Seasonal variation of polychlorinated biphenyl concentrationsin the southern part of the Baltic Sea

Regina Bruhn*, Michael S. McLachlan

Baltic Sea Research Institute, Seestrasse 15, 18119 Rostock, Germany

Abstract

The impact of seasonal fluctuations in forcing factors such as atmospheric concentration, temperature, and biological produc-

tivity on the concentration of polychlorinated biphenyls (PCBs) in the surface water of the southern part of the Baltic Sea was

investigated. Water samples were collected on eight cruises over 2 years. A clear seasonal variability in dissolved PCB concentrations

was observed with higher levels in summer than in winter and spring. This was attributed to changes in atmospheric concentrations

and water temperature, based on measurements showing the PCB levels in the atmosphere and surface water to be close to a

partitioning equilibrium. Concentrations in the suspended particulate material (SPM) fraction were also variable, and when the

quotient of the organic carbon normalised concentration in SPM and the dissolved concentration was calculated (i.e., the bioac-

cumulation factor (BAF)), a seasonal pattern was observed which was consistent with kinetic limitations on partitioning into

particles caused by plankton growth. However, seasonal variability in the partitioning properties of the SPM may also contribute to

this variability. � 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Polychlorinated biphenyls; Seawater; Bioaccumulation factors; Baltic Sea; Seasonal variability

1. Introduction

Concentrations of persistent organic pollutants(POPs) such as polychlorinated biphenyls (PCBs) inmarine surface water are subject to numerous seasonalinfluences. For instance, gaseous concentrations of manyPCBs have been shown to have a pronounced seasonalcycle with maximum levels in summer and minimumlevels in winter (Manchester-Neesvig and Andren, 1989;Haugen et al., 1999). This can be expected to affect at-mospheric deposition and air/sea gas exchange of thesecompounds. In addition, the depth of the mixed surfacelayer and with it the vertical mixing are determined bysolar radiation and wind speed, both of which fre-quently possess a distinct seasonality. Solar radiationalso influences photochemical and – via water temper-ature – chemical degradation rates. Finally, the primaryloss mechanism for persistent hydrophobic contami-nants in surface water, namely vertical export withsinking particles, has a strong annual cycle in temperate

and sub-polar waters. As a result, it would seem rea-sonable to expect a pronounced seasonal variability inconcentrations of POPs in marine surface water. How-ever, most of the published data have been collected onjust one or two cruises and give at best only a rudi-mentary indication of possible seasonal influences. Aknowledge of the seasonal variability in POP levels isessential in order to judge the representativeness of ex-isting and future data sets.

The marine environment of the Baltic Sea, a semi-enclosed coastal sea with a water residence time ofapproximately 20 years (Rheinheimer, 1995), is con-taminated with many POPs including PCBs. The bio-logical cycle is characterised by three bloom eventswhich occur in spring, summer and autumn. There is apronounced annual cycle in solar radiation, water tem-perature, wind speed, mixing depth, and atmosphericconcentrations of PCBs (Rheinheimer, 1995; Agrell etal., 1999). As such, this area is particularly suited forinvestigation of the seasonal variability in surface waterconcentrations of PCBs. In an earlier study in thesouthern and central Baltic, much higher PCB concen-trations were found in the suspended particulate mate-rial (SPM) fraction in the spring of 1991 compared to

Marine Pollution Bulletin 44 (2002) 156–163

www.elsevier.com/locate/marpolbul

* Corresponding author.

E-mail address: regina.bruhn@io-warnemuende.de (R. Bruhn).

0025-326X/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S0025-326X(01 )00198 -9

the autumn of 1988 and 1989, and the authors suggestedthat this was due to PCB uptake during the springplankton bloom (Schulz-Bull et al., 1995). In addition toseasonal variability, regional differences in contamina-tion have to be considered in the Baltic Sea due to theintense industrialisation of the drainage basin and theresulting likelihood of a heterogeneous source distribu-tion.

In order to investigate the regional and seasonalvariation of PCB concentrations we undertook eightcruises during different seasons in the southern BalticSea. Water samples were taken and the concentrationsof eight PCB congeners were measured separately in thedissolved phase and in SPM. The data were employed toexplore the different factors controlling the seasonalvariability in PCB levels.

2. Material and methods

2.1. Sample collection

Samples were collected at several stations and along anumber of transects in the Arkona Sea, a region of thesouthern part of the Baltic Sea, during eight cruises withthe research vessels A.v. Humboldt and P.A. Penck inFebruary, March, May, and August 1998 and in Feb-ruary, March, May, and June 1999. Detailed informa-tion on the sampling locations and sample volumes aregiven in Fig. 1 and Table 1.

Water samples were taken from the mixed surfacelayer at a depth of 5–10 m at seven stations in theArkona Sea during the first seven cruises using anin situ-pump system. When the results showed no con-sistent regional variations in the PCB distribution, it wasdecided to also sample along transects in June 1999 toget more representative measures of the concentrationsin the region. The transect sampling was performed withthe aid of a catamaran floating beside the research vesseloutside the wake. A PE-tube was fixed at a depth of 2 mto the side of the catamaran and connected to the insitu-pump (also on the catamaran).

The in situ-pump system consisted of a filter holder, aresin column, a pump, a flow meter, an electronic con-trol unit and a power supply. The system is described indetail in Petrick et al. (1996). To obtain separate mea-sures of dissolved PCBs and PCBs associated with SPM,seawater was filtered through a glass fibre filter (GF/F52, Schleicher and Schuell, Dassel, 14.2 cm diameter, 0.7lm nominal cut off) and the filtrate was extracted onXAD-2 resin columns. The volume sampled per minutewas chosen as five times the bed volume (1.25 l/min).Typically 200–600 l of water were sampled both at thestations and along the transects. After sampling theXAD-2 columns were stored at 4 �C and the filtersat )20 �C.

All glass fibre filters were precleaned by heating for12 h at 400 �C in a muffle furnace after which they werewrapped in aluminium foil and stored at room temper-ature. The XAD-2 resin columns were precleaned bysoxhlet extraction with acetone (see below). They werethen rinsed with water, filled with water, sealed, andstored at 4 �C in a refrigerator.

For the determination of the organic carbon contentof the particulate material, samples of 2 l were takenfrom the CTD rosette and filtered through precleanedglass fibre filters (GF/F 52, Schleicher and Schuell,Dassel, 0.42 cm diameter).

2.2. Analytical procedure

Solvents (analytical grade) were purified by distilla-tion. Aluminium oxide (neutral, MERCK, Darmstadt)was heated for 12 h in a muffle oven at 850 �C, Florisil(0.150–0.250 mm, MERCK, Darmstadt) and sodiumsulphate (MERCK, Darmstadt) for 12 h at 450 �C. Allchemicals were checked for impurities.

Fig. 1. Map showing the location of the sampling stations.

R. Bruhn, M.S. McLachlan / Marine Pollution Bulletin 44 (2002) 156–163 157

For the XAD-2 columns a method published by theIOC (1993) was used. The XAD-2 columns were ex-tracted for 12 h with 300 ml acetonitril and 30 ml waterin a modified soxhlet extractor (Ehrhardt, 1987). Priorto the extraction internal standards (PCBs 65 and 207)were added to the solvent. The acetonitril was removedwith a rotary evaporator, and the remaining aqueousfraction was extracted three times with n-hexane. Thecombined hexane fractions were dried over sodiumsulphate. After concentration with a rotary evaporatorthe samples were subjected to fractionation with HPLCaccording to the method of Petrick et al. (1988). A Li-Chrospher Si 100-5 column (Merck, Darmstadt, 250 mmlength) was used and the PCBs were eluted in 6 ml of

100% n-hexane. The volume was reduced to 100 ll foranalysis.

In the first samples the recovery of the internalstandard PCB 65 was unsatisfactory. Difficulties wereexperienced in evaporating the acetonitril, which mayhave been caused by the high DOC levels in Baltic Seawater. Consequently, the method was modified for theanalysis of the samples from March, May, and June1999. The XAD-2 columns were extracted for 1 h with180 ml acetone and subsequently for 12 h with 300 mlacetone and 30 ml water in a modified soxhlet extractor.Prior to the second extraction internal standards (PCBs65 and 207) were added to the solvent. The acetone wasremoved with a rotary evaporator, and the remaining

Table 1

Details of the water samples

Date Sampling station Water temperature (�C) POC content (mg/l) Water sample volume (l)

10/02/98 140 3.43 0.06 449

11/02/98 202 3.99 0.06 605

12/02/98 150 2.89 0.13 512

12/02/98 109 3.31 0.05 601

21/03/98 RB1 3.56 0.14 415

21/03/98 30 3.51 0.15 483

22/03/98 140 3.53 0.12 426

22/03/98 109 3.67 0.16 396

23/03/98 202 3.57 0.11 367

23/03/98 202 3.57 0.12 349

24/03/98 150 3.65 0.20 346

06/05/98 30 7.97 0.16 366

06/05/98 RB1 6.36 0.21 398

07/05/98 109 7.54 0.53 296

01/08/98 30 15.56 0.43 371

02/08/98 RB1 15.26 0.35 371

04/08/98 202 16.10 0.36 320

08/02/99 113 2.58 0.12 367

09/02/99 RB1 2.25 0.12 212

10/02/99 140 2.87 0.08 256

11/02/99 109 2.70 0.12 376

21/02/99 150 2.01 0.15 516

21/02/99 150 2.01 0.15 427

19/03/99 30 2.56 0.12 418

20/03/99 RB1 2.46 0.20 401

22/03/99 150 2.99 0.43 381

07/05/99 30 6.98 0.17 426

07/05/99 RB1 7.00 0.32 375

08/05/99 109 7.03 0.20 414

08/05/99 140 5.93 0.29 389

09/05/99 202 6.56 0.29 399

10/05/99 150 9.57 0.31 231

01/06/99 30 14.94 0.25 340

02/06/99 RB1 13.77 0.25 419

02/06/99 109 15.19 0.27 373

02/06/99 transect 109-115 14.34 0.41 362

03/06/99 transect 115-102 14.34 0.22 273

10/06/99 140 18.42 0.31 443

11/06/99 transect 111-30 18.42 0.37 400

158 R. Bruhn, M.S. McLachlan / Marine Pollution Bulletin 44 (2002) 156–163

aqueous fractions were combined and extracted threetimes with n-hexane. The further processing of thesamples was done as described for the extraction withacetonitril.

The filter samples were extracted using an acceleratedsolvent extractor (ASE 200, DIONEX). The frozen fil-ters were transferred into the cartridges and the internalstandard was added. The extraction was performed witha n-hexane/acetone solvent mixture 3/2 (two static cy-cles, 100 �C and 140 bar). The aqueous and organicsolvent phases were separated and the aqueous phaseextracted three times with n-hexane. The further pro-cessing of the combined organic phases was done asdescribed for the XAD-2 columns.

PCBs 28, 31, 101, 118, 138, 149, 153, and 180 weredetermined using gas chromatography with electroncapture detection (GC–ECD). An HP 5890 GC with aDB-5 capillary column (60 m � 0:25 mm, 0.25 lm filmthickness, J&W Scientific) was employed. The systemconfiguration is described in detail elsewhere (Dannen-berger and Lerz, 1996).

For the analysis of POC, the filters were dried for 12h at 60 �C, transferred to tin cups (VARIO CHN cups),and analysed for the carbon content by combustion andIR CO2 detection using an Elementar VARIO ELanalyser (Grasshoff et al., 1999). The previous elimina-tion of particulate inorganic carbon (PIC) was notnecessary as the PIC content is negligible in SPM in theBaltic Sea.

3. Results and discussion

3.1. Quality control and quality assurance

The method blanks contained in general between 10%and 20% of the substance quantity present in the sample.The results were corrected for the blank values. Therecoveries of the internal standard PCB 65 in the XADsamples from February 1998 through February 1999were very poor. A thorough investigation revealed thatthe problem could be remedied by extracting with ace-tone instead of acetonitril, and the analytical protocolwe had adopted from the IOC (1993) was modified ac-cordingly. As a result of this problem, the results for themore volatile congeners PCB 28, 31, and 101 in the earlyXAD samples had to be discarded. For all other sam-ples, both XAD and filters, the recoveries of the internalstandards PCB 65 and 207 exceeded 50%. The resultswere corrected for the recoveries. Duplicate samplesyielded coefficients of variance for the PCB concentra-tions between 10% and 40% in the dissolved fractionand between 10% and 30% in the particle fraction.Given the very low concentrations, we consider this tobe satisfactory.

3.2. PCB concentrations in the dissolved phase and boundto particles

The dissolved concentration of PCBs 28, 31, 101, 118,138, 149, 153, and 180 during the different expeditionsranged from 0.1 to 4.5 pg/l. The concentration of par-ticle bound PCBs in the water varied between 0.2 and7.1 pg/l or between 0.7 and 16.5 pg/(mg OC) whennormalised to the organic carbon content of the parti-cles. The most abundant congeners of those measuredwere PCBs 28 and 31 in the dissolved phase and PCBs138 and 153 in the particle phase. The results are com-parable with levels found by other groups in the centralBaltic Sea in August 1995 (Axelman et al., 2000) and inthe western and central Baltic Sea in April/May 1991(Schulz-Bull et al., 1995). In general the PCB concen-trations in the Baltic Sea are approximately one to twoorders of magnitude higher than in the northern NorthAtlantic (Schulz-Bull et al., 1998).

3.3. Regional variation

In order to investigate the regional variation of thePCB concentrations in the southern Baltic Sea, the meanconcentrations of each congener were calculated foreach expedition. The percent deviation of the concen-tration at each station from the mean of the 3–6 stationsfor that expedition was then determined. The transectsamples were not included in the calculation. In Fig. 2the percent deviations from the eight expeditions areplotted for each station for PCB 138 in the dissolved andparticle bound fractions. Some 90% of the data pointslie within �40% of the expedition mean, and there areno systematic differences between the stations. Similarresults were obtained for the other PCB congeners.Considering that the reproducibility of duplicate sam-ples was of the order of 10–40%, we concluded that thedata provide no evidence of systematic regional varia-tion of PCB levels in the water column. Consequently,mean values from each expedition are used for the re-mainder of the data interpretation.

3.4. Seasonal variation

The seasonal variation in the dissolved phase con-centrations is shown in Fig. 3(a), which plots the meanconcentrations and standard deviation for each of theeight expeditions. PCBs 28, 118, and 138 were selectedfor the figure to cover a large portion of the range inhydrophobicity of the studied congeners.

The particles in the study area are largely of biolog-ical origin (due to the depth of the water, resuspension isnot a major source of particles in the surface water).These particles develop in the surface water, and as aresult the PCBs associated with these particles must

R. Bruhn, M.S. McLachlan / Marine Pollution Bulletin 44 (2002) 156–163 159

originate from the dissolved phase. Consequently, theparticle bound concentration of PCBs in the surfacewater can be expected to be related to the quantity of theparticles in the water and the dissolved concentrationsof the PCBs. It is known that PCBs and other hydro-phobic organic chemicals associate primarily with theorganic material in the particles. As a result, it was de-cided that a bioaccumulation factor (BAF) calculated asthe quotient of the organic carbon normalised concen-tration in the particle bound phase CPorg and the dis-solved concentration CD would form the best basis forinterpreting the seasonal development of the particlebound concentrations of the PCBs.

BAF ½l=kg� ¼ CPorg ½pg=kg�=CD ½pg=l�: ð1Þ

The BAFs are presented in Fig. 3(b). The maximumvalues were observed during winter (February 98 andFebruary 99). During spring the values decreased, fol-lowed by an increase again in summer. Certain differ-ences were observed between the two years, namely thelower level of the winter BAF maximum in 1999 andthe comparable values for the March and May expedi-tions in 1999, whereas in 1998 the decrease in BAFpersisted into May. No pronounced differences betweenthe congeners are apparent.

As mentioned above, the particle development in thestudy area is governed primarily by biological produc-tion. The biological production in this part of the BalticSea displays a strong seasonal cycle. Conditions duringthe winter are stagnant due to light limitation and in-tensive mixing of surface water. This is followed by anintensive plankton bloom in spring when the uppermixed layer becomes shallower than the euphotic zone(Wasmund et al., 1998). The spring bloom typicallybegins in mid-March and continues into April until theavailable nutrients have been exhausted. Late spring andearly summer are characterised by relatively low levelsof production due to nutrient limitation. A small bloomof nitrogen fixing cyanobacteria often occurs in August.This is followed by a third bloom – in this case primarilydiatoms – in September/October. The amount of bio-mass produced during the summer and autumn bloomsis less than the amount produced during the springbloom (Wasmund et al., 1999).

In light of these conditions, two hypotheses can bepostulated for the seasonal variability of the BAFs. Oneis that the BAF is controlled by the plankton growthwhich causes disequilibrium between the dissolved andparticle phases, effectively diluting the particle boundconcentration. The second is that the sorption or par-titioning properties of the particles vary as the compo-sition of the plankton community changes during theyear.

The fact that the highest BAFs were measured inFebruary is consistent with the hypothesis that the BAFis controlled by disequilibrium induced by planktongrowth. In February the surface water had experiencedat least three months of very low growth conditions, andhence the particles had had a long time to approachequilibrium with the dissolved phase. This is the time ofyear when a partitioning equilibrium (and hence maxi-mum BAF) is most likely to exist. The subsequent de-crease in the BAF during the spring could be explainedby the development of new particles during the springbloom which would have had only a very short timeto approach equilibrium. Post-bloom, during the latespring and summer, the average age of the particulateorganic matter in the surface water increases, and onewould expect the BAF to rise again, assuming thatthe particle bound concentrations are limited by thekinetics of uptake from the dissolved phase. Labora-tory measurements of the uptake of PCBs by phyto-plankton during periods of rapid growth indicate thatan equilibrium partitioning state is not approached(Swackhamer and Skoglund, 1993). It has been pointedout that the approach to equilibrium may be size de-pendent, with a much more rapid equilibration for smallplankton (0.2–2 lm) (Axelman et al., 1997). However,the majority of the phytoplankton biomass in the BalticSea is formed by phytoplankton between 10 and 200 lm(Kahru et al., 1991), which is comparable with the size

Fig. 2. The percent deviation of the concentration of PCB 138 (dis-

solved and particle bound) at each station from the mean of the 3–6

stations for that expedition.

160 R. Bruhn, M.S. McLachlan / Marine Pollution Bulletin 44 (2002) 156–163

of the plankton used in the laboratory studies. Hencethis first hypothesis would seem to be consistent withcurrent knowledge of PCB uptake kinetics.

The second hypothesis is that the sorption propertiesof the particles change with time. The nature of theparticles in the surface water undergoes pronouncedseasonal changes. The primary production starts inspring with a bloom of diatoms, followed by dinofla-gelattes. In summer cyanobacteria are the dominantgroup and in autumn mainly diatoms occur (Wasmundet al., 1999). It has been shown that different planktonspecies display different BAFs. For instance, in a labo-ratory study of PCB uptake, Stange and Swackhamer(1994) found that BAFs for a given PCB congenervaried by more than a factor of 5 between three phy-toplankton cultures. Hence the second hypothesis wouldseem plausible.

A linear relationship between the BAF and KOW of aclass of chemicals (e.g. a slope of a log BAF vs. logKOW

plot of 1) has been used as a criterion for the presenceof equilibrium partitioning between the particle boundand dissolved phases (Mackay, 1982; Swackhamer andSkoglund, 1993). This is based on the assumptions that

when all compounds are at equilibrium the slope isalways 1 and that when the equilibrium is disturbed theless hydrophobic compounds will establish a new equi-librium more quickly, causing the slope to differ from 1.Log BAF is plotted against logKOW (taken from Haw-ker and Connell, 1988) for the last three expeditions inFig. 4. The slopes range between 0.63 and 0.76. Ac-cording to the above criterion, a partitioning equilib-rium was not present in the water column in the springand early summer of 1999; a slope less than 1 is con-sistent with dilution of the particle bound fractionwhich would support Hypothesis 1. However, the sci-entific evidence supporting the use of this criterion isscanty. The BAF vs. KOW relationship was originallydeveloped for soil/water mixtures. There have been fewcontrolled measurements of partitioning equilibriumbetween phytoplankton and the dissolved phase, andsome of these show slopes <1 (e.g. Stange and Swa-ckhamer, 1994). Studies of PCB partitioning between airand terrestrial plants have shown that the analogousslopes for this system (log BAF vs. logKOA) can rangefrom 0.57–1.15 depending on the species (K€oomp andMcLachlan, 1997). As a result, we believe that the

(a) (b)

Fig. 3. (a) Dissolved concentrations [pg/l] and (b) BAFs [l/mg] for PCBs 28, 118 and 138 for the eight cruises. The arithmetic means and standard

deviations are shown.

R. Bruhn, M.S. McLachlan / Marine Pollution Bulletin 44 (2002) 156–163 161

presence of a slope <1 in Fig. 4 is not sufficient to refuteHypothesis 2.

The temporal variability in the dissolved concentra-tions is characterised by relatively constant low levelsduring the winter and spring of 1998, higher levels inAugust, low levels again in winter 1999, but then anincrease in March followed by a decrease. However, thevalues in 5/98 and 5/99 are comparable (see Fig. 3(a)).

The dissolved concentrations of PCBs in the surfacewater are largely a result of two competing processes:air/sea gas exchange and sorption to particles. In someenvironments – especially during plankton blooms – theair to water flux of PCBs may be lower than the water–particle flux, and the dissolved phase can be depleted(Dachs et al., 1999). Such a depletion might be an ex-planation for the decrease in the dissolved concentra-tions observed between March and June, 1999, but wasnot clearly present for the same time period in 1998.Furthermore, investigations of the air/sea gas exchangeof PCBs conducted in parallel to this study showed thatthe air/water system was close to a partitioning equi-librium between March and June 1999 (Bruhn et al., inprep). No evidence of a pronounced depletion of thedissolved phase with respect to the atmosphere wasfound. Hence the dissolved concentrations are morelikely to have been driven by the factors governingequilibrium partitioning with the atmosphere, namelythe gaseous air concentration and the water tempera-ture. The more or less gradual increase in CD during thespring and summer of 1998 reflects the seasonal increasein gaseous air concentrations of PCBs that has beenobserved in this region (Agrell et al., 1999) and thatcan be attributed to warming of the land masses sur-rounding the Baltic. The influence of the increasingair concentration on the dissolved water concentration

is compensated by the gradual increase in water tem-perature and the corresponding increase in the air/water equilibrium partition coefficient. The differencesbetween CD measured in March 1998 and in March 1999indicate that other factors besides the surface watertemperature and the season influence CD. One possibleexplanation is that in March 1998 (before the cruise) thewind blew almost exclusively from the west, whereas inMarch 1999 easterly winds were measured 50% of thetime (Matth€aaus et al., 1999, 2000). Other studies haveshown that air masses that have passed over easternEurope have higher levels of PCBs than air massesoriginating from the west (Wittlinger and Ballschmitter,1987).

In conclusion, it would appear that the dissolved PCBconcentrations in the surface water of the Baltic Sea aredetermined mainly by the gaseous concentrations in theatmosphere and the water temperature. The sorption toparticles is in turn driven by the dissolved concentra-tions, but it is also influenced by other factors as seenby the variability in the BAFs. It is not currently pos-sible to establish whether this variability is due primarilyto non-equilibrium partitioning conditions induced byparticle growth or to seasonal differences in the sorptionproperties of the particles. More investigation of theequilibrium partitioning and contaminant turnover dy-namics in marine surface water is needed to resolve thisissue. Nevertheless, this study demonstrates that thedissolved and particle bound concentrations of PCBs inthe surface water of the southern Baltic Sea have apronounced seasonal variability. This has consequencesfor contaminant research and monitoring. For instance,when studying long term temporal trends in contami-nant levels, it is prudent to sample in the winter whenthe conditions are most stable.

Acknowledgements

We are grateful to the captains and crews of the A.v.Humboldt and P.A. Penck for their contribution to thiswork. We express our thanks to B. Koßurok, A. Lerzand E. Trost for their skilful assistance in taking andworking up the samples. P. K€oomp and D. Wodarg arekindly acknowledged for their help during sampling. Weexpress our thanks to K. Nagel and A.-M. Welz for thePOC measurements.

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