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PCB Levels in Laminated Coastal Sediments of the Baltic Sea along Gradients

of Eutrophication Revealed by Stable Isotopes (δ15N, δ13C)Author(s): Markus Meili, Per Jonsson, Rolf CarmanSource: AMBIO: A Journal of the Human Environment, 29(4):282-287. 2000.Published By: Royal Swedish Academy of SciencesDOI: http://dx.doi.org/10.1579/0044-7447-29.4.282URL: http://www.bioone.org/doi/full/10.1579/0044-7447-29.4.282

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282 © Royal Swedish Academy of Sciences 2000 Ambio Vol. 29 No. 4–5, August 2000http://www.ambio.kva.se

INTRODUCTIONPolychlorinated biphenyls (PCBs) are highly toxic and persist-ent organic compounds that have been produced and dispersedwidely since the 1930s. Since the early 1970s, the use of PCBhas been substantially reduced in the Baltic Sea area after thegradual implementation of a ban in the west and economicchanges in the east, but environmental PCB levels remain a con-cern (1). Since most of the PCB congeners have a high affinityfor particles in natural waters, they have a tendency to accumu-late in sediments, which therefore may form a major reservoirof PCB (e.g. 2, 3). Coastal sediments are therefore likely to beimportant in controlling the fate and turnover of many contami-nants that are released into coastal waters. On the other hand,the accumulation of sediments and associated contaminants incoastal areas may be expected to be low or absent because ofstrong erosion forces causing a gradual focusing from shallowto deep areas by repeated resuspension. A more rarely consid-ered aspect is the inverse pathway, i.e. the pollution of coastalareas from the open sea, in particular after reduction of coastalemissions. However, these simultaneous fluxes are difficult toquantify, since little is known about the turnover of coastalsediments and associated pollutants. This is the case particularlyfor heterogeneous semi-enclosed areas such as the Baltic archi-pelagos. In the case of PCB, even basic information such as con-centrations in coastal sediments is scarce (4) and needs to be up-dated.

In recent years, potential interactions of eutrophication andcontaminant cycling have been highlighted (e.g. 5) and amongother things, resulted in the EUCON research program of theSwedish Environmental Protection Agency (6). One of the mainissues in focus is the question whether local or regional changes

in nutrient load (eu-/oligotrophication) can cause measurablechanges in the concentration (e.g. dilution) or in the fluxes (e.g.burial) of contaminants in the sediments, changes that may belinked either to to scavenging and recycling in the water col-umn or to diagenesis and retention in the sediment. Such changesare most likely to be found in coastal areas with local nutrientemissions. Therefore, and as part of the EUCON framework, weinitiated the investigation of levels, composition, and fluxes ofboth PCB and nutrients in coastal sediments of the Baltic Sea.

Our first efforts were focused on a systematic mapping of PCBlevels in sediments on bottoms with undisturbed fine sedimentaccumulation in semi-enclosed coastal areas (bays) with vary-ing anthropogenic impact. This included a detailed analysis ofbasin morphometry, sediment characteristics, geochronology,and the distribution of contaminants and nutrients. Particular at-tention was paid to a systematic collection of samples, includ-ing site selection and description, sediment core description, andselection of subsamples. Instead of using traditional samples ofsurficial sediments down to a given depth, we extracted datedsubsurface samples that were designed to represent sedimentsdeposited over a given time span. In this way, we were able tominimize bias induced by pollution history by using sedimentsof equal age, to minimize the effect of seasonal and interannualchanges by covering an equal time span of several years, and tominimize differences in initial diagenesis by excluding thesurficial sediments deposited during at least the first year beforesampling in order to obtain an assessment of truly buried PCB.

MATERIALS AND METHODS

Selection of Sampling SitesSediment samples were collected along the Swedish coast of

the NW Baltic proper from the northern part of the Stockholmarchipelago to the St. Anna archipelago southeast of Norrköping(Fig. 1). This region is rich in archipelago areas, which consti-tute a geomorphological transition zone between a lake-rich landand the sea, and which are common also along the Finnish coast.The study covered about 200 km of coastline (58.3–59.6°N) andincluded undisturbed reference areas as well as areas with knownemissions of PCB and/or nutrients. The locations of the sam-pling sites were chosen to cover both hydrodynamically shel-tered (semi-enclosed) bays surrounded mainly by land as wellas more exposed archipelago areas. Before sampling, hydrody-namic boundaries of each bay were determined from nauticalcharts, and the bottoms within the boundaries were mappedacoustically using a low-frequency sediment echo-sounder anda side-scan sonar to identify and characterize soft sediments (7,8). Finally, the sampling positions within each bay were selected,usually in the deepest areas with presumably undisturbed accu-mulation of fine sediments. Within each bay, usually 5 and atfew sites 1–6 cores were collected across the accumulation area.Sampling depths ranged from 11 to 107 m.

Sediment Collection and CharacterizationFrom the research vessel Sunbeam, 88 sediment cores were col-lected during summer 1996 from 19 bays in different archipelagoareas along the Swedish coast of the Baltic (Table 1). Position-

PCB Levels in Laminated Coastal Sediments ofthe Baltic Sea along Gradients of EutrophicationRevealed by Stable Isotopes ( δ15N, δ13C)

Markus Meili, Per Jonsson and Rolf Carman

A systematic investigation of PCB in coastal sediments ofthe Baltic Sea was initiated in 1996 by retrieving datedsubsurface layers (1992–1994) from 88 sites in 19 shelt-ered archipelago areas, where laminated sediments werefound to be common. PCBs7 (HELCOM’s standard sumof 7 congeners, ≈1/6 of total PCB) was quantified, as wellas the abundance and isotopic composition of nutrients (N,C), in order to trace effects of eutrophication and potentialinfluences on the turnover of PCB. In most of the areas,the median of PCBs7/C was within 70 to 120 ng g–1 C. Thisis similar to recent values in regional offshore sediments,and thus consistent with a rapid exchange between coastaland open waters. Elevated PCBs7 levels up to around1000 ng g–1 C were found within 50 km from Stockholm.Here, but not in other areas, PCBs7/C increased linearlywith the degree of eutrophication. This was evident fromthe isotopic composition of sediment N and C, reflectinglocal eutrophication as well as its link to freshwater input.In contrast, concentrations or ratios of nutrients in sedi-ments were poor indicators of eutrophication. Our resultsdemonstrate that the Stockholm watershed still is a sourceof both PCB and excess nutrients to the Baltic Sea, butdo not support any enrichment or dilution of PCBs7/Cratios caused by eutrophication.

283Ambio Vol. 29 No. 4–5, August 2000 © Royal Swedish Academy of Sciences 2000http://www.ambio.kva.se

ing was based on DGPS and depth echo sounding. Sedimentcores were collected using a Gemini twin gravity corer equippedwith two transparent Perspex tubes with an inner diameter of 80mm (9). The corer was slightly modified in order to use tubesof different length (89 or 120 cm). Only cores with an undis-turbed sediment surface overlain by clear water were retained;most of them also showed vertical features such as regular visualstructures and concentration gradients suggesting minimal han-dling artifacts. One core of each twin was sectioned horizontally(usually on board ship) into 1 cm (sometimes 0.5 cm) thicksubsamples that were stored frozen in 60 ml vials from the timeof sectioning until further treatment in the laboratory. The sec-ond core was cut longitudinally into 2 halves and examined onboard the ship or in the laboratory, to document vertical struc-tures such as lamination as well as other features (7, 8).

Sample Preparation and AnalysisFrozen and preweighed subsamples were freeze-dried to constantweight for at least 72 hrs at 210 Pa (0.1 mbar) and reweighed todetermine water content. All dry samples were homogenized andtransferred to scintillation vials for 137Cs analysis and further stor-age.

137Cs was analyzed using a multichannel gamma-spectrometer(Intertechnique Model 4000 Gamma Counting System) equippedwith 3 x 3 inch NaI well detector. Readings after 100 min count-ing time were converted to 137Cs activities by comparison withdry and wet standards containing pure 137Cs and Chernobylradionuclide mixtures, and by correcting for background radia-tion, radiation from other radionuclides, sample geometry, andradioactive decay. Counting error (1 SD) was usually around 2%for surface samples and ≤ 1% for the most contaminated strata,and detection limit for deep samples (3 SD of background) wasaround 0.03 Bq g–1 dw.

Total carbon and total nitrogen were analyzed by high-tem-perature combustion (1000°C) using a LECO CHNS-932, whereCO2 and SO2 are measured by infrared absorption and NO2 bythermal conductivity. After about every 12th sample (3 to 8 mgdw), two blanks and two sulfomethazine standards were analyzedas control samples and used for drift corrections of the initialcalibration if necessary. Analyses with deviations of standardsby more than ± 0.2 µg for blanks and ± 1% (relative) for C, and

± 0.4 µg for blanks and ± 3% (relative) for N, were usually re-jected. For typical samples, detection limits were 0.02% dw forboth C and N, and coefficients of variation (c.v.) about 1–2%of the reading. The c.v. in samples was similar to the c.v. instandards suggesting that much of the variability was due to sam-ple weighing rather than sample inhomogeneity. Accordingly,the c.v. of C/N ratios was only about 0.5% among standards,and as low as 1% even among different samples adjacent withincores. It was assumed that total C values represent organic Csince the carbonate content in surficial Baltic sediments is neg-ligible (10), and that total N contains mainly organic N and typi-cally less than 10% inorganic nitrogen (10, 11).

Preparation of Dated Subsurface SedimentsSediment layers were dated with two independent methods: from137Cs measurements at intervals of usually 1 cm and from laminacounts in vertically sectioned cores (see above). Radiocesiumdating was based on usually very distinct 137Cs signals relatedto fallout from Chernobyl (1986) and nuclear weapon testing(1963) (12). Where lamination of sediments was distinct, a dat-ing of different sediment depths or at least an estimate of sedi-mentation rate was obtained by counting and measuring the dark/light layers in the description cores. Apart from initial notes,color slides transferred to CD format were used for reexamina-tion, analysis, and interpretation based on digital images.

To obtain data from sediments of the same age for all sites,subsamples representing the sediment deposited during the en-tire 3-year period of 1992–1994 were selected and pooled forfurther analyses to obtain data from sediments of the same agefor all sites. For variables with data available for all layers,weighted mean values for the same strata were calculated forcomparisons.

Stable IsotopesAnalyses of carbon and nitrogen isotopic composition were madeusing a Carlo Erba NC 2500 element analyzer connected to aFinnigan MAT Delta Plus isotope ratio mass spectrometer atcontinuous flow and a combustion temperature of around1000°C. Results are reported versus atmospheric nitrogen (N)

Figure 1. Sediment sampling along the Swedish coast of the Balticproper in June-September 1996. Four different areas can bedistinguished (cf. Table 1): the Stockholm archipelago (A and B), theSödermanland archipelagoes (C), and the Östergötland archipelagoes(D). Crosses denote the location of the 88 coring sites that aredistributed among 19 bays (usually 5 cores each). Arrows show themost important rivers and their mean water discharge (m 3 s–1; nationalsurvey data from 36). National grid coordinates (rectangular, km) aswell as longitude/latitude ( °) are given.

Table 1. Locations of sampling sites (bays) in different coastalregions of the Baltic Sea (labelled A–D, from north to south).Distinct hydrological chains are indicated with numbers (1–5)following the flow of freshwater and pollutants from areas ofdischarge to areas characterized by exchange with the open Balticproper (o). Coordinates (latitude and longitude, decimal degrees)and depths refer to the location of the deepest sampling site.

Region Bay Lat. Long. Depth Label(°N) (°E) (m)

Northern and central Stockholm archipelagoes:Torsbyfjärden 59.36 18.45 47 A1Solöfjärden 59.38 18.44 50 A2Östra Saxarfjärden 59.42 18.54 67 A3Gälnan 59.53 18.76 30 A4Kanholmsfjärden 59.35 18.78 101 A5Skatfjärden 59.57 18.76 38 oTräsköfjärden 59.46 18.82 46 oSkagsfjärden 59.38 18.76 45 oMöja Söderfjärd 59.36 18.85 107 o

Southern Stockholm archipelago:Baggensfjärden 59.29 18.32 54 B1Erstaviken 59.22 18.40 72 B2

Södermanland archipelagoes (Södertälje/Nyköping)Näslandsfjärden 59.07 17.68 39 C1Himmerfjärden 58.99 17.72 43 C2Tvären 58.78 17.43 75 o

Östergötland archipelagoes (Norrköping)Slätbaken 58.45 16.55 42 D1Trännöfjärden 58.42 16.69 20 D2Gropviken 58.33 16.70 34 D3Orren 58.24 16.81 64 oDalvåmmen 58.23 16.85 41 o

284 © Royal Swedish Academy of Sciences 2000 Ambio Vol. 29 No. 4–5, August 2000http://www.ambio.kva.se

or PDB limestone (C) as standards and calculated as δ15N or δ13C(‰) = [Rsample / Rstandard – 1] x 1000, where R is (15N/14N) or (13C/12C), respectively. Regular duplicate determinations on samplesshowed a mean difference of around 0.10‰ and 0.15‰ for Cand N, respectively.

PCBSubsamples representing the sediment deposited during 1992–1994 were pooled and sent for PCB analyses to Trent Univer-sity, Environmental and Resource Studies, Peterborough, Canada(13-15). Freeze-dried sediment samples (0.5–1 g) were extractedby sonication into acetone. The extracts were filtered through aBuchner funnel packed with a Whatman glass-fiber filter andtopped with Celite. The filtrates and acetone rinsings were ro-tary evaporated to a volume of approximately 1 ml and the ex-tract solvent was changed to hexane by successive additions ofhexane and rotary evaporation. The extract volume was adjustedto 1 ml prior to silica gel clean-up/subfractionation by columnchromatography with activated silica gel (Baker, 60–200 mesh).Elution with 40 ml of hexane produced a fraction containing allPCBs and some pesticides. Sulfur compounds were removed byprecipitation with Hg and filtration through a Whatman glass-fiber filter. After addition of 2,2,4'-trimethylpentane, the extractswere evaporated to a volume of 0.1 ml.

PCB congeners were analyzed by high resolution gas chro-matography (HRGC) using a Varian model 3500 GC with a 60m DB-5 (J&W) capillary column (0.25 mm i.d.) and an elec-tron capture (63Ni) detector. Quantification of all PCB congenerswas done using a standard mixture of PCB congeners (CLB1,National Research Council, Halifax, Canada) supplemented withcongeners 28, 47, 52, 66, 99, 105 and 149 (Ultra Scientific,Rhode Island). The congeners analyzed represent approximately50% by weight of the total congeners in Aroclors 1242, 1254,and 1260. The detection limits were 0.3–1.0 ng ml–1 for theanalytes, with the sensitivity increasing with the degree of chlo-rination. In typical samples, detection limits for individual con-geners were 0.02–0.08 ng g-1 dw.

Quality assurance included regular extraction and analysis ofprocedural blanks and certified reference materials. Extractionefficiencies for all quantified PCB congeners were determinedto be > 90% based on spiked samples, and no correction wasmade.

Of the 90 congeners detectable in the environment, up to 26congeners were quantified: IUPAC #18, #31, #28, #52, #49, #47,#44, #66, #101, #99, #119, #87, #110, #151, #118, #149, #153,#105, #138, #156, #180, #170, #199, #195, #194, #209 . Here,we report the sum of 7 congeners, in accordance with HELCOMroutines: IUPAC #52, #101, #105, #118, #138, #153, #180.These 7 congeners typically accounted for about 30–45% of theanalyzed congeners, and an estimated 15–20% of total PCB, al-though this fraction may vary with source and recipient.

RESULTS AND DISCUSSIONSediment characteristicsLaminated sediments indicating long periods of anoxia werefound to be common in sheltered archipelagoes. Lamination cancover a substantial fraction of coastal basins (often roughly 20%)and has in some areas been recorded even at water depths of lessthan 20 m (see 7). In these sediments, bioturbation can be ne-glected as a confounding factor for radiometric dating. Datingwas further facilitated by comparatively high concentrations offallout 137Cs (0.3 to 2 Bq g–1 dw) originating from the Chernobylaccident (1986) and to some extent from nuclear weapon test-ing (1963). Distinct vertical profiles of 137Cs allowed dating withexceptional accuracy, in some cases even with seasonal resolu-tion (12). Steep concentration gradients of 137Cs extending some-times less than 0.1 cm after 10 years at natural conditons sug-gest that the 137Cs migration by diffusion is minimal in these

laminated sediments despite anoxic conditions and brackish wa-ter. Radiometric dating usually coincided with lamina counts,suggesting annual varves.

High mean sedimentation rates of 0.5 to 5 cm yr–1 were ob-served, based on the location of the distinct Chernobyl 137Cs ho-rizon at depths of 5 to 50 cm below the sediment surface in lami-nated sediments where post-depositional redistribution appar-ently is minimal (12). This implies that the sediment accumula-tion in deep offshore areas of the Baltic proper (0.05–0.3 cmyr–1 as recently compiled by Ilus et al. (16) is exceeded by up to2 orders of magnitude in semi-enclosed coastal areas where lo-cal conditions favor inflow and retention of settling particles. Adetailed regional investigation of the southern part of our studyarea (7) also showed one order of magnitude higher sedimenta-tion in semi-enclosed coastal areas than in rather deep offshoreareas. Similar patterns have recently been observed also in theGulf of Finland (17), but were partly attributed to river input,which is negligible in most of our areas (Fig. 1). We attributeour findings to erosion and resuspension of sediments in wind-exposed coastal and offshore areas of the region, wheresediments are increasingly exposed to erosion because of thegradual land uplift caused by the ongoing crustal rebound afterdeglaciation (18). This is consistent with the finding that the sedi-mentation rates along the coast of central Sweden are not onlymanifold higher than those in offshore areas, but also higher thanin the Gulf of Finland (0.2–2 cm yr–1 including coastal sediments(17) where land uplift is slower (19). The contribution of recenteutrophication to the observed coastal sedimentation fluxes canbe regarded as negligible, since the mean organic content no-where exceeded 20% dw.

PCB Concentrations and SourcesThe concentration of PCBs7, the sum of the 7 HELCOM con-geners, in subsurface sediments deposited 1992–1994 rangedfrom 2–33 ng g–1 dw when considering single cores, and from3–22 ng g–1 dw when considering bay medians. All bay medi-ans above 10 ng g–1 dw were found in areas close to Stockholm(A1, A2, A3, B1; Table 1), suggesting local pollution sources.All of the few medians below 5 ng g–1 dw were found in en-closed areas that are affected by coastal eutrophication (C1, C2,D1; Table 1). This may be interpreted as an effect of dilutionwith organic matter, although sediment organic contents of <20% dw do not allow for much variability in dry-matter accu-mulation. However, the sediment organic content in these areasof eutrophication was unusually low, with C contents around 4%dw even in areas of undisturbed fine sediment accumulation,whereas all other bays showed values around 6–7% dw. Thesefindings suggest a reduced sedimentation rather than a dilutionof PCB on a dry-weight basis and are consistent with PCB con-centrations being related or even proportional to the organic con-tent of the sediments.

Following the common assumption that PCB is associatedmainly with organic matrices, all PCBs7 were normalized withrespect to the carbon content of the sediment. This can be con-sidered to reflect the degree of contamination of the organic ma-trix better than dry-weight related concentrations, which are af-fected by the dilution with variable amounts of minerogenic par-ticles. The resulting ratios are expressed as PCBs7/C, with unitsof ng g–1 C. These normalized concentrations were quite uniformamong most areas, showing bay medians of 70–120 ng g–1 C ex-cept near Stockholm where bay medians reached values up to370 ng g–1 C.

The high values near Stockholm are well in line with a re-cent study of lake and marine sediments within about 10 km fromthe inner city (20; PCBs7 recalculated excluding congener #28).In the innermost part of the Stockholm archipelago, surface sam-ples from cores with an apparently undisturbed stratigraphyshowed PCBs7 values of around 100 ng g–1 dw, corresponding

285Ambio Vol. 29 No. 4–5, August 2000 © Royal Swedish Academy of Sciences 2000http://www.ambio.kva.se

around Stockholm at present.Comparison of the data from our coastal study and the urban

study shows that the contamination of recent sediments is about10 times higher in central Stockholm than in the neighboringcoastal areas. Interestingly, the PCB concentrations decline veryrapidly to the east, even when following the large freshwater cur-rent carrying pollutants in this direction. Background values arereached within about 50 km, i.e. within the archipelago. Appar-ently, pollutants from Stockholm are efficiently diluted by ma-rine sediments, even within the archipelago. The influence of ma-rine currents on the distribution of contaminants also in shelteredcoastal areas is supported by among other things the intrusionof Chernobyl 137Cs that can be traced in the sediments of theStockholm archipelago within a few months after deposition onthe Bothnian Sea and by a continuing accumulation in largeamounts (12, 22). The spatial distribution of PCB and other con-taminants such as 137Cs suggests that much of the pollution ofcoastal archipelagoes can be attributed to the retention and ac-cumulation of contaminants originating from the open Balticrather than from coastal emissions within the area. This continu-ing transport of contaminants from the open sea to coastal areasevidently deserves our attention, which is commonly focused onthe transport in the opposite direction.

The degree of contamination of most coastal sediments (70–120 ng g–1 C) may indeed be the same as in offshore areas ofthe northern Baltic proper and the Bothnian Sea, where PCBs7values of 80–140 ng g–1 C have been reported (1, 2). This wouldbe consistent with the earlier notion that contaminants from pointsources are rapidly distributed over large areas and also with adynamic exchange of sediments between coastal and offshoreareas. Other studies, however, suggest higher values in offshore

as well as coastal sediments, even if normalized to organic matter(e.g. 22). Unfortunately, comparisons among studies are ham-pered not only by differences in sampling strategy, but also byanalytical inconsistencies. Two of our sediment samples wereanalyzed in three different well-established laboratories with ap-parently rigorous quality control. The subsamples analyzed wereall taken from the same freeze-dried sample to preclude differ-ences in sample collection and preparation. Values of PCBs7 re-ported by laboratories A, B and C were 5, 10 and 13 ng g–1 dwfor the first sample, and 13, 16 and 19 ng g–1 dw for the secondsample, respectively. Evidently, spatial differences or temporalchanges based on comparison of different published studiesshould be interpreted with care if they are less than threefold,apparently even if based on different reports from the same labo-ratory. Trends observed within a given study on the other handmay be more reliable.

Coastal areas showing similar PCB levels but a 10-100 timeshigher sediment accumulation than in offshore areas may, de-spite their limited extension, be important as a temporary orquasi-permanent store of PCB, and may even influence the turn-over of PCB in the Baltic Sea as a whole despite their limitedextension. A detailed assessment is, however, beyond the scopeof this report.

In conclusion, the comparison of concentration gradients re-veals that even near urban centres, much or most of the PCBload to coastal areas originates from remote point sources lo-cated anywhere in the Baltic watershed, or from atmosphericdeposition of globally dispersed PCB (1–3), and correspondinglythat point source discharges are rapidly dispersed over large ar-eas. The observation that steep local gradients in PCB concen-tration are maintained around Stockholm despite intense coastal

Different views of the Stockholm Archipelago,showing the variability of urban, agricultural,and marine influences, all of which affect thewater quality in sheltered coastal areas likethese. Photos: M. Meili.

to roughly 1000 ng g–1 C according to conversionsbased on reported loss on ignition (LOI ≈ 2…3 C).In deeper sediment layers, which were dated radio-metrically to around 1965-75, PCBs7 values peakedat around 1000 ng g–1 dw, corresponding to around10␣ 000 ng g–1 C (20). Accordingly, undisturbed sedi-ment cores suggest that the PCB exposure in themain watercourse through central Stockholm hasdeclined by roughly one order of magnitude over thepast 3 decades. In sediment cores with irregular or-ganic matter profiles and indications of sedimentmixing based on blurred profiles of 137Cs from theChernobyl fallout in 1986, peak values of PCBs7were lower and surface values were several timeshigher than in apparently undisturbed cores. Thissuggests that vertical mixing and resuspension ofsediments are major sources of PCB in the waters

286 © Royal Swedish Academy of Sciences 2000 Ambio Vol. 29 No. 4–5, August 2000http://www.ambio.kva.se

Figure 3. Concentrations of PCB in sediment, given as PCBs7 per unitof organic carbon, vs the relative degree of eutrophication in 19 shelt-ered areas along the Swedish coast of the Baltic Sea (Table 1; Fig. 1).The relative degree of eutrophication is based on the isotopic composi-tion of sediment nitrogen ( δ15N) and carbon ( δ13C) and given as thedeviation of δ15N (in ‰) from a baseline value that is determined by δ13C(Fig. 2). Data represent dated subsurface sediments (whole 3-year core

Figure 2. Median isotopic composition of nitrogen ( δ15N) and carbon(δ13C) in dated subsurface sediments (whole 3-year core sectiondeposited 1992–1994) from 19 sheltered areas along the Swedish coastof the Baltic Sea in 1996 (Table 1; Fig. 1). Distinct hydrological chainsin different areas are indicated (A-D), by numbers (1–5) following theflow of freshwater and pollutants from areas of coastal discharge toareas characterized by exchange with the open Baltic (circles on black

mixing suggests that the Stockholm watershed remains as a largesource of among other PCB to the Baltic Sea, in spite of sig-nificant emission control over the past decades. Outside such gra-dients, the uniformity of PCB levels over large areas may fa-cilitate the study of locally varying factors influencing thebiogeochemical cycling of PCB. This includes the effects ofeutrophication in areas where it is maintained by continuous in-put of excess nutrients.

Indicators of EutrophicationIn order to quantify the effect of local and regional eutrophicationon sediment compositions and potential influences on PCBturnover, various nutrient-related aspects were quantified. Theelemental composition of bulk sediment was spatially very uni-form, in particular when correcting for variations in inorganicmatter content. Carbon concentrations usually varied within 4–7% in areas of undisturbed fine sediment accumulation. This cor-responds to about 10–20% (dw) organic matter in thesesediments, which is in line with data from offshore areas of theBaltic proper (e.g. 11, 23). Ratios of C/N, providing informa-tion about the composition and potentially, also the source andfate of organic matter, were fairly constant among areas and werein most sediments around 6.5 to 7 g g–1. This suggests that thesupply of organic matter is mainly from pelagic production(Redfield C/N ≈ 5.7 g g–1 in marine sedimentation) rather thancontinental input (cf. C/N ≈ 10–20 g g–1 in the sediment ofpolyhumic lakes), even in most coastal areas. Total P concen-trations were typically 1–1.5‰ dw, and the organic P contentcorresponded to around 3‰ of the organic dry mass in the sedi-ment, without systematic variation among areas (unpubl. data).

The Stockholm watershed is known as a source of excess nu-trients to the Baltic. Municipal sewage is today discharged partlyeast of the city close to some of our study areas (A1, A2), partlysouth of the city close to some of our other study areas (C1, C2,Table 1). The effects of the resulting local eutrophication areeasily traced in the water, e.g. in urban monitoring programs.However, the elemental composition of our sediments showslittle or no effects such as nutrient enrichment, neither in the con-centrations of the major nutrients (C, N, P) nor in any of theirratios (N/C, P/C). Apparently, the consumption, decompositionand/or dilution of locally produced organic matter is very effi-cient.

In contrast to elemental composition, the isotopic composi-tion (δ15N, δ13C) of coastal sediments reveals distinct influencesof local eutrophication. In several areas with high nutrient lev-els in the water and close to known nutrient emission sources,e.g. Stockholm, the sediments show mean δ15N values of 5–8‰that are elevated relative to more remote areas showing valuesaround 4–5‰ (Fig. 2). High δ15N values are typical of watersin densely populated areas with domestic and/or agricultural nu-trient or sewage discharge (24–28) and are probably to some ex-tent the result of enhanced denitrification due to nutrient excessand expansion of anoxic conditions (25, 29). Since almost allof our samples were taken in sediments that showed at least par-tial and usually continuous lamination, the high δ15N values can-not be explained solely by anoxic conditions in deep waters with-out the presence of a continuous local nutrient discharge in thearea. Indeed, high δ15N values a often coincide with compara-tively low δ13C values which clearly reflect the local influenceof continental (terrestial and/or freshwater) production and itsdispersal into the Baltic Sea. Similar isotopic gradients have alsobeen reported from the southern Baltic (25). In our least per-turbed archipelago areas, the isotopic composition of recentsediments is very similar to that in offshore deeps of the Balticproper (δ13C ≈ –23... –24‰; δ15N ≈ 3...5‰; 30). This applieseven to areas in the proximity of pollution sources, which sug-gests a rapid dilution of local discharges, and which is again con-sistent with a dynamic exchange between archipelago and off-shore areas. The isotopic signature of the sediments in offshoreareas is fairly similar to that inremote coastal and shelf areas ofthe Atlantic and other oceans (δ13C ≈ –20…–23‰; δ15N ≈4…7‰; 26, 27, 31, 32), but is slightly shifted towards valuesfound in sediments from humic forest lakes which probably arerepresentative of the riverine input of primary or recycled ter-restrial matter and some planktonic production to the Baltic (δ13C≈ –28…–31‰; δ15N ≈ 1…3‰; 33, 34). Accordingly, the organicmatter in these coastal sediments and possibly also in the off-shore Baltic proper can be regarded as a mixture of marine andcontinental primary production, with marine production clearlydominating (Fig. 2) (cf. 25, 30). The line between these endpointscan be regarded as the present baseline, and the deviation fromthis line as the anthropogenic impact. Indeed, the influences ofeutrophication and freshwater input covary and are most pro-nounced close to areas with municipal or agricultural discharges

PC

Bs7

(ng

gC

–1)

area, see Table 1).The variationamong cores fromthe same bay wasusually within 1‰,and often within0.3‰ in bayswithout signs oflocal pollution, buthigher nearpollution sourcesas also indicated bythe highest singlevalue (A2). Theinfluences ofcontinental inputfreshwater inflowand eutrophica-tion are indicated.

section deposited1992–1994)collected in 1996;medians fromusually 5 cores.Shaded areashighlight the linearrelationship inareas aroundStockholm and theabsence of such arelationship in otherareas.

287Ambio Vol. 29 No. 4–5, August 2000 © Royal Swedish Academy of Sciences 2000http://www.ambio.kva.se

References and Notes

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3. Jonsson, P. 2000. Sediment burial of PCBs in the offshore Baltic Sea. Ambio 29, 260–267.4. Reutergård, L. 1988. Identification and distribution of chlorinated organic pollutants

in the environment. National Swedish Environmental Protection Board, Report 3465.ISBN 91-620-3465-0. 74 p.

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6. Jonsson, P. (Ed.). 1996. EUCON—Interactions Between Eutrophication and Contami-nants in the Aquatic Environment. Research programme for the period 1995–1999.Swedish Environmental Protection Agency, Report 4690, ISBN 91-620-4690-x.

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17. Vallius, H., Kankaanpää, H., Niemistö, L. and Sandman, O. 1998. Sedimentation andwithin-basin variations in the Gulf of Finland as determined by 137Cs tracer. In: Ilus,E. (Ed.), Dating of Sediments and Determination of Sedimentation Rate. Proceedingsof a seminar held in Helsinki 2–3 April 1997. Report STUK-A145, pp. 131–135.

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24. Cabana, G. and Rasmussen, J.B. 1996. Comparison of aquatic food chains using nitro-gen isotopes. Proc. Natl. Acad. Sci. USA 93, 10844-10847.

25. Voss, M. and Struck, U. 1997. Stable nitrogen and carbon isotopes as indicator ofeutrophication of the Oder river (Baltic Sea). Mar. Chem. 59, 35–49.

26. McClelland, J. W., Valiela, I. and Michener, R. H. 1997. Nitrogen-stable isotope sig-natures in estuarine food webs: A record of increasing urbanization in coastal water-sheds. Limnol. Oceanogr. 42, 930–937.

Markus Meili is associate professor in biogeochemistry atStockholm University. This work was initiated while he wasassistant professor in geoecology at the Institute of EarthSciences, Uppsala University, Sweden, after graduation atthe Institute of Limnology, Uppsala University. His researchinterest is the coupled cycling of contaminants andnutrients in aquatic ecosystems and includes the tracing ofbiotic and abiotic processes in food webs and sediments.His address: Institute of Applied Environmental Research,Stockholm University, SE-106 91 Stockholm, Sweden.E-mail: Markus.Meili@itm.su.se

Per Jonsson has been project manager for the EUCONframework. He works part-time as a principal scientificofficer at the Swedish Environmental Protection Agencyand part-time as adjunct professor in recent marinesedimentology at Uppsala University. His main scientificinterest is in sediment and contaminant dynamics in themarine environment.His address: Institute of Earth Sciences, Uppsala University,SE-752 36 Uppsala, Sweden.E-mail: Per.Jonsson@geo.uu.se

Rolf Carman is assistant professor at Stockholm University.His main focus is on geochemical processes related to thenutrient cycling in the Baltic Sea.His address: Department of Geology and Geochemistry,Stockholm University, SE-106 91 Stockholm, Sweden.E-mail: Rolf.Carman@geo.su.se

(Fig. 2). Accordingly, the isotopic composition of sediments canbe used to quantify the impact of eutrophication as the devia-tion of δ15N from a baseline value that is a function of δ13C. Thisprovides an easily measured eutrophication index. Since a sin-gle sediment sample can represent several years or decades oforganic input, a survey of sediment isotopic compositions canbe a convenient and reliable way of quantifying long-term andlarge-scale changes of nutrient dynamics in coastal areas or en-tire watersheds. The biological relevance of our sediment iso-tope values is supported by comparison with an earlier isotopestudy focusing on the food web in one of our study areas (35)and by similar studies in other coastal areas (e.g. 26–28).

PCB and EutrophicationIncreased primary production due to eutrophication might beexpected to affect the settling and burial of PCB, either by a di-lution of PCB within the expanded matrix of organic particlesif both their standing stock and their PCB sorption affinity, e.g.distribution coefficient KOC, are sufficiently high to deplete thedissolved phase of PCB, or by an enrichment of PCB if newly

27. Tucker, J., Sheats, N., Giblin, A. E., Hopkinson, C. S. and Montoya, J. P. 1999. Usingstable isotopes to trace sewage-derived material through Boston Harbor and Massa-chusetts Bay. Mar. Environ. Res. 48, 353–375.

28. Fry, B. 1999. Using stable isotopes to monitor watershed influences on aquatic tro-phodynamics. Can. J. Fish. Aquat. Sci. 56, 2167–2171.

29. Altabet, M. A., Francois, R., Murray, D. W. and Prell, W. L. 1995. Climate-relatedvariations in denitrification in the Arabian Sea from sediment 15N/14N ratios. Nature373, 506–509.

30. Struck, U., Emeis, K. C., Voss, M., Christiansen, C. and Kunzendorf, H. 2000. Recordsof southern and central Baltic Sea eutrophication in δ13C and δ15N of sedimentary or-ganic matter. Mar. Geol. 164, 157–171.

31. Fry, B., Hopkinson, C.S., Nolin, A. and Wainright, S. C. 1998. 13C/12C composition ofmarine dissolved organic carbon. Chem. Geol. 152, 113–118.

32. Peters, K. E., Sweeney, R. E. and Kaplan, I. R. 1978. Correlation of carbon and nitro-gen stable isotope ratios in sedimentary organic matter. Limnol. Oceanogr. 23, 598–604.

33. Meili, M., Fry, B. and Kling, G. W. 1993. Fractionation of stable isotopes (13C, 15N) inthe food web of a humic lake. Verh. Internat. Verein. Limnol. 25, 501–505.

34. Meili, M., Kling, G. W., Fry, B., Bell, R. T. and Ahlgren, I. 1996. Sources and parti-tioning of organic matter in a pelagic microbial food web inferred from the isotopiccomposition (δ13C and δ15N) of zooplankton species. Arch. Hydrobiol. 48, 53–61.

35. Hansson, S., Hobbie, J. E., Elmgren, R., Larsson, U., Fry, B. and Johansson, S. 1997.The stable nitrogen isotope ratio as a marker of food-web interactions and fish migra-tion. Ecology 78, 2249–2257.

36. SNA. 1995. The National Atlas of Sweden: Climate, Lakes, and Rivers (Eds) Raab, B.and Vedin, H., 176 p. (In Swedish).

37. We acknowledge the excellent work by the crew of R/V Sunbeam as well as PiaHolmberg and numerous other field and laboratory assistants. PCBs were analyzed byChris and Tracy Metcalfe, Trent University, Canada. Stable isotopes were analyzedby Magnus Mörth, Stockholm University. Two referees provided valuable commentson an earlier version. This study was financially supported by the EUCON frameworkof the Swedish Environmental Protection Agency.

produced organic sediments have a higher sorption affinity thanin pre-eutrophication sediments (e.g. due to elevated lipid con-tent). In our study, PCBs7/C increased linearly with the degreeof eutrophication in the Stockholm area, but appeared independ-ent of eutrophication in other areas (Fig. 3). This confirms thatthe Stockholm watershed still is a source of PCB and excess nu-trients to the Baltic Sea, and is consistent with a co-transport ofPCB and organic matter. However, our results from severalcoastal gradients of eutrophication do not show any measurabledilution or enrichment of sediment PCBs7/C ratios that can beattributed to coastal eutrophication. The absence of such a rela-tionship cannot be explained solely by regional marine mixingof coastal and offshore sediments and a resulting dilution of lo-cal gradients, since isotopic signatures of sediment organic matterindicate not only that coastal eutrophication has a significant lo-cal influence on organic matter cycling, but also that this influ-ence is preserved locally in sediments, similar to PCB gradients.We conclude that even a substantial eutrophication may have anegligible influence on the long-term burial of PCB in sedimentsunder natural conditions.