
chlorophyll a was 40 mg∙m-3 that was at its highest in comparison with the next
five-year observations. Phytodetritus reached sediments after massive development
of dinoflagellate species according to hypothesis of Tamerlander and Heiskanen
(2004) that cells of dinoflagellates sink slower than cells of diatoms and bacterial
decomposition of these cells occurs in water column. It probably caused anoxic
conditions of the sediment surface.
Fig. 6. Seasonal dynamics of phytoplankton biomass and chlorophyll a in photic layer (0 10 m) of the water column at station 119 in the time period from January 2001 to
December 2003.
In spring 2002, the dynamics of phytoplankton was relatively weak and short in
comparison with spring 2001 and 2003. The biomass was 30% less and concentration
of chlorophyll a was 25% less than in 2001, diatom species dominated (85%; particularly Chaetoceros wighamii) in samples. In spring 2003 phytoplankton massive
development was relatively long and intensive and diatom species dominated (95%;
particularly Achnanthes taeniata) but concentration of chlorophyll a was as low as
in 2002.
Particulate suspended matter in water of the Daugava River and Gulf of Riga
During the observation period from February 2001 till December 2005, the concentration range of suspended particulate matter in the water column of the Daugava
River was 1.4 - 31.0 mg∙L-1 (mean 6.6 mg∙L-1; median 3.6 mg∙L-1). From February
2001 untill October 2006, the concentration of suspended particulate mater in the
water column of the Gulf of Riga was 0.5 - 13.7 mg∙L-1 (mean 3.0 mg∙L-1; median 2.4
mg∙L-1). The highest concentration of suspended particulate matter was observed in
April 2004 when the highest run-full was also recorded (see p. 53 Fig. 5). There was
significant correlation between concentration of suspended particulate matter and
the Daugava River run-full (0.92; p <0.001).
______________________________________________________________ 55
In early spring, concentration of suspended particulate matter observed at stations165,101 A, 103 located close to the Daugava River was higher than at stations in
the central part of the Gulf of Riga. The same situation was observed at station 163
and 167 located close to the Gauja River and the Lielupe River respectively (Fig. 7).
Fig. 7. Distribution pattern of suspended particulate matter in the surface water (1 m) of
the Gulf of Riga in April 2005.
In the central part of the Gulf of Riga suspended particles are distributed evenly
in the surface water. In contrast, vertical distribution of suspended particles in the
water column of some stations was inhomogeneous. Every August (particularly in
2005) at stations 119,121,135,137A, 121A and 102A, concentrations of suspended
particulate matter observed in the water column below 30 m was higher (3 13 mg∙L-1) than in the surface water (0.5 - 3 mg∙L-1).
The principal component analysis (PCA) of particulate conservative and semiconservative metals (Fe, Al, Cr and Ni) in the water column of the Daugava River
estuary in 2001 and 2002 showed well pronounced seasonal dynamics because
the first principal component explained 85% of the total variations (Poikāne et al.,
2005). The first principal component had significant correlations with water temperature (R2 = 0.58) and the Daugava River run-full (R2 = 0.62). Since significant
correlation was observed between concentration of particulate conservative metals
and concentration of suspended particulate matter in the water column of the
Daugava River as well as in the water column of the Gulf of Riga (both approximately
r ≈ 0.9), it showed that suspended particles enter the Gulf of Riga mainly during
spring floods.
56 ______________________________________________________________
The PCA for particulate conservative and semiconservative metals in the water
column of the Gulf of Riga shows that 56-91% of the total variation depends on
the station of observation. Furthermore, no correlation of the first principal component with salinity or water temperature was observed. Therefore, it seems that the
hydrological dynamics in the Gulf of Riga does not influence seasonal changes of
concentration of particulate matter in the water column of central part of the Gulf
of Riga.
According to observation at station 119, the significant increase of concentration
of particulate matter expected in spring was the sum of resuspended material, terrestrial material supplied by river run-off and sinking phytoplankton. The next significant increase of concentration of particulate matter could be expected in autumn
due to specific hydrological character of the Gulf of Riga (Raudsepp, Kõuts, 2001)
and autumn phytoplankton sinking (Leivuori, Vallius, 1998). Firstly, pycnocline
which rises above sediments in autumn after saline water inflow in summer and
thermal stratification in the Gulf of Riga (Omstedt, Axell, 2003) can reduce the
speed of particles sinking as well as increase the concentration of particulate matter
on this water layer as discovered in the Baltic Proper (Pohl et al., 2004). Secondly,
in autumn, underwater currents caused by local wind and pycnocline in the Gulf
of Riga were stronger than in spring or summer, which amplified underwater undulation (Raudsepp, Kõuts, 2001). It created resuspension of the fresh bottom sediments. Thirdly, significant negative correlation (-0,50; 0,001<p<0,01) was observed between the concentration of suspended particulate matter and dissolved
oxygen in the water column of the Gulf of Riga. This indicated that the reduced
dissolved fluxes of redox sensitive metals Fe and Mn were transformed to solid Fe
and Mn oxihydroxides by oxidation in the 30 - 40 m water layer of the Gulf of Riga
which was similarly observed in other areas (Pohl et al., 1998, 2004; Neretin et al.,
2003; Pohl, Hennings, 2005). This was supported by results of a cruise to station
119 in August 2003, when in the 30 and 40 m water layers Mn concentration in
suspended particles was 20% and 10% respectively in contrast to the mean value
of the Gulf of Riga (1.4%) or of the Daugava River (0.8%). Similar discoveries and
hypothesis about seasonal processes in the Gotland basin and in the Gulf of Finland were completed by German (Pohl et al., 2004) and Finnish (Leivuori, Vallius,
1998) scientists.
Suspended metals in the water column of the Gulf of Riga and the
Daugava River
The range of concentrations of suspended metals in the water column of the
Gulf of Riga and the Daugava River was dependent on the season and the year of
observation. The range, mean and median values of concentrations of suspended
metals in the water column of the Gulf of Riga and the Daugava River are shown
in Table 1. The range, mean and median values of concentrations of metals in the
suspended matter of the Gulf of Riga and the Daugava River are shown in Table 2.
______________________________________________________________ 57
Table 1.
The range, mean and median values of concentrations
of suspended metals in the water column of the Gulf of Riga and
the Daugava River in the time period from February 2001 till October 2006.
(Explanations:a - sample was contaminated probable;b - values below determination limit)
The highest concentrations of suspended Fe and Al in the water of the Daugava
River were observed during spring floods, usually in March and April. The highest
values of Al concentration in suspended particles of the Daugava River was observed
on 10 April 2002 and the highest values of Fe concentration in suspended particles of
the Daugava River was observed on 25 February 2001 (Table 2). Significant (p <0.001)
correlation between suspended Fe and Al concentrations (0.99) as well as correlation
58 ______________________________________________________________
between these suspended metals and concentration of particulate matter (0.95) in the
water of the Daugava River was observed. Correlation between concentrations of Fe
and Al in suspended particles of the Daugava River was 0.79 (p <0.001).
The concentration maximum of suspended Fe and Al in the water of the Gulf of Riga was observed in the spring at stations located opposite to the mouth of the Daugava
River (165,101A, 165A and 103), the mouth of the Gauja River (163) and the mouth
of the Lielupe River (167). For example, the cruise in April 2005 to stations located
2 km from these river mouths discovered concentrations of suspended Fe and Al in
the surface water at station 165 (the Daugava River mouth) were 416 and 428 µg∙L1
respectively, at station 163 (the Gauja River mouth) 293 and 294 g∙L-1
respectively and at station 167 (the Lielupe River mouth) 235 and 183 g∙L-1
respectively. The results indicate that the Daugava River provided most of
suspended particles in the Gulf of Riga. During the same cruise, the observed
concentrations of suspended Fe and Al in the surface water at station 119 were 36
and 54 [ig-L' respectively.
Vertical distribution pattern of suspended Fe and Al concentrations in the whole
water column at the most frequently observed station 119 was homogeneous in
winter and spring but exhibited clear stratification in summer (Fig. 8). During autumn maximum concentrations of suspended Fe and Al as well as concentration of
suspended particulate were observed in the water layers above the sediment.
A sediment trap experiment completed in 2001 indicated a change in proportion
of sedimentation rate between suspended Al and suspended total C during phytoplankton spring bloom in the whole water column (see p. 60 Fig. 9) at station 119.
Similarly, a change was observed in the sedimentation rate proportion between suspended Fe and suspended total C that agrees with findings in the Gulf of Finland
(Leivuori, Vallius, 1998). High sedimentation rate of suspended Al and significant
sedimentation of particulate total C without phytoplankton activity (see p. 54 Fig. 6)
in early spring clearly showed deposition of resuspended material. Sedimentation
rate of particulate total C increased during phytoplankton activity in April.
Results showed that material of different origin could be expected in different
seasons in the water column of the Gulf of Riga - material of organic origin in
spring, but material of mineral origin in autumn. And proportions of suspended
element in suspended material could vary by seasons and water layers.
Several authors (Gobeil et al., 1997; Kremling et al., 1997; Brügmann et al., 1998;
Pohl et al., 1998; Pohl, Hennings, 2005; Saulnier, Mucci, 2000, Neretin et al., 2003)
describe findings about Fe behavior in anoxic water layers. Dissolved Fe diffused
from anoxic sediments to the water column, and through oxidation processes in
oxic water layers, Fe precipitated as suspended Fe oxyhydroxide. It is high probable
that oxic conditions in the bottom water layer of the Gulf of Riga caused oxic
conditions in the sediments surface. Therefore, fluxes of reduced and dissolved
Fe from deeper sediment pore waters were oxidized in the sediment surface. As a
result, mean bentic fluxes from sediments to water column were negligible.
59
Fig. 8. Suspended Al and Fe (µg∙L-1) distribution patterns in the water column from
February to August 2001 at station 119. Temperature isothermoline (4 °C) is
shown with a dashed line.
Similarly to that observed in the Daugava River, there is significant correlation
between suspended Fe and Al (0.97; p <0.001) in the water column of the Gulf of
Riga. Furthermore, the correlation between concentrations of suspended particulate
matter of these metals is also significant (0.90; p <0.001). The highest concentrations
of Fe and Al in suspended particulate matter in winter and early spring were observed in the whole water column of the Gulf of Riga, while in summer and autumn,
highest concentrations were observed in water layer above sediments.
The concentrations of Fe and Al in suspended particulate matter are widely used
as indicators of terrestrial material in marine water (Pohl et al., 1998,2004; Kersten,
Smedes, 2002; Kanellopoulos et al., 2006). Investigations within this work showed
that concentration of Fe and Al in suspended particulate matter can also be used
as indicator of particles of mineral origin, which enters the Gulf of Riga through
rivers, the atmosphere and bottom resuspension.
Fig. 9. Suspended total C and suspended Al sedimentation rate (mg∙m-2∙d-1) in spring and
summer 2001 in the 5 m, 10 m, 15 m and 30 m water layer at station 119.
Significant correlation was observed between concentrations of suspended Mn
and suspended particulate matter in the water column of the Daugava River (0.60; p
<0.001) as well as between concentrations of suspended Mn and the Daugava River
run-full (0.42). The lowest concentrations of suspended Mn in the water column
of the Daugava River were observed in May 2002 and the highest, in April 2004
(see p. 57 Table 1). Investigations showed that concentrations of suspended Mn
in the suspended matter and in the water of the Daugava River tended to increase
from June to September in 2002. The highest concentration of suspended Mn was
observed in September when Daugava River runoff (run-full) was at its lowest.
The maximum concentrations of suspended Mn in the Gulf of Riga were observed
annually in the water layer below 30 m starting from August (Fig. 10). Several years
the concentration of suspended Mn in the water layer above sediments was 60 times
higher than in the surface water. The absolutely highest concentrations of suspended
Mn were observed in August 2003 at stations 119 and 120 in 40 m and 42 m water
layers. Significant negative correlations (p <0.001; -0.57) between concentrations of
suspended Mn and concentrations of dissolved oxygen, concentrations of suspended
Mn andpH (p <0.001; -0.60) as well as a positive correlation between concentrations
of suspended Mn and concentrations of ammonium (p <0.001; 0.62) were observed.
At the same time, no significant correlations between concentrations of suspended
Mn and concentrations of suspended particulate matter or other suspended metals
in the water column of the Gulf of Riga were observed.
The highest concentrations of Mn in particulate matter of the Gulf of Riga were
observed in bottom waters in August 2003. For example, the concentration of Mn in
suspended particulate matter in 30 m water layer at station 119 was approximately
20%, but in the water layer below 35 m at stations 119,120, 135 and 121, Mn concentrations in suspended particulate matter varied from 3% to 10% which is two
to seven times higher than the mean value. Similar findings in the Baltic Sea were
described by other scientists (Kremling et al., 1997; Brügmann et al., 1998; Pohl et
al., 1998,2004; Pohl, Hennings, 1999,2005; Kremling, Streu, 2000; Sokolowki et al.,
2001). It shows that Mn behaviour in the water of the Gulf of Riga depends on redox
changes induced by oxygen depletion in surface sediments what in turn is caused by
digenesis of organic matter in the sediment.
Fig. 10. Suspended Mn distribution pattern in the water column from February to
August 2001 at station 119. Temperature isothermoline (4˚C) is shown
with a dashed line.
The observed concentration gradient of suspended Mn in the water column of
the Gulf of Riga (Fig. 10) demonstrates a decreasing trend from sediment surface
due to reduction and dissolution of Mn in the pore waters and diffusion to the water
layer above sediments caused by decrease of dissolved oxygen in the sediments.
Thereafter, Mn formed particulate oxihydroxides in the oxic water through the
oxidation processes. As a result, the benthic fluxes of dissolved Mn increased and
oxidation of Mn was observed higher in water column with decrease of oxygen in
the water above sediments in summer and/or autumn.
The highest concentration of particulate Cd in Daugava River water was observed
on 18 May 2001 at station DG, but it seems that this concentration is artifact because
this was observed at this single observation point in the Daugava River during this
cruise. The highest mean concentration of suspended Cd in water between the
Daugava River stations was observed on 6 April 2004 (24.2 ng∙L-1) but at the same
time the Cd concentration in particulate matter was the lowest (0.7 µg∙g -1). Specific
situation was observed in April 2002 when concentrations of suspended Cd in the
whole water column and in the particulate matter at station DG were higher than at
stations DG-1 and DG-2 (see p. 62 Fig. 11).
Fig. 11. Suspended Cd concentration (ng∙L-1) distribution pattern in the water of the
Daugava River from Vanšu Bridge (DG2) to the Gulf of Riga (station 165) in
April, May, June and September 2002.
Mixing zones of fresh and brackish water in the Daugava River estuary depend on
meteorological conditions. Previous investigations conducted in estuaries of other
rivers and experiments with suspended particulate matter in brackish waters (L'her
Roux et al., 1998; Gerringa et al., 2001; Ciffroy et al., 2003; Turner et al., 2004; Waeles
et al., 2005), which found that up to 90% of total Cd can be found in ionogenic form
(Schneider, Pohl, 1996; Pohl, Hennings, 1999; Pohl et al., 2002), support the assumption
that the mechanism of dissolved Cd chloride complex formation in the mixing zone
of the Daugava River estuary was blocked due to mixing of fresh and brackish water,
and, as a result, Cd forms insoluble compounds and complexes. Additionally, Cd could
be transformed to particulate phase as a nutrient-like element due to phytoplankton
metabolism processes (Sunda, Huntsman, 1998; Wang et al., 2001).
Statistical analyses of data show that there is a correlation between suspended Cd
concentration and the Daugava River run-full (0.40; p <0.001), but we did not find
evidence that suspended Cd enters the Gulf of Riga mainly during high river runoff.
It seemed that Cd was brought in the gulf as dissolved Cd and/or was involved in
different biogeochemical processes which is in agreement with other findings (Moor,
Ramamoorthy, 1984; Schneider, Pohl, 1996; Pohl et al., 1998; Dippner, Pohl, 2004).
The highest concentrations of suspended Cd were observed in May 2001. The
results obtained 16-18 May 2001 showed that concentrations of suspended Cd
______________________________________________________________ 63
in the surface water layer (1 m) at all observed stations were 2 - 3 times higher
than mean value observed in the whole Gulf of Riga (7.9 ng∙L-1) (Fig. 12). Due to
unusual spring phytoplankton development in 2001, and according to Schneider
and Pohl (1996) Sunda and Huntsman (1998) Wang et al. (2001), we assumed that
Cd was probably involved in phytoplankton metabolism processes with continuous
suspended Cd concentration increase in the photic water layer of the Gulf of Riga.
But, at the same time, significant correlation between suspended Cd and suspended
total C sedimentation rates did not appear.
Fig. 12. Suspended Cd concentration (ng∙L-1) distribution pattern form February to
August 2001 at station 119.
The highest concentration of suspended Pb in the water of the Daugava River
was observed in April 2004 (see p. 57 Table 1). Significant correlation between suspended Pb concentration in Daugava River water and Daugava River run-full (0.85;
p <0.001) showed that suspended Pb entered in the Gulf of Riga mainly via the
Daugava River spring floods similar to the Odera River (Pohl et al., 2002).
The higher Pb concentrations in the suspended particulate of the Daugava River
were observed in June 2002 (see p. 57 Table 2). Two times higher Pb concentrations
in particulate matter were observed in June and September 2002 in the surface
water of station DG-2, which is located close to Vanšu Bridge. It probably indicates
that there is a local pollution source, however, the data were insufficient to draw
definite conclusions.
The concentration of suspended Pb in the water column of the Gulf of Riga
did not show well-pronounced seasonality although in summer the concentration
of suspended Pb was lower than during the rest of the year. In the summer
concentrations of Pb in suspended particulate matter in the surface water
layer (1 m) of the Gulf of Riga occasionally were two to ten times higher than in
particles in the bottom water layer. The increase of suspended Pb sedimentation rate
in August 2003 in 30 m water layer and in August - November 2003 in 15 m water
layer could
64 ______________________________________________________________
be explained by resuspention and fluvial particles transportation by underwater
currents. A similar situation was observed in the Baltic Proper in November 2000
(Pohl, Hennings, 2005), where the authors argue the suspended Pb was transported
by currents from the Kuronian lagoon and/or deposited from the atmosphere.
Suspended Cu and Zn concentrations in Daugava River water had significant
(p <0.001) correlations with particulate matter concentration (0.81 - Cu; 0.74 - Zn)
and with Daugava River run-full (0.77 - Cu; 0.63 - Zn). The highest concentrations
of these suspended metals in Daugava River water were observed in March 2001
and April 2004. This indicates that suspended Cu and Zn entered in the Gulf of Riga
mainly in spring (see p. 57 Table 1).
Suspended Cu and Zn concentrations show weak seasonal changes in the water
column of the Gulf of Riga. The highest concentrations of suspended Cu and Zn were
observed in the photic water layer in spring during phytoplankton development
time. Occasionally, high concentrations of these suspended metals in photic water
layer were observed in August.
Research (Gerringa et al., 1991; Schneider, 1995; Pohl, Hennings, 1999, 2005;
Sokolowski et al., 2001; Dippner, Pohl, 2004) indicates that Cu and Zn tend to form
dissolved compounds in oxic waters. As a result, less than 50% of total Cu and Zn
were found in suspended form. However, these metals can transform from dissolved
to insoluble forms due to different processes, the main being the involvement of Cu
and Zn in biological processes (Pohl et al., 2004; Pohl, Hennings, 2005, Gélabert
et al., 2006), scavenging of Cu and Zn by Mn oxihydroxide (Lōffler, 1997) and insoluble Cu and Zn sulphide formation (Öztürk, 1995).
In experiments with sediment traps, well-pronounced sedimentation of suspended Cu and Zn in photic water layer of the Gulf of Riga were not observed. This may
indicate that Cu and Zn sorption on and desorption from suspended particles are in
balance. The results of experiments with sediment traps suggest that Zn could attach
to suspended Mn oxihydroxides in 30 m water layer. This agrees with findings in the
Gotland Basin (Pohl et al., 2004) and the Gulf of Finland (Leivuori, Vallius, 1998).
The highest concentrations of suspended Ni and Cr in Daugava River water were
observed in March 2001 and April 2004 (see p. 57 Table 1). Significant correlations
(p <0.001) between the concentrations of these suspended metals and Daugava River
run-full (0.83 - Ni; 0.85 - Cr) shows that suspended Ni and Cr were transported to
the Gulf of Riga mainly during spring floods.
The results of experiments with sediment traps at station 119 in the Gulf of Riga
in 2001 demonstrate that sedimentation dynamics of suspended Ni was similar
to sedimentation dynamics of suspended Fe, except in April when suspended Ni
sedimentation rate increased. It shows that Ni could be involved in biological processes and reach sediments together with biological material, which agrees with
research results in the Gotland basin (Pohl et al., 2004). The results of experiments
______________________________________________________________ 65
with sediment traps in the Gulf of Riga in 2003 show that suspended Ni sedimentation dynamics is similar to suspended Fe dynamic the similar to the Gotland basin
(Pohl et al., 2004) and the Gulf of Finland (Leivuori, Vallius, 1998).
Although, suspended Cr did not show seasonal dynamics within experiments
with sediment traps, a significant correlation was observed between concentration of
suspended Cr and suspended Al (0.88; p <0.001) in 40 m water layer at station 119 in
the Gulf of Riga. This shows that Cr attached to particles of mineral origin similar to
observations in the coastal waters of England and France (Auger et al., 1999).
The concentrations of metals in the sediment surface (nepheloid) layer
During my three-year investigation, significant changes of Al concentration in
the nepheloid layer were observed only in 2001. Regularly concentrations of Al in
nepheloid layer decreased from 7% to 5.5% from March to July 2001, although, the
most drastic decrease of Al concentration to 1.7% was observed in August 2001.
This value was the lowest observed within whole study period. There were observed correlations between concentrations of Cu and Pb and concentrations of Al
(0.81 and 0.89, respectively; 0.05>p>0.01; n=6) from March to August 2001.
In the summer of 2003, the concentrations of Al in nepheloid layer did not change
substantially (6 - 7.5%) in contrast to an expected decrease of Al concentration
due to spring phytoplankton sedimentation. Salinity increase in the bottom layer
waters at station 119 appeared from July 2003. Probable inflow from the Baltic
Proper of more saline water most likely induced underwater currents, which in turn
created bottom sediment resuspension and re-sedimentation. That was recorded by
sediment traps at station 119 in autumn 2003, and, furthermore, was in agreement
with findings in the Gulf of Finland (Leivuori and Vallius, 1998). Al exhibited
significant correlations with Cd (0.75; 0.001<p<0.01; n=13), Cr (0.78; 0.001<p<0.01;
n=13) and Ni (0.61; 0.01<p<0.05; n=13) in the nepheloid layer.
Concentration changes of redox sensitive metals Fe and Mn differed at station
119 in the Gulf of Riga (see p. 66 Fig. 13) in the nepheloid layer. Substantial Fe
concentration changes in the nepheloid layer were observed in 2001, however,
no such changes were observed in 2002 and 2003. Substantial Mn concentration
changes were observed in 2002.
In 2001 and 2002 the concentration dynamics of Fe and Al differed in the nepheloid layer, in contrast to that observed in suspended particles where correlation
between Fe and Al concentrations was (0.90; p <0.001). However, significant correlation (0.85; p <0.001; n=13) between Fe and Al concentrations was observed in
2003 in the nepheloid layer.
Correlations between Fe and nutrients in the nepheloid layer were not observed
in 2002 (see p. 67 Fig. 14). In contrast, significant correlations of Fe with inorganic P
(0.79; 0.001<p<0.01; n=13), chlorophyll a (-0.71; 0.001<p<0.01; n=13), total C
66 ______________________________________________________________
(-0.59; 0.01<p<0.05; n=13), total N (-0.68; 0.01<p<0.05; n=13) and biological
available Si (-0.63; 0.01<p<0.05; n=13) were observed in 2003. Negative correlations
indicate sedimentation of material of mineral origin.
Fig. 13. Fe and Mn concentration changes in sediment nepheloid layer at station 119in
the Gulf of Riga in time period from March 2001 to December 2003.
Manganese concentration increased from December 2001 to September 2002 in
the nepheloid layer. In the following month, Mn concentrations decreased rapidly
to values observed in January 2002. No significant correlations between Mn and
other metals or Mn with nutrients were observed, except a correlation between Mn
and Cd (0.81; 0.001<p<0.01; n=9), from July to December 2002.
Several different factors can explain the dynamics of Fe and Mn in the nepheloid
layer during the three-year observation period - specific phytoplankton development
in 2001 (see p. 54 Fig. 6), relatively week spring development in 2002 and relatively
strong saline water inflow from the Baltic Proper in 2003.
In 2001 Fe sedimentation and accumulation in sediment was governed by several
factors. From March to May Fe precipitated with material of mineral origin, but in
67
June and July Fe precipitated with material of organic origin due to the involvement
of Fe in phytoplankton development processes (Chen et al., 2003; Ho et al., 2003;
Kustka et al., 2003; Gerringa et al., 2006; McKay et al., 2006).
Fig. 14. Total C and chlorophyll a concentration changes in the nepheloid layer at
station 119 in time period from March 2001 to December 2003.
Oxygen deficiency was not observed in 42 m water layer at station 119 in 2001 and
2002 (see p. 52 Fig. 4) (ULIAE, 2002,2003), what indicates that the nepheloid layer
was oxic in the summer. Redox conditions in sediments and water was suitable for
Mn benthic flux oxidation and formation of Mn oxihydroxides in nepheloid layer
or in water close to sediment surface (Öztürk, 1995; Gobeil et al., 1997; Neretin
et al., 2003; Pohl, Hennings, 2005). Rapid accumulation of Mn oxihydroxides in
the nepheloid layer was observed in August and September 2002. According to
results, accumulation of Mn oxihydroxides occurred until thermal water turnover
in autumn.
More saline water inflow from the Baltic Proper in 2003 created a stagnant
situation in the sediment surface and the water layer above sediments in the Gulf
of Riga. Conditions were suitable for Mn dissolution in sediment pore waters and
diffusion to water overlying sediments. It caused the decrease of Mn concentration in
the nepheloid layer and an increase of Mn concentration of up to 20% in suspended
particulate matter in the 30 m water layer.
Other metal (Zn, Cu, Cd, Pb and Cr) concentration changes in the nepheloid
layer showed that accumulation mechanisms of these metals were similar in 2001. It
is probable that these metals reached sediments associated with material of mineral
origin because there were significant correlations of Cu and Pb with Al. Since Al
had significant correlations with Cd, Cr and Ni in the nepheloid layer in 2003, this
indicates that in 2003 Cd, Cr and Ni reached sediment with material of mineral
origin.
68 ______________________________________________________________
Significant correlations between chlorophyll a and Ni (0.86; p<0.001; n=20), as
well as Cr (0.84; p<0.001; n=16) were observed in 2002 (see p. 67 Fig. 14 and 15) in
the nepheloid layer. For Ni this correlation was observed from January to December
2002, but for Cr from February to October 2002. It is possible that this indicates
dissolved Ni and Cr involvement in phytoplankton development and metabolism
processes (Moor, Ramamoorthy, 1984; Sunda, Huntsman, 1998, Achterberg et
al., 1999; Auger et al., 1999). The involvement of these metals in phytoplankton
development and metabolism processes may depend on phytoplankton species
composition. Nickel and chromium were released in the water column as dissolved metals due to phytoplankton decomposition in the nepheloid layer (Moor,
Ramamoorthy, 1984; Santschi, 1988; Öztürk, 1995; Brügmann et al., 1998;
Achterberg et al., 1999; Auger et al., 1999; Morse, Luther III, 1999).
Fig. 15. Cr and Ni concentration changes in the pheloid layer at station 119 from March
2003 to December 2003.
Metal concentrations in the nepheloid layer of the Gulf of Riga depend on the
origin of particulate matter precipitated in different seasons. The dominance of
material of different origin varied annually in the nepheloid layer. In 2001 material of
organic origin precipitated after spring phytoplankton development as observed in
the Gulf of Finland and the Gotland Basin (Leivouri, Vallius, 1998; Pohl et al., 2004).
______________________________________________________________ 69
In 2003 all observations showed that the dominant material was of mineral origin.
Findings in 2002 showed three periods - precipitation of spring phytoplankton in
spring and summer, precipitation of Mn oxihydroxides in summer and autumn and
sedimentation of mineral origin particles in autumn, which supports observations
in the Gulf of Finland (Leivouri, Vallius, 1998). Since the Mn concentration increase
was observed together with an increase in Cd concentration, significant correlation
between these metals (0.81; 0.001<p<0.01; n=9) in 2002 supports the hypothesis
of other research (Brügmann et al., 1998; Pohl, Hennings, 1999; Sokolowski et al.,
2001; Koschinsky et al., 2003) on the role of Mn oxihydroxides in scavenging and
transportation of microelement to sediment.
Metal vertical profiles in the sediment of the Gulf of Riga
The vertical profiles of sediment at station 119 in the Gulf of Riga show that
composition of precipitated material in different time periods is inhomogeneous.
The most substantial differences were in the upper 10 cm of the sediment core
which correspond with the time period from 1970s to today. There was observed
decrease of Al concentration in the upper 10 cm in contrast to an increase of total
C, biologically available Si and loss of ignition. Aluminum concentrations had a
significant (p <0.001; n=30) negative correlation with total C (-0.71) and biologically
available Si (-0.75) concentrations. Similar findings appeared in the Gotland
Basin (Hille, 2005). The results of eutrophication and anthropogenic pollution
are well reflected on the lithogenic material background. The vertical profiles of
several elements in sediment core demonstrate that there is a need to normalize
these elements against Al and subsequently calculate the enrichment factor (EF) for
better data interpretation.
Two recently deposited sediment layers at station 119 were chosen for calculation
of enrichment factor in the Gulf of Riga - the top centimeter and 4 - 5 cm layer
reflecting a period of intensive and stable industry in Latvia in the 1980s (see p. 72
Table 3) and where nutrient, Fe, Zn, Cu, Cd and Cr relations to Al showed maximum
levels in the sediment vertical profile (see p. 70 Fig. 16 and p. 71 Fig. 17).
Fig. 16. Normalized against Al Fe, Mn, total C and biologically available Si vertical
profiles in sediment core of the Gulf of Riga at station 119.
71
Fig. 17. Normalized against Al microelement vertical profiles in the sediment core of the
Gulf of Riga at station 119. Legends: r - coefficient or regression; p - significance;
EF0-1 - enrichment factor (against sediment layer in 1809) in the top sediment layer;
EF4-5 - enrichment factor (against sediment layer in 1809) in 4 - 5 cm sediment
layer which reflect 80ties of last century (against sediment layer in 1809).
72
Table 3.
Enrichment factor for metals in the sediments at station 119 in the Gulf of Riga,
in the Gotland Basin (in sediment layer of 1995 and 1983), in the Ventspils Harbour
(1998), in the Arkona Basin (1994), in the Pomeranian Bight (1995), in the Gulf of
Finland (1993), in the Botnia Sea (1993) and in the Gulf of Botnia (1993).
Explanatory notes: T - EF ratio to crustal material according to Taylor (1964);
L - EF ratio to element concentration in local preindustial sediments
The vertical profile of Cd describes the history of anthropogenic pollution of the
Gulf of Riga. According to Gobeil et al. (1997), Brügmann et al. (1998), Dippner and
Pohl (2004), Pohl and Hennings (2005), Metzger et al. (2007) and others, Cd forms
insoluble CdS and accumulates in sediments. Correlations between Cd/Al ratios
and C/Al indicated changes in the ecosystem of the Gulf of Riga - the significant
increase of C/Al and Cd/Al between 1928 and 1985 (0.90; p <0.001; n=10) and the
significant decrease of C/Al and Cd/Al in time period from 1985 (0.89; 0.01 <p<0.05;
n=6). According to Dippner and Pohl (2004) and Pohl et al. (2004), Cd is an element
that reaches sediments associated with particles of organic origin. This tendency also
appears in the sediment of the Gulf of Riga between 1924 and 1985.
Copper and Zink are not redox sensitive metals but their accumulation in sediment
depends on sulphide formations of these metals (Öztürk, 1995). The proportion of
the total amount of these metals attached to marine particles in oxic water is low (Pohl
et al., 1998; 2004). The formation of insoluble Cu and Zn forms and precipitation and
accumulation in sediments occurs in anoxic environment with hydrogensulphide
______________________________________________________________ 73
presence (РоЫ et al., 2004, Pohl, Hennings, 2005). However, hydrogensuphide in the
water column of the Gulf of Riga was not observed during this study. This indicates
that Cu and Zn have not reached the sediment as sulphides since 2001.
In comparison with other regions of the Baltic Sea (Table 3), well-pronounced
historical accumulation of Cr and Ni was not observed in the sediment of the Gulf
of Riga. Possible explanations for this are that the specific geochemical behaviour
of Cr in oxic water - formation of thermodynamically stable Cr (VI) (Moor,
Ramamoorthy, 1984) and Cr can not form sulphide in sediments (Morse, Luther III,
1999), and redox processes weakly affect Ni behavior, in oxic water Ni was mostly
in dissolved form and could reach sediments attached to Mn and Fe oxihydroxides
(Öztürk, 1995; Brügmann et al., 1998; Sokolowski et al., 2001).
Metals in the sediments of the Ventspils harbour
Data of metal concentrations in Ventspils Harbour indicates the anthropogenic
pollution was not observed here (Table 4). The highest metal concentrations were
observed in the lower reaches of the Venta River close to the river mouth in the
territory of the outer harbour (see p. 74 Fig. 18). The highest Al concentration
(5.54%) in sediments was detected here. These sediments can be classified as old
glacial sediments. The harbor was dredged in 1998, however, since then new sediment
accumulated on the surface of glacial sediment, mixing marine and Venta River deposits (Müller-Karulis et al., 2003). In comparison with sediments in territory that
was not dredged (V40), new sediments in dredged territory did not show traces of
pollution although intensive ferry traffic, regular dredging and relatively intensive
marine particle deposits in the harbor territory, particularly when the wind comes
from the Southwest (Bethers, Senņikovs, 1997), could interfere in detection of local
pollution and its sources.
74 ______________________________________________________________
Investigation of the sediment of Ventspils Harbour in 1998 offered good background for further research in this area and other harbours such as Liepāja, Klaipēda and Riga. This data could also be used for calculation of metal budgets in the
Gotland Basin.
Fig. 18. Al, Cr un Pb distribution patterns in the sediment surface (0-2 cm) of
the Ventspils harbour.
75
Conclusions
1. The mechanisms of distribution of suspended particulate matter in the central part
of the Gulf of Riga differ by season, location and depth. Intensive sedimentation of
suspended particulate matter and sequential change or simultaneous sedimentation
of dominant settling material occurs in spring: sedimentation of resuspended
material caused by winter-spring water masses convection, sedimentation of fluvial
material and organic material of phytoplankton blooming. In autumn, suspended
particulate matter is evenly distributed in surface layer of the water column. More
complicated mechanisms of suspended particulate matter distribution are in
the water column below thermocline, and includs transfer and sedimentation of
resuspended sediment particles by underwater currents, sedimentation of phy
toplankton cells as well as new particle formation caused mostly by redox condi
tions in the sediment and water column of the Gulf of Riga.
2. Vertical profile of the sediments at station 119 reflects the history and progress of pollution of the Gulf of Riga. Decrease of Al concentration and increase of total C and biologically available Si concentration reflects eutrophication in the Gulf of Riga since the 1970s.
3. The most intensive Zn, Cu, Cd, Pb, Cr and Ni accumulation in the sediments of
the Gulf of Riga was occurred during the 1980s. However, accumulation of Ni
and Cr was not observed and accumulation of Zn, Cu, Cd and Pb decreased in
recent samples of surface sediment. Metal accumulation in the recent samples of
surface sediment of the Gulf of Riga can be described by accumulation intensity:
Ni < Cr < Cu < Pb < Zn < Cd. Anthropogenic pollution of the sediments of
the Gulf of Riga with metals was less in comparison to the Gotland Basin, the
Pomeranian Bight and the Gulf of Botnia.
4. Dissolved oxygen concentration changes in the water column and sediments
of the Gulf of Riga affect dissolved or particulate forms of redox sensitive Fe
and Mn. The processes, dissolution of reduced Fe form suspended particles and
immediate formation of oxidized particulate Fe, are in balance. These processes
are in balance throughout the water column of the Gulf of Riga. Furthermore,
the benthic flux of dissolved Fe is negligible because it was oxidized in sediments
or in the water column very close to sediment surface and so indicates oxic
conditions in the sediments and water column of the Gulf of Riga. Oxidation of
benthic flux of Mn occurred in the water column, which is the main and most
important mechanism of new particle formation.
5. Metal concentrations in the nepheloid layer of the Gulf of Riga are dependent
on seasonal proportions of settling materials. Quantity of precipitating organic
material and phytoplankton species composition affect oxygen depletion in
the sediment surface and play a role in the transfer of Fe, Ni and Cr from the
water column to sediments. Particles of mineral origin play an important role
as a metal carriage due to metal sorption on particle surfaces and distribution
by underwater currents. The Mn oxyhydroxides are partially responsible for
76 ______________________________________________________________
scavenging of Cd and Zn from the water column and thereafter for transporting
them to the sediment surface. Manganese oxihydroxides with scavenged metals
can reach sediments if oxidation of Mn benthic fluxes occurs close to sediment
surface, otherwise Mn oxihydroxides dissolve while sinking.
6. The dynamics of particulate Cu and Zn show low seasonality in the water co
lumn of the Gulf of Riga and so indicate that these elements are involved in
phytoplankton metabolism. Copper and zinc sorption and desorption processes
in the water column are in balance, however, the concentration of particulate Zn
is affected by the presence of particulate Mn oxihydroxides. These metals reach
sediments scavenged by material of organic origin and/or Mn oxihydroxides,
and are a less effective way of metal "burying" than the formation of insoluble
metal sulphides. There is a high probability that Cu and Zn are being released
back in the water column through dissolved metals due to decomposition of
particles of different origin in the sediments of the Gulf of Riga.
7. Distribution and concentration of particulate Cd in the water column of the Gulf
of Riga and the Daugava River are dependent on salinity, biological processes
within the whole period of phytoplankton activity and formation of Mn oxi
hydroxides particles in the water and sediments. In the Gulf of Riga Cd reach
sediments scavenged by particulate organic material and Mn oxihydroxide.
8. Particulate Ni and Cr are brought in the Gulf of Riga with fluvial material.
Nickel is involved in biological processes in the water column of the Gulf of Riga
and reaches sediments scavenged by material of mineral and biological origin.
Chromium transfer from the water column to sediments occurs mostly with
particles of mineral origin. Nickel and chromium accumulation is negligible in
the sediments of the Gulf of Riga. These elements circulations on the water sediment surface are in balance because of oxygen presence in water.
9. In comparison with the Gulf of Finland and the Botnia Sea, the anthropogenic
pollution level with Pb in the sediments of the Gulf of Riga is equal in level, but
in comparison with the Gotland Basin, the Arkona Basin, the Pomeranian Bight
and the Gulf of Bothnia, the sediment of the Gulf of Riga is less polluted with
Pb. Results of my research do not clearly show the progress of anthropogenic
pollution. The particular scenario of suspended Pb behaviour or distribution in
the water column of the Gulf of Riga was not found.
10. Aluminium, iron and manganese from rivers of the drainage basin of the Gulf of
Riga and from the atmosphere are buried mainly in the sediments of the Gulf of
Riga because these metals are in particulate form in oxic water. Research results
showed indirectly that Cu, Zn, Ni, Cr and Cd occurred mostly in dissolved form
in the water column of the Gulf of Riga and only small quantities were buried in
the sediments of the Gulf of Riga. This indicates that the Gulf of Riga is one of
the potential sources of pollution in the Baltic Sea as it brings the dissolved Cu,
Zn, Ni, Cr and Cd to the Baltic Proper through water exchange.
______________________________________________________________ 77
11. Recent sediment samples from Ventspils Harbour did not show anthropogenic
pollution. The data from new and glacial sediments are a good point of reference
and basis for sequential research in harbour territory as well as good basis for
sediment quality comparison and assessment in territories and dumping sites of
other ambient ports - Liepaja, Klaipeda and Riga.
78
Acknowledgements
I am very grateful to all who helped me on each step to this work. I would like
to thank to my scientific supervisors Dr. Juris Aigars and Dr. Arturs Vīksna; my
colleagues in Latvian Institute of Aquatic Ecology, especially, Mintauts, Miķelis,
Ņina, Alla, Iveta, Vadims, Maija, Dr. Anda, Baiba, Sveta, Mikus, Solveiga, Bärbel.
I am grateful to scientific grants coordinators Dr. habil. Aivars Yurkovskis and
Dr. Andris Andrušaitis; captains and crews of research vessel "Antonija" and
Latvian Naval Forces flagship A90 "Varonis"; colleagues from international project
"SeaDataNet" and from Denmark National Environmental Research Institute Jacob
Carstensen, Ingela Dähllof, Ole Manscher and Martin Larsen. I thank to my parents,
sisters Aina and Gunta, niece Baiba, my friends Guna, Indra, Gunta and Dace.
This work was supported by European Social Fund, and I am grateful to project
coordinator Ingrīda Kalviņa.
______________________________________________________________ 79
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