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Estuarine, Coastal and Shelf Science 82 (2009) 152–160

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Estuarine, Coastal and Shelf Science

journal homepage: www.elsevier .com/locate/ecss

The spreading of eutrophication in the eastern coast of the Gulf of Bothnia,northern Baltic Sea – An analysis in time and space

Cecilia Lundberg*, Britt-Marie Jakobsson, Erik BonsdorffEnvironmental and Marine Biology, Department of Biology, Åbo Akademi University, FI-20500 Åbo, Finland

a r t i c l e i n f o

Article history:Received 27 August 2008Accepted 5 January 2009Available online 13 January 2009

Keywords:eutrophicationnutrientslong-term changescoastal zonemultivariate analysisBaltic SeaGulf of Bothnia

* Corresponding author.E-mail addresses: cecilia.lundberg@abo.fi (C. Lund

abo.fi (B.-M. Jakobsson), erik.bonsdorff@abo.fi (E. Bon

0272-7714/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.ecss.2009.01.005

a b s t r a c t

In the Baltic Sea, the Gulf of Bothnia is the only sub-basin with only minor effects of eutrophicationmainly due to physical factors. Most evaluations of the state of the Gulf of Bothnia are based on offshoreinvestigations. In the present study the coastal zone of the eastern Gulf of Bothnia is analysed. Long-termdata (1980–2007) of total nitrogen and phosphorus, turbidity and oxygen are analysed using principalcomponent analysis (PCA) for spatial and temporal patterns in the trophic situation. The coastal zone isdivided into six regions: inner and outer areas of the Bothnian Sea and the Quark, and the outer areas ofthe southern and northern Bothnian Bay. The results show a degradation of water quality from north tosouth, and from outer to inner coastal areas. Eutrophication changes from an almost non-existingproblem in the Bothnian Bay in the north to clear signs of nutrient over-enrichment in the Bothnian Sea.This shows that even if eutrophication in the Gulf of Bothnia is not serious, the increasing trends innutrient levels should be seen as warning signals for the future, and remedies to combat eutrophicationshould be taken rapidly.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction Clear signs of eutrophication are lacking in the open areas, even

The first undisputable signs of pollution in the Baltic Sea wereregistered in the 1960s (e.g. Jansson, 1997; Elmgren, 2001). Today,nutrient over-enrichment, i.e. eutrophication, is arguably the mostserious threat to the Baltic Sea ecosystem (e.g. special issues ofAmbio, 1990, 2007). Similar physico-chemical drivers are involvedin the eutrophication process in the entire Baltic Sea, but the effectsand consequences vary in different sub-basins and regions (Lund-berg, 2005). The issue of eutrophication has reached a wider publicawareness due to the legislation in the European Union, namelythrough the Water Framework Directive (WFD), which aims toprotect and achieve ‘‘good’’ ecological status in all waters withinthe EU until the year 2015 (EC, 2000).

The Gulf of Bothnia (Fig. 1) constitutes ca. 30% of the area ofthe Baltic Sea and is the region that is so far least influenced bynutrient over-enrichment (Bernes, 1988; Håkansson et al., 1996).The water exchange time in the Gulf of Bothnia is about 7 years(Myrberg and Andrejev, 2006), compared to 25–30 years for theentire Baltic Sea, which helps to keep the effects of eutrophicationlow. The low salinity (<6) gives a weak stratification and thewater mass is effectively turned over twice a year (HELCOM,1996).

berg), britt-marie.jakobsson@sdorff).

All rights reserved.

though the amounts of nutrients have doubled in 30 years (HEL-COM, 1996, 2002; Fleming-Lehtinen et al., 2008) and cyanobacterialblooms have been reported (Hansson and Håkansson, 2007). In thecoastal zone point sources have an impact on the nutrient levels,and eutrophication problems have been observed locally. A well-mixed water column promotes good oxygen conditions andrestricts the influence of loadings from land to the narrow archi-pelago zone along the coast (HELCOM, 1996; Humborg et al., 2003;Kirkkala and Oravainen, 2005). In the Finnish coastal waters, thesituation has partly improved during the last 15–20 years. Thereasons are mainly efficient waste water treatment, and reductionsin point source loads (Pitkanen et al., 2001a).

The properties of the eastern (Finnish) and western (Swedish)coastlines differ in the Gulf of Bothnia. On the western side, thecatchments mainly consist of forests and mountains, whereas theeastern side of the Gulf is dominated by forests and peat bogs(Pettersson et al., 1997). The eastern, Finnish, coast is flat with smallrivers, enclosed embayments and shallow waters, which favourlocal eutrophication. The western or Swedish coast is more openand steep with large and mainly oligotrophic rivers. The signs ofcoastal eutrophication along the western coast are therefore few(HELCOM, 1996; Håkansson et al., 1996).

Almost all research done in the Gulf of Bothnia is focused on theoffshore areas (e.g. HELCOM, 1996, 2002; Fleming-Lehtinen et al.,2008), from which the state of eutrophication is assessed. In thispaper we analyse coastal eutrophication through the following

Bothnian Sea

Bothnian Bay

Quark

SWEDEN

FINLAND

9

8

76

54

321

4241

4039

38 3736

3534

3332

3130

2928

2726

2524

2322

2120

1716

1514

13

1211

10

25°0'0"E

25°0'0"E

20°0'0"E

20°0'0"E15°0'0"E

66°0'0"N

64°0'0"N

64°0'0"N

62°0'0"N

62°0'0"N

60°0'0"N

60°0'0"N

0 50 100 Km25

6

5

4

2

1819

Tornio

Vaasa

Uusikaupunki

Fig. 1. The Gulf of Bothnia with the locations of the 42 selected stations along the Finnish coast. The division into three regions, Bothnian Sea, Quark and Bothnian Bay, is due to thedeclining salinity (practical salinity scale). :¼ inner costal locations, C¼ outer coastal locations.

C. Lundberg et al. / Estuarine, Coastal and Shelf Science 82 (2009) 152–160 153

questions: How influenced by eutrophication is the Finnish coastalarea of the Gulf of Bothnia? Can a period of about thirty years showreliable trends of eutrophication? Are there differences in the north-to-south gradient as well as between the inner and outer archipelagozones? The special characters of the eastern coast of the Gulf ofBothnia with small rivers flowing through cultivated areas shouldfavour a process of nutrient over-enrichment at least locally. In thisstudy we have analysed temporal and spatial trends in eutrophica-tion using multivariate methods. The data originate from a spectrumof databases and reports covering the entire coastal zone of theeastern Gulf of Bothnia, and the long-term trends are analysed bysimilar approaches as in Lundberg et al. (2005).

2. Material and methods

2.1. Study area

The Gulf of Bothnia consists of two major basins, the BothnianSea (60.5�N–63.5�N, mean depth 68 m), and the Bothnian Bay(63.5�N–66�N, mean depth 43 m; Fig. 1). These two basins areseparated by the Northern Quark (hereafter the ‘‘Quark’’), a shallowsill of only 20 m depth between Sweden and Finland (HELCOM,1996; Håkansson et al., 1996). The freshwater discharge to the

Bothnian Bay and to the Bothnian Sea is of the same magnitude(Wulff et al., 2001), but the water volume in the Bothnian Sea isalmost three times the volume in the Bothnian Bay (Håkanssonet al., 1996). The brackish water from the surface of the Baltic Properinfluences the conditions in the Bothnian Sea, especially in thesouthernmost part (HELCOM, 1996). The open Gulf of Bothnia hasa decreasing salinity from 7–5 in the Bothnian Sea to 4–3 in theBothnian Bay. In the river mouths the freshwater content can beeven higher (Håkansson et al., 1996; HELCOM, 2002).

We have divided the Gulf of Bothnia into three regions: theBothnian Sea (BS), the Quark (Q) and the Bothnian Bay (BB), due todecreasing salinity. The Bothnian Sea and the Quark are furtherseparated into inner and outer coastal/archipelago areas, BSi, BSo,Qi and Qo. As there are no representative public data from the innerareas of the Bothnian Bay, we divided the outer area into two zones,a southern and a northern part (BBo-s, BBo-n), based on thegeographical positions of the monitoring stations (betweenstations number 33 and 34; Fig. 1, Table 1).

2.2. Data material and selection

This study is based on data from the Finnish coastal waters ofthe Gulf of Bothnia. The coastal waters are defined according to the

C. Lundberg et al. / Estuarine, Coastal and Shelf Science 82 (2009) 152–160154

WFD (EC, 2000). 42 monitoring stations have been selected (ofa total of over 1000) in order to cover the coastal waters of theentire area (Fig. 1, Table 1). The selection criteria were thefollowing: the sampling series must be continuing for minimumone decade of which data for not more than three years are missingand the stations should not be situated in contact with an outletfrom a point source. The stations are situated from the town ofTornio/Torneå (65�4005700N, 24�1404900E) in the north to the town ofUusikaupunki/Nystad (60�4401500N, 21�1602800E) in the south. Thesampling stations are included in water quality monitoringprograms in the Regional Environment Centres of Finland (Lapland,North Ostrobotnia, West Finland, and Southwest Finland; SYKE,

Table 1The 6 regions and the 42 stations with geographical position, depth (m), tempera-ture (t �C; average from year 1998), degree of exposure (Exp., �), water exchange rate(W.e.r.), and salinity (psu).

Region Station Location Depth(m)

t (�C) Exp. W.e.r. Salinity(psu)

BSi 1 60�4401500N, 21�1602800E 11.1 16.4 90 2 –2 60�4700200N, 21�1900400E 24.9 16.6 80 2 5.73 60�4801700N, 21�2103000E 12.5 16.7 10 2 6.24 61�1304700N, 21�2400100E 13.4 18.7 80 1 –5 61�1500400N, 21�3003200E 8.4 19.1 90 1 –6 61�3600400N, 21�3303600E 5.0 18.1 200 2 0.57 61�4000600N, 21�2804900E 5.3 16.8 30 2 3.38 62�1901200N, 21�1703100E 17.3 15.9 90 2 5.4

BSo 9 61�0103100N, 21�1702700E 16.9 15.1 160 1 5.610 61�0503400N, 21�2200700E 11.8 16.8 190 1 –11 61�0605000N, 21�2502900E 11.2 16.9 160 1 5.812 61�0900400N, 21�2300800E 14.7 16.6 190 1 6.013 61�3001700N, 21�2205300E 20.6 15.8 360 1 5.214 61�4303200N, 21�2604200E 15.5 16.0 230 1 5.115 61�4904100N, 21�2401200E 10.2 16.3 230 1 5.3

Qi 16 62�5403000N, 21�2102500E 5.1 17.4 150 3 5.017 63�5500600N, 21�5900200E 5.4 17.9 10 3 3.718 63�0603500N, 21�3104800E 5.7 17.6 100 3 4.419 63�0706900N, 21�2700800E 11.5 16.9 70 2 4.620 63�1003100N, 21�2805100E 12.3 16.3 200 2 4.421 63�1905900N, 21�1900000E 9.7 14.6 150 2 3.4

Qo 22 62�5603300N, 21�1601100E 9.9 16.8 330 2 5.123 62�5900500N, 21�0201200E 18.6 16.0 310 1 5.024 63�0704200N, 21�1900000E 15.0 16.3 310 1 4.825 63�2401300N, 21�5903600E 26.5 12.1 300 1 3.526 63�2703400N, 21�4502800E 12.2 12.7 60 1 3.7

BBo-s 27 63�4005600N, 22�2501400E 10.3 13.4 360 1 –28 63�4304200N,

22�3702500E12.1 14.1 150 1 3.3

29 63�4402200N,22�3302200E

15.6 16.7 310 1 3.4

30 63�5002700N, 22�3801800E 11.9 13.7 180 1 –31 63�5205400N,

23�0303500E4.9 13.2 130 1 3.5

32 63�0203400N, 21�2705800E 18.7 13.0 310 1 3.333 63�5703800N,

22�5505500E15.3 12.9 340 1 3.3

34 64�3904900N,24�2303600E

8.6 14.3 170 1 –

35 64�4305900N,24�3005900E

3.9 15.2 260 1 –

BBo-n 36 64�5800000N,25�0400000E

10.0 14.8 360 2 –

37 65�0400000N,25�0400000E

12.0 13.0 320 1 –

38 65�0800000N,24�3600000E

23.9 11.5 360 1 2.7

39 65�3505400N,24�5500700E

13.8 15.8 310 1 1.9

40 65�3604600N, 24�2100200E 16.9 14.9 360 1 1.641 65�3802000N, 24�2905100E 16.5 14.2 350 1 1.442 65�4005700N, 24�1404900E 11.1 16.2 280 1 1.1

2008), in urban regions or for specific point sources. The selecteddata is available on the Environmental Information System (Hertta)database, which is maintained by the Finnish Environment Insti-tute (SYKE, 2008). The long-term trend was chosen to comprisea time series of 27 years, i.e. from 1980 to 2007.

2.3. Study parameters

For the division of the sampling stations into inner and outerarchipelago areas depth, water exchange rate, exposure andtemperature were analysed. The rate of water exchange is calcu-lated based on the morphology and the distance of the water area ofthe sampling station from the open sea, according to the SwedishEnvironmental Protection Agency (Anon., 1999) and exposure isestimated as in von Numers (1995). We randomly selected August1998 to represent the average temperature in the water column forthis analysis, as Appelgren and Mattila (pers. comm.) pointed outthat this parameter is almost constant from year to year in thisnorthern Baltic Sea region.

To study temporal and spatial trends in the water quality inrelation to eutrophication, phosphorus, nitrogen, oxygen andturbidity were used. The parameters were analysed according toFinnish Standard Association (SFS, 2008) and the studied timeperiod was based on the period used for summer monitoring byFinnish environmental authorities. The sampling stations had beenvisited 1–8 times a year during the summer months studied.

The total amounts of phosphorus (TP) and nitrogen (TN) in thesurface water (1 m depth) are measures of both organic and inor-ganic contents of each nutrient in the water and were thereforechosen. In this case, only the summer values (July–August) wereused. Turbidity is affected by insoluble particles in the water andvaries with type of suspended matter. In the Gulf of Bothnia, largeinflows of humic and clay particles from rivers have a considerableimpact on the transparency of the water. Correlation betweenphytoplankton and transparency in the water is hence a nonsig-nificant parameter, and makes Secchi disc readings in the Gulf ofBothnia unreliable as direct indications of water quality (Anon.,1999). We used the turbidity values (1 m depth) from the summerperiod (July–August). For oxygen we have used the annualminimum in the bottom water (0.5–1 m above the bottom surface).

Chlorophyll a could not be used in this study as too many datawere missing in the material in the national database.

2.4. Numerical analyses

Principal component analysis (PCA) is used both to divide thesampling localities into inner and outer archipelago zones and forthe analysis of the eutrophication parameters in order to detectspatial and temporal changes in the data. This method requiresa normal distribution of the variables and creates correlationstructures in the data plot (Jenerette et al., 2002). As PCA allowsdifferent parameters and measurement scales in the same analysisand transforms them into dimensionless scales, the method is wellsuitable for analysis of environmental data (Clarke and Warwick,1994). It is important to bear in mind that different PCAs are notdirectly comparable with each other (Chatfield and Collins, 1980).The clusters are due to a similarity matrix with normalisedEuclidian distance and are here expressed from the third level ofdichotomisation of the dendrogram. The principal componentscores and the eigenvectors of the original variables for the first twoprincipal component axes are correlated with Pearson’s (bivariatenormal distribution) or Spearman’s tests (variables transformed toranks included; Quinn and Keough, 2002). The correlation coeffi-cients are expressed as vectors for each analysis.

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34

35

3536

37

38

3940

41

42

Outer coastal area

Inner coastal area

Depth

Exp.

W.e.r.

Temp.

1825 38 2 22 36 35 26 31 34 28 30 27 37 40 41 32 35 39 24 29 13 23 9 15 42 10 11 12 14 4 5 7 20 21 8 3 1 19 6 17 16

0

1

2

3

4

Eu

clid

ean

d

istan

ce

Outer coastal area Inner coastal area

Fig. 2. The principal component ordination for the division of the stations into inner and outer coastal areas. A) The PCA plot. The vectors stand for the correlations between the PCAscores and the descriptive variables (Exp.¼ exposure, Temp.¼ temperature, W.e.r.¼water exchange rate). B) The dendrogram of the clusters with normalised Euclidian distance ofthe 42 sampling stations.

C. Lundberg et al. / Estuarine, Coastal and Shelf Science 82 (2009) 152–160 155

In the division into inner and outer archipelago regions, all 42sampling stations and their physical parameter values were ana-lysed. For the assessment of the degree of eutrophication, thedivision into the six regions (BSi and BSo, Qi and Qo, BBo-s, andBBo-n) was followed. For each region the data set was divided intofive five-year time periods and the final period to three years:1980–1984, 1985–1989, 1990–1994, 1995–1999, 2000–2004,2005–2007. The values are given as averages for each time periodand region and thereby the influence of any missing values isminimised. Requirements for the analyses were that every timeperiod had values from three or more of totally five years for eachsampling station. For the final period (2005–2007) data from atleast two out of three years were required. All sampling stationswithin a certain region and from a certain time period were thenpooled and their relative mean values were used in the final PCA.Data from all regions and all six time periods, from 1980 to 2007,are included in the same graph. A similar analysis was done withthe data material divided into only three geographical groups inorder to receive possible changes in a north–south direction toincreasing nutrient-enrichment. I.e. all stations from the Bothnian

Sea (BS), the Quark (Q) and the Bothnian Bay (BB), respectively,were pooled. Besides that, we used the same requirements as in theanalysis with six different groups.

For the PCAs the PRIMER software (Clarke and Gorley, 2006) wasused. The correlation calculations were done in SPSS.

3. Results

3.1. Inner and outer coastal areas

The salinity limit in the open areas between the Bothnian Seaand the Quark is 5. North of the Quark, the salinity has a sharpdecline below 4, which is the boarder between the Quark and theBothnian Bay (Håkansson et al., 1996). The division of the samplingstations into the Bothnian Sea, the Quark and the Bothnian Bay waspartly based on the geographical position, partly on the salinityvalues at the stations (Fig. 1).

The division into inner and outer archipelago areas is based onthe results from a PCA with depth, temperature, exposure rate andwater exchange for each sampling station. The result depicts

Table 2Spearman’s correlation coefficients between the descriptive parameters and theprincipal component scores for axes 1 and 2 in Fig. 2. Upper values are correlationcoefficients (r2) and lower values probabilities (p). Significant correlations in bold(0.001).

PCA 1 PCA 2

Depth �0.664 �0.6870.000 0.000

Temperature 0.781 �0.2160.000 0.169

Exposure �0.769 0.2470.000 0.114

Water exchange rate 0.748 �0.1380.000 0.383

C. Lundberg et al. / Estuarine, Coastal and Shelf Science 82 (2009) 152–160156

a division into two main groups (Fig. 2). The vectors in Fig. 2 expressthat the stations in the right-hand group are characterised byhigher temperatures and a low water exchange rate, i.e. have moresheltered localities (inner coastal areas) compared to the stations inthe left-hand group, where the depths and degrees of exposure arehigher (outer coastal areas). The vectors are the results of thecorrelation of the variables and the principal component scores.The first two principal component axes explain approximately 73%of the variations (57.0% for axis 1, 16.3% for axis 2). The coefficientsand significances for the correlations (Spearman’s) between axes 1and 2 in the PCA and the four parameters used in the analysis aregiven in Table 2 and show that depth gives the strongestcorrelations.

Stations 2, 25 and 38 are the deepest (>24 m; Table 1). Thereforethese stations are singled out in Fig. 2. Due to its exposure andwater exchange rate, we have included station number 2 in theinner coastal areas. Of similar reasons, stations 25 and 38 are sortedwith the outer areas (Fig. 2, Table 1).

3.2. The state of eutrophication

The spatial means of the nutrient concentrations, turbidity andoxygen content in the bottom water in the six geographical groupsduring the 27-year period from 1980 to 2007 are presented in Table3. Summer values of nitrogen are varying from high mean values(330–420 mg l�1) in the inner areas of the Bothnian Sea, the Quarkand in the southern Bothnian Bay, to low values (275–290 mg l�1) in

Table 3An overview of the descriptive parameters during the period 1980–2007 (mean� standarof TN, TP and turbidity, and the annual mean minimum of O2 in bottom water) on the s

Parameters Region

BSi BSo Qi

TN (mg l�1)Mean� SD 419.4� 176.7 275.3� 55.7 138.2� 1Ranges 160–910 110–580 130–870N 188 272 197

TP (mg l�1)Mean� SD 26.1� 12.5 15.7� 5.8 13.9� 3.Ranges 5–87 2–59 5–24N 202 256 196

Turbidity (fnu)Mean� SD 2.8� 3.1 1.5� 1.0 1.6� 0.8Ranges 0.2–22 0.1–6.5 0.6–6.7N 191 246 158

O2 (mg l�1)Mean� SD 9.7� 2.6 10.5� 1.8 10.4� 1.6Ranges 0.0–14.9 0.0–15.0 6.0–14.0N 849 1099 413

the rest of the area. The phosphorus limitation in the Gulf ofBothnia is evident in the summer values of TP, which are low tovery low (10–16 mg l�1). The inner areas of the Bothnian Sea are anexception, where the mean value is high (26 mg l�1). Besides, forregions BSi and BBo-n turbidity is moderate according to Anon.(1999). In BSi the value is high (3 fnu; formazine turbidity units),and in the BBo-n low (1 fnu). The means of the oxygen concen-trations are high (>9 mg l�1) in all the investigated areas, and thelower ranges reach periodic hypoxia or anoxia only in the inner andouter regions of the Bothnian Sea.

3.3. Detection of spatial and temporal trends

The PCA for patterns of eutrophication in the six study areas ispresented in Fig. 3, and for the larger sub-regions in Fig. 4. Theeigenvalues for the first two axes in Fig. 3 explain over 91% of thevariation (axis 1: 76.2%, axis 2: 15.2%). In Fig. 4 the correspondingvariation is also 91% (axis 1: 73.1% and axis 2: 18.5%). Tables 4 and 5present the coefficients and significances for the correlations(Pearson’s) between PCA axes 1 and 2 in Figs. 3 and 4 respectively,and the analysed eutrophication-related parameters are presented.The variables from Table 4 are used for the direction of the vectorsin Fig. 3, and from Table 5 in Fig. 4.

In Fig. 3 the state and development of the stations in the innercoastal areas of the Bothnian Sea (BSi) differ markedly from that ofthe five other areas analysed. The largest differences between theinner areas of the Bothnian Sea and the other regions are found inthe high concentrations of nutrients and in the high level ofturbidity. The changes in the environmental conditions, as in theordination in Fig. 3, can be seen as a cascade-effect or a gradualchange in a northward direction and from inner to outer areas. Thenegative changes begin in the inner parts of the Bothnian Sea andthe Quark already in the 1980s, whereas the corresponding outerareas have been more stable over time.

The position of the southern part of the Bothnian Bay in Fig. 3 isexplained in Table 3, as the nitrogen concentration is at a high meanlevel for this region. It is worth noticing that the two regions north ofthe Quark have developed towards a more oligotrophic state (Fig. 3).

All stations from the Bothnian Sea, the Quark and the BothnianBay, respectively, are pooled into three groups or regions in order toillustrate the large-scale overall patterns due to graduallyincreasing eutrophication in Fig. 4. No distinction between outerand inner areas is made. The differences between the sub-basins in

d deviation (SD), ranges and total number (N) of observations for the summer valuestudied stations, grouped into six regions.

Qo BBo-s BBo-n

02.6 279.8� 52.0 330.5� 116.1 289.4� 58.0140–560 116–1100 190–820221 553 438

9 10.9� 4.1 11.1� 8.7 10.4� 4.13–33 1–140 1–33221 563 435

1.2� 0.6 1.3� 1.0 0.9� 0.50.3–4.1 0.3–9.2 0.0–5.2211 357 383

11.0� 1.9 11.6� 1.6 10.8� 1.81.8–15.2 4.5–15.8 2.7–15.8990 1867 1256

-5 -4 -3 -2 -1 0 1 2PC axis1

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2.6

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Fig. 3. The principal component ordination for the data split into six groups. A) PCA plot: Bothnian Sea inner (BSi) and outer (BSo), Quark inner (Qi) and outer (Qo), and BothnianSea outer south (BBo-s) and north (BBo-n), and six time periods: 1980–1984 (¼I), 1985–1989 (¼C), 1990–1994 (¼-), 1995–1999 (¼utrif;), 2000–2004 (¼A), 2005–2007 (¼<). Thecorrelations between the PCA scores and the analysed variables indicated with vectors. B) The dendrogram of the clusters with normalised Euclidian distance: Bothnian Sea (1),Quark (2) and Bothnian Bay (3), and six time periods: 1980–1984 (¼0.1), 1985–1989 (¼0.2), 1990–1994 (¼0.3), 1995–1999 (¼0.4), 2000–2004 (¼0.5), 2005–2007 (¼0.6).

C. Lundberg et al. / Estuarine, Coastal and Shelf Science 82 (2009) 152–160 157

the Gulf of Bothnia appear even clearer compared to Fig. 3. TheBothnian Sea corresponds to the highest nutrient values over theinvestigated time period 1980–2007, which is also evident in Table3. The time period with the most marked deterioration of theenvironmental state appears to be the beginning of the 1980s andagain from 1990 to 2004, which corresponds to the situation ininner areas of the Bothnian Sea in Fig. 3.

The Quark region shows a clear increase in nutrients, especiallynitrogen, during these three decades (Fig. 4). The patterns of theQuark in Fig. 4 and the inner Quark in Fig. 3 are similar, whichshows that the inner areas govern the changes in water quality. Thelack of eutrophication symptoms in the Bothnian Bay is also illus-trated in Fig. 4. The trend for the Bothnian Bay is positive, towardsbetter quality of the water, with the greatest changes alreadyduring the 1980s and 1990s (Fig. 4).

4. Discussion

The Gulf of Bothnia differs from the rest of the Baltic Sea in thesense that the region lacks unambiguous signs of eutrophication(HELCOM, 2003; Kirkkala and Oravainen, 2005; Lundberg, 2005).This is in contrast to the nearby Gulf of Finland, which is regardedas the most heavily eutrophied sub-basin of the Baltic Sea (e.g.HELCOM, 2002; Lundberg, 2005). The loads of nutrients and theamount of phytoplankton in the Gulf of Finland are about 8 timesand 20–25% higher, respectively, compared to the average in theGulf of Bothnia (Vehvilainen, 2005). The poor oxygen conditions inthe sediment–water interface in the Gulf of Finland make thesituation even worse by accelerating the process of leakage ofphosphorus from the sediment (Pitkanen et al., 2001b). When long-term data from the Finnish coast to the Gulf of Finland were

-3 -2 -1 0 1 2PC axis1

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BS

Q

TN

TP

O2

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1.1

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1.6

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2.1

2.2

2.3

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2.5

3.1

3.6

3.4

3.5

3.2

3.30

1

2

3

4

Eu

clid

ean

d

istan

ce

Fig. 4. The principal component ordination for the data split into three groups. A) PCA plot: Bothnian Sea (BS), Quark (Q) and Bothnian Bay (BB), and six time periods: 1980–1984(¼I), 1985–1989 (¼C), 1990–1994 (¼-), 1995–1999 (¼:), 2000–2004 (¼A), 2005–2007 (¼<). The correlations between the PCA scores and the analysed variables indicated withvectors. B) The dendrogram of the clusters with normalised Euclidian distance: Bothnian Sea (1), Quark (2) and Bothnian Bay (3), and six time periods: 1980–1984 (¼0.1), 1985–1989(¼0.2), 1990–1994 (¼0.3), 1995–1999 (¼0.4), 2000–2004 (¼0.5), 2005–2007 (¼0.6).

C. Lundberg et al. / Estuarine, Coastal and Shelf Science 82 (2009) 152–160158

analysed in a comparative study, a clear difference in the eutro-phication situation between coastal and offshore areas was detec-ted. In the gradient west to east, no trends were evident (Lundberget al., 2005).

However, when the eutrophication situation in the Gulf ofBothnia was assessed based on information in available literature inLundberg (2005), increasing trends of eutrophication-relatedparameters were indicated. Such parameters are nutrient concen-trations, primary production and dissolved organic material. Theseincreasing trends should not be neglected, and taken as earlywarning signals for possible forthcoming eutrophication. Thereason for the increasing nitrogen load can possibly be found inincreased river discharges (Sonesten, 2007). According to Savchuk(2005), all the basins of the Baltic Sea exported more nitrogen than

imported from a nearby basin. Thus, the ecosystem is not ina steady state as the sinks do not increase with the loads (Wulffet al., 2001). The amounts of nutrients transferred between the Gulfof Bothnia and the Baltic Proper are negligible. There seems to bea small export of nitrogen to the Baltic Proper and an import ofphosphorus to the Gulf of Bothnia (Wulff et al., 2001; Savchuk,2005; Rahm and Danielsson, 2007).

In this study we have focused on the coastal zone on the easternside of the Gulf of Bothnia. The coastal area is important forbiogeochemical cycles and estuaries are efficient filters for materialdelivered from land, which makes the production higher near thecoast compared to the open sea (Humborg et al., 2003; Vahteraet al., 2007). The effects of environmental pollution are of particularinterest in the coastal areas of the Gulf of Bothnia, as many

Table 4Pearson’s correlation coefficients between the effect parameters and the principalcomponent scores for axes 1 and 2 in Fig. 3. Upper values are correlation coefficients(r2) and lower values probabilities (p). Significant correlations in bold (0.001).

PCA 1 PCA 2

TN �0.807 �0.5340.000 0.001

TP �0.947 0.0970.000 0.581

Turbidity �0.921 0.1110.000 0.527

Oxygen 0.808 �0.5470.000 0.001

C. Lundberg et al. / Estuarine, Coastal and Shelf Science 82 (2009) 152–160 159

industries (e.g. paper, pulp and metal) and harbours are situatedalong the coasts (HELCOM, 1996; Håkansson et al., 1996). However,after June 2007, only one single hot spot (mining waste to theBothnian Sea from the River Dalalven in Sweden) of the originaleight in 1992 is left on HELCOM’s list of serious environmentalthreats in the Gulf of Bothnia catchment area (HELCOM, 2008).

4.1. Comparisons of the inner and outer coastal areas

According to Humborg et al. (2003), most of the estuaries in theGulf of Bothnia behave differently from the eutrophied estuaries inthe southern Baltic Sea. Both the phosphorus-limited Bothnian Bayand the nitrogen-limited Bothnian Sea have higher concentrationsof dissolved inorganic nitrogen and phosphorus, respectively,compared to the discharging rivers. The estuaries in boreal andarctic waters import nutrients from the open sea. Thereforea depletion of nitrogen during summer in rivers and estuaries isboth natural and common in the Gulf of Bothnia (Humborg et al.,2003). This phenomenon is seen in Table 3 for BSo, Qi, Qo and BBo-n. Eutrophied estuaries in the southern Baltic have much higherconcentrations of nutrients than localities offshore (Jørgensen andRichardson, 1996), whereas in the Gulf of Bothnia the concentra-tions of TN and TP in the sediments are comparable with the rangesoffshore (Humborg et al., 2003). In spite of that, the effects of theland-delivered nutrients have most influence in the shelteredcoastal areas, which is clearly seen when inner and outer areas inthe Bothnian Sea are compared (Fig. 3, Table 3). However, theeffective water exchange in the whole Gulf of Bothnia promotesgood oxygen conditions, which in turn reduces the effects ofinternal phosphorus loading. From Table 3 it is evident that theinvestigated stations in the Bothnian Sea have sporadically expe-rienced hypoxia, whereas critical oxygen values are not detected inthe Quark and in the Bothnian Bay.

Table 5Pearson’s correlation coefficients between the effect parameters and the principalcomponent scores for axes 1 and 2 in Fig. 4. Upper values are correlation coefficients(r2) and lower values probabilities (p). Significant correlations in bold (0.001), bolditalic (0.05).

PCA 1 PCA 2

TN �0.714 0.6790.001 0.002

TP �0.940 �0.1220.000 0.631

Turbidity �0.915 0.0600.000 0.812

Oxygen 0.834 0.5100.000 0.030

According to the Swedish national monitoring program in theGulf of Bothnia, the nutrient situation has been stable during thelatest decade. The annual mean values of chlorophyll a andphytoplankton biomass decrease from south to north in the Gulf ofBothnia, due to the lower concentrations of nutrients (Wikner andAndersson, 2006).

4.2. Eutrophication trends from south to north

The overall main contributor of nutrients to the Gulf of Bothniais agriculture and forestry. Further, the amount of natural back-ground losses is larger than in other catchment areas to the BalticSea (HELCOM, 2004). Almost half of the amount of freshwaterdischarges from rivers to the Baltic Sea have their origin in the Gulfof Bothnia, especially in the Bothnian Bay (Myrberg and Andrejev,2006). Rivers from Finland and Sweden contribute equal amountsof water to the Bothnian Bay (Bergstrom and Carlsson, 1994), butthe amount of organic material in the Finnish rivers is the doublecompared to the Swedish levels (Pettersson et al., 1997). Aquacul-ture, i.e. fish farms, has local impact in the coastal areas of theeastern Bothnian Sea (HELCOM, 2004; Oravainen, 2005).

The Bothnian Bay is effectively separated from the Bothnian Seaby the Quark, which forms a productive mixing area. The watercirculation in the Bothnian Bay is isolated (Myrberg and Andrejev,2006), and the cold water and the long duration of the ice coveragein combination with low phosphorus concentrations in the waterprevent the development of a distinct spring bloom (Graneli et al.,1990; Andersson et al., 1996). The benthic system in the BothnianBay is not driven by the sedimentation of primary producers andinstead organic material is utilized from the river discharges(Kuparinen et al., 1996; Stockenberg and Johnstone, 1997). Neitherthe outer coastal areas in the northern nor the southern BothnianBay show any signs of increasing eutrophication (Fig. 3). On thecontrary, any possible correlation to nutrient over-enrichment hasweakened since the 1980 (Figs. 3 and 4). According to Wikner andAndersson (2006), the concentration of phosphorus has decreasedin the open water of the Bothnian Bay followed by a drop in thephytoplankton biomass.

These evaluations are based on 42 sampling sites. The timeperiod studied covering almost three decades gives a reliablepicture of the situation and development of eutrophication in theGulf of Bothnia. However, the positions of the sampling stations arescattered. BS, Q and BB are represented by 11–16 sampling stationseach, of which 5–9 belongs to either inner and outer (BS and Q) orsouthern and northern (BB) areas. It is noteworthy that a distanceof approximately 130 km between stations number 15 and 16 isrepresented by only one single site (number 8) in the northern partof the BS. Similarly, more than 100 km separate stations 33 and 34in the BB, which forms the boarder between BBo-s and BBo-n in thisstudy. Also in the northernmost part of BB the stations occur inpatches (Fig. 1, Table 1). This is all due to the fact that a commoncoordinated long-term national monitoring program is lacking inFinland. The data are collected from different monitoring projectsconducted for different purposes, and hence a comprehensiveevaluation over the entire area is hard to make. As Erkkila andKalliola (2007) state, a synchronization and strategic planning tomeet both short-term and long-term goals is urgently needed.

5. Conclusion

Compared to the rest of the Finnish coastal waters and the BalticSea, the Gulf of Bothnia is in good environmental condition. Thenutrient levels of the 42 stations studied along the coast duringalmost thirty years are generally low. As expected, the signs ofeutrophication increase in a southward direction, and close to the

C. Lundberg et al. / Estuarine, Coastal and Shelf Science 82 (2009) 152–160160

mainland. Few studies of the nutrient situation and long-termdevelopment along the eastern coast of the Gulf of Bothnia havebeen done. The multivariate assessment in this study points outa degradation for the Bothnian Sea, a status quo situation for theQuark and slight improvements for the Bothnian Bay. However, thePCAs reveal that most of the stations, which show no direct signs ofeutrophication, are undergoing a slow gradual degradation,possibly governed by the overall trends in the coastal Baltic Sea.

Acknowledgements

We are grateful to the staff at the West Finland Regional Envi-ronment Centre in Vaasa and Tapio Saario at the Southwest FinlandRegional Environment Centre in Turku for help with the Herttadatabase. Patrik Kraufvelin assisted with the PRIMER analyses andMikael von Numers with the map to Fig. 1. Part of the work wasfinancially supported by the MISTRA program MARE.

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