Estuarine, Coastal and Shelf Science (1997) 44 (Supplement A), 63-72

Coastal Eutrophication: Causes, Consequences andPerspectives in the Archipelago Areas of theNorthern Baltic Sea

E. Bonsdorff'", E. M. Blomqvist", J. Mattila" and A. Norkko"

"Husii Biological Station, Department of Biology, Abo Akademi University, FIN-22220 Emkarby, Aland Islands,FinlandbWater Protection Association of SW Finland, FIN-20360 Turku, Finland

Coastal eutrophication has, since the early 1970s, become the foremost threat to the marine ecosystem of the ArchipelagoSea (the Aland Islands and the SW Finnish archipelago) in the northern Baltic Sea. Nutrient levels (N, P) have risensignificantly both in coastal areas and basin-wide, which has led to increased primary production (both pelagic andbenthic), decreased transparency, increasing amounts of oxygen-consuming drift-algal mats at shallow and intermediatebottoms, and changes in zoobenthos and fish communities.

Local nutrient input originates mainly from agriculture, riverine input, municipal wastewaters, aquaculture andairborne loading. Levels indicate an even distribution of nutrients from the inner areas to the open coast, reducing thenatural diluting or filtering effects of the mosaic archipelago system.

Future prospects for the archipelago and coastal ecosystem are poor unless local and regional measures to drasticallyreduce nutrient levels of the archipelago are undertaken. Even then, positive effects are unlikely to show immediately.

© 1997 Academic Press Limited

Keywords: archipelago and coastal waters; eutrophication; aquaculture; long-term trends; Baltic Sea

Introduction

The Baltic Sea, with a short and dramatic geologicalhistory (about 10 000 years with alterations betweenlimnic, marine and brackish conditions) and markedphysical, chemical and biological gradients character­izing the system (Voipio, 1981; Leppakoski &Bonsdorff, 1989), is one of the most severely affectedand best studied sea areas of the world. The awarenessof environmental hazards stems from the 1960s and1970s, when the effects of pollution (heavy metals,organochlorines, oil spills) and eutrophication becameobvious. Since the mid-1970s, the state and conditionof the Baltic Sea and its sub-regions have been moni­tored closely under the supervision of the BalticMarine Environment Protection Commission­Helsinki Commission (HELCOM, 1990). As oneresult of this awareness, and of measures taken conse­quently, the levels of ' traditional' pollutants in thesystem have decreased, and some endangered specieshave recovered both in terms of levels of toxicantsand decreased habitat disturbance (Elmgren, 1989;HELCOM, 1990). Not as much attention has beenfocused on the remedary measures associated witheutrophication, however, although the phenomenonhas been documented in some detail at all levels in themarine ecosystem both in coastal waters and in the

0272-7714/97/44A063+ 10 $25.00/0

open sea (e.g. Cederwall & Elmgren, 1980, 1990;Larsson et al., 1985; Kautsky et al., 1986; KiHiriii et al.,1988; Elmgren, 1989; Hansson & Rudstam, 1990;Kautsky, 1991; Bonsdorff & Blomqvist, 1992;Nehring, 1992, 1994; Schulz et al., 1992; Pitkanen,1994; Wulff et al., 1994; Bonsdorff et al., 1997).The state of the coastal areas was emphasized byHELCOM (1993 a,b), and two archipelago areas (theArchipelago Sea and the Aland archipelago) werelisted as environmental hot-spots, with eutrophicationas the main threat to the ecosystem, and agricultureand fish farming (net cages with rainbow trout;Oncorhynchus mykiss) as the main sources. As theeffects of eutrophication can be found at all levels ofcoastal ecosystem organization, the authors feel that itis of importance to analyse some pathways of thechain reaction caused by increased nutrient levels(Bonsdorff et al., 1997).

Eutrophication has recently been defined as theeffects of' an increase in the supply of organic matterto an ecosystem' (Nixon, 1995), which is largelygenerated by an increase of nutrient input followed byan increased primary (and secondary) production.Eutrophication has also been recognized as one of themajor threats to (coastal) marine ecosystems on aglobal scale (Nixon, 1990; Gray, 1992; Pearl, 1995),and some comparisons have been made on European

© 1997 Academic Press Limited

64 E. Bonsdorff et al.

and North Atlantic scales (Dederen, 1992; de jongeet al., 1994). For the Baltic Sea, the coastal areas havebeen recognized as specifically vulnerable (Cederwall& Elmgren, 1990; Schulz et al., 1992; HELCOM,1993b; Anonymous, 1994; Bonsdorff et al., 1997).

As the archipelago of SW Finland (northern BalticSea) is the most extensive and island-rich archipelagoof the Baltic Sea and possibly the world (v, Numers,1995), the aims of this paper are to briefly describe thepresent status of eutrophication in this area, analysethe causes of eutrophication (local vs basin-wide),illustrate some ecological consequences, and discusspossible perspectives in the light of the presentsituation.

Study area and methods

The Baltic Sea is an enclosed basin, connected to theworld ocean only through the narrow and shallowDanish Sounds (Voipio, 1981; Leppiikoski &Bonsdorff, 1989). The drainage area is large, andpopulated by an estimated 70-80 million people(Figure 1). The geological history of the Baltic basinis still influenced by the previous glaciation, some10-15000 years ago, undergoing relatively rapidchanges (Voipio, 1981). Thus, in the northern parts,land uplift prevails at 50-100 cm century- \ con­stantly forming new coastal and archipelago areas.Water exchange is slow (long retention time), theinflow of freshwater is large, and salinity is low withextreme vertical and horizontal gradients (including apermanent halocline at about 50-70 m depth in theBaltic proper) as a consequence (Miilkki & Tamsalu,1985; Leppakoski & Bonsdorff, 1989; de Ionge et al.,1994). The salinity ranges from 4 to 8 from the innerarchipelago to the open coast. In the archipelagoareas, bottom topography is characterized by sills andtrenches that further reduce the exchange of deepwater. For the entire Baltic Sea, nutrient concen­trations in the water and organic content of thesediments have increased significantly during thiscentury through increased sedimentation (Ionsson &Carman, 1994; Wulff et al., 1994).

The Archipelago Sea and the Aland archipelago(59°45'-60045'N and 19Q30'-23QOO'E; Figure 2) arecharacterized by an enormous topographic complex­ity (Figure 3), including some 30 500 islands, over20 000 km shoreline covering an area of more than15 000 km2

. The average water depth is only 23 m,but has some deep trenches reaching over 100 m.The mosaic structure and the sharp environmentalgradients (salinity, temperature, oxygen, exposureetc.) create numerous biotopes and complicatedecological webs (Bonsdorff & Blomqvist, 1993;

SLOREP

FIGURE 1. The Baltic Sea basin and its catchment area,drawing water from 14 nations, and affected by an estimated70-80 million inhabitants. The Aland archipelago and theArchipelago Sea are located in the central northern Baltic(A) at the junction between the Baltic proper, the Gulf ofFinland (GF), and the Gulf of Bothnia (GB).

v. Numers, 1995). As the topography is complexand the water is shallow, much of the primaryproduction is linked to the benthic system (Kautsky& Kautsky, 1995). The archipelago is affectedby nutrient inflow from a multitude of sources, andhydrographically, physically and biologically, suchareas may act as a buffer or filter between thecoastline and the open sea. This has long been thecase for the Archipelago Sea, where the widespreadmosaic archipelago has had a diluting effect onnutrient levels. However, with the high diversity ofnutrient sources to the system, the diluting effectstoday are less evident (lumppanen & Mattila, 1994;Bonsdorff et al., 1997).

The material for this analysis is collected fromrecent literature for the area (Table 1), and datafrom unpublished reports.

Coastal eutrophication in the Baltic Sea 65

20° E 22°

Finland

**

*

• 0

* ~~**~.*

a 10 20 30 40 km .fl I!),), I

60°30'

59045'N 1,--LI --'''-- -'- _

FIGURE 2. The Aland and Archipelago Sea areas in the northern Baltic Sea. A, long-term monitoring stations forhydrography and nutrients (Station Kumlinge encircled); *, a network of zoobenthos sampling stations (20-80 m) to serve asa common baseline for the entire area. Local monitoring stations/areas not included.

Coast

2040

] 60oS 80fr

100i=I

120140

..

Archipelago

5-30km

Open Baltic Sea

..FIGURE 3. Generalized bottom profile in the archipelago of SW Finland, with some pathways for nutrient import indicated.The most rapid increase in nutrient levels is recorded in the archipelago and transition zones, where the input throughextensive aquaculture (A) is high.

Results and discussion

Nutrient increase affects the ecosystem

The state of the marine archipelago ecosystem hasundergone rapid changes since the late 1960s(lumppanen & Mattila, 1994; Bonsdorff et al., 1997),and consequences of eutrophication are recordedthroughout the system (Table 1; references therein).Thus, increased nutrient levels have lead to alteredNIP ratios, increased sedimentation rates and in­creased input of organic matter to the benthic system.This has lead to increased pelagic and benthic primaryproduction with both structural and functionalchanges in the system (Bonsdorff et al., 1997), in-

creased turbidity and reduced transparency of thewater, reduced oxygen reserves even above thehalocline, increased occurrences of benthic sulphurbacteria indicating benthic anoxia, and increasedfrequency and amounts of drifting benthic algal mats(Bonsdorff, 1992; Norkko & Bonsdorff, 1996a,b).These changes have significantly affected bothzoobenthos and fish (Table 1; Bonsdorff et al., 1997).Although similar changes (when measured at severaltrophic levels) have also been recorded in the openBaltic Sea, the effects of eutrophication are generallymore pronounced in the coastal areas (Schulz et al.,1992). Further, in the open sea, nitrogen is generallyacknowledged as the limiting nutrient, but in the

66 E. Bonsdorff et aI.

TABLE 1. Examples of indications and consequences of eutrophication in the archipelago and coastal waters of the northernBaltic Sea

Parameter

Nutrients (N,P)

NIP ratioSi pool

Sedimentation, organic matterTransparency

Oxygen

Sulphur bacteria (benthic)Pelagic primary production

Frequency of toxic bloomsPhytobenthos

Growth of annual algaeFucus uesiculosusDrift algal mats

Zoobenthos

Dead bottomsFish standing stocks

Trend

Increasing

DecreasingDecreasingIncreasingDecreasing

Decreasing

IncreasingIncreasingIncreasingIncreasing

IncreasingDecreasingIncreasingIncreasing

IncreasingIncreasing

References

[umppanen & Mattila (1994), Pitkanen (1994), Wulff et al. (1994),Bonsdorff et at. (1997)[umppanen & Mattila (1994), Bonsdorff et al. (1997)Wulff et al. (1994), Bonsdorff et al. (1997)Jonsson & Carman (1994) (entire Baltic basin)Anonymous (1994), Jumppanen & Mattila (1994),Bonsdorff et at. (1997)Anonymous (1994), [umppanen & Mattila (1994),Bonsdorff et al. (1997)Rosenberg & Diaz (1993)Gronlund & Leppanen (1990), Jumppanen & Mattila (1994)Schulz et at. (1992), Leppanen et al. (1995)Makinen & AuIio (1986), Bonsdorff et al. (1995),Kautsky & Kautsky (1995)Makinen & AuIio (1986)Ronnberg et al. (1985), Kautsky et al. (1986)Bonsdorff (1992), Norkko & Bonsdorff (1996a,b)Cederwall & Elmgren (1980, 1990), Bonsdorff et al. (1991, 1997),Mattila (1994)Anonymous (1994)Hansson & Rudstam (1990) (entire Baltic basin)Bonsdorff et al. (1997)

Baltic Sea, phosphorus becomes more important to­wards the gulfs of Bothnia and Finland, and in thearchipelago, the limiting nutrient is often phosphorus(Iumppanen & Mattila, 1994). Thus, the pelagicecosystem of the inner and middle archipelago func­tions in a different manner than that of the opencoastal zone, with intricate consequences (Smith,1984) .

Relevance of local sources; the importance of aquaculture

Nutrient load and organic enrichment from the mainpopulation and industrial centre in the area, the Cityof Turku with 160 000 inhabitants, and a surroundingarea with about 40 municipal and industrial waste­water treatment plants, has decreased dramatically forBOD7 (80% reduction) and phosphorus (reductionby over 90%), whereas the nitrogen loading hasremained almost constant since the early 1970s(Figure 4). At present, the nitrogen reduction inmunicipal wastewater treatment plants is about 30%with further reduction anticipated in the near future.As a direct consequence of this, the trend in nutrientlevels in the inner archipelago waters of theArchipelago Sea is decreasing (Iumppanen & Mattila,1994; Bonsdorff et al., 1997).

In the central and outer parts of the entire archi­pelago area, the trend is the opposite (Figure 5). At

monitoring stations in the outermost archipelagoareas, least affected by loading from land, the increasehas been significant for both phosphorus and nitro­gen. This can partly be explained by the overallincrease in the open Baltic, but local sources arealso important. In this case, fin fish culture (mainlyrainbow trout; Oncorhynchus mykiss) was used as anexample of a local source of nutrients and organicenrichment of visible importance. These farms affectthe ecosystem by a constant, year-round input ofnutrients, which prolongs the season for primaryproduction, and eliminates the natural nutrientlimitation. The distribution of fin fish farms in thearchipelago of SW Finland covers the entire areaconcentrated to the middle and outer regions(Figure 6). In the Aland archipelago, 35-40 fish farmsproduce about 5000 tonnes year - I, with an averageproduction of some 150 tonnes year - I per unit(1995), whilst about 100 operating units in theArchipelago Sea produce about the same amount(average farm size about 50 tonnes year-I). All thesefarms may be considered as ' local point sources', butit is evident that their pooled area of influence is muchlarger (Figure 6). The loading of nutrients (nitrogenand phosphorus) from the farms to the Aland archi­pelago area is estimated at 40 tonnes of phosphorusand 270 tonnes of nitrogen year- I (ProvincialGovernment of Aland). This equals the loading from

Coastal eutrophication in the Baltic Sea 67

-----

10000

'i'a9000

8000Q)

»'" 7000 -Q)

§ 60000~ 5000~Q)

eo 40000b

3000~ I

I

"" 2000 I,

~ ,0

1000pq

0

FIGURE 4. Municipal and industrial wastewater loading into the sea off Turku 1970-1993. ---, BOD7; ---, nitrogen;.. " phosphorus.

30 500000

0

1: 20~

i,

-5 bll2-

0.. Z'3 10 ~~

100

o I 01965 1970 1975 1980 1985 1990 1995

FIGURE 5. Long-term (1968-93) trends (winter values) innutrient concentrations (phosphorus, 0, y=0'51x-19'8,1'=0'82, P<O·OI; nitrogen, .; y=7'36x-288'31, 1'=0'74,P<O'OI) in the productive water layer (0-10 m) at StationKumlinge in the outer Archipelago Sea (encircled triangle inFigure 3).

treated municipal wastewaters (90 % reduction forphosphorus and 30% reduction for nitrogen) ofapproximately 370 000 persons for phosphorus, and90 000 persons for nitrogen. The entire local popu­lation in the Aland area is 25 000 inhabitants. The35-40 operating fish farms thus contribute 15 timesthe load from municipal wastewater for phosphorus,and 3·6 times the nitrogen input through treatedwastewater. For the entire Archipelago Sea and Alandarchipelago, aquaculture alone stands for an in­put of phosphorus corresponding to about 740 000person-equivalents, and for nitrogen, 180 000 person­equivalents. Thus, in the archipelago environment,the impact of aquaculture in terms ofeutrophication ishighly significant. For the marine coastal and archi­pelago ecosystem, the gross effects of eutrophicationare shown in Table 1. Such effects are also clearlydetectable for specific sites or local clusters of fish

farms, where the increase in fish production isinversely related to the state of the zoobenthiccommunities at both shallow (less than 10 m) anddeep (below 20 m) bottoms (Figure 7; [umppanen &Mattila, 1994).

The < healthy zone ' gets narrower

As the archipelago ecosystem is stressed from alldirections (Figure 3), the need for accurately selectingmonitoring stations or areas becomes more important(Figure 2). The innermost areas have been studied fordecades, and proven to be affected by pollution andto be highly variable over time (Leppakoski, 1975;[umppanen & Mattila, 1994; Bonsdorff et al., 1997).The open sea area has been shown to be influenced bythe halocline, causing long periods of hypoxia oranoxia (Andersin & Sandler, 1989, 1991). Simul­taneously, increased phytobenthic production in thearchipelago has lead to an increase in benthic driftingalgal mats, presently covering large areas of inter­mediate depths in the archipelago. These mats, withan average biomass of 300 g dry weight m - 2, or2000 g wet weight m - 2 (1995), are highly oxygen­demanding, and have serious effects on bothzoobenthos and fish (Bonsdorff, 1992; Norkko &Bonsdorff, 1996a) . The intermediate depth zone inthe outer archipelago is normally not affected byhypoxia or anoxia, but the increasing amounts ofdrift algal mats induce hypoxia to the sediment,with effects on nutrient dynamics and zoobenthiccommunity development (Bonsdorff, 1992; Norkko& Bonsdorff, 1996b). Similarly, negative effects ofalgal mats and loose-lying macroalgae have also beendescribed from other sea areas, and the need formonitoring them has been emphasized (Everett, 1994;

68 E. Bonsdorff et al,

200E

A:iand Sea

59°45'N L- _

FIGURE 6. The distribution of fish farms (., open-water net cages; .. , land-based hatcheries) in the SW Finnish archipelagoin 1993-1995. Fish farm production in the Aland Sea (35-40 operating farms) is ~ 5000 tonnes year - 1 and ~ 5000tonnes year- 1 in the Archipelago Sea (about 100 operating farms). H, Houtskar area.

K.olbe et al., 1995). Thus, the transition zone betweenthe archipelago and the open sea becomes ecologicallymore important as a recruitment area for both thedeeper offshore areas and the archipelago system.Sampling zoobenthos in depth strata along thearchipelago gradient indicates that the communitycomposition is changed and the number of species isslightly reduced with depth, whereas abundance, pri­marily due to increased abundance of amphipods,increases with depth. Biomass values are highest in thecentral archipelago zone, and significantly lower withincreasing depth as a consequence of amphipod domi­nance over bivalves (Elmgren et al., 1984; Bonsdorff,1988). Benthic biomass in the archipelago has almostdoubled since the 1970s (Bonsdorff et al., 1991,1997). The intermediate zone (and depth stratum)offers a mixture of both the characteristic archipelagobenthos and the open-sea benthos, and should there­fore be included in coming monitoring. This zone hasso far been neglected in much of the monitoringin Finnish waters, even though biomass changes inbenthic macrofauna along the depth gradient wereindicative of eutrophication in Swedish coastal watersin the 1970s (Cederwall & Elmgren, 1980, 1990) andother areas of the Baltic Sea (Brey, 1986; Badenet al., 1990; Weigelt, 1991; Prena, 1995). Long-termmonitoring in the archipelago has concentrated onhydrographic parameters (including nutrient concen­trations) and zoobenthos, with time sets starting in the1960s (Leppakoski, 1975; Iumppanen & Mattila,1994; Bonsdorff et al., 1997). Meiofauna has rarely

Cal

600l-ad'I;; 500~ 400

J~ "',",Jill,1977 1979 1981 1983 1985 1987 1989 1991

(b)

500~450400350'3 300

..... 250«t 200

IS is8'"d 50Jt 0- 1981 1984 1987 1991

(el

{illLL:~i~~ 1982 1987 1991<11

FIGURE 7. The development of fish farming in one area(Houtskar in the Archipelago Sea; Figure 6) in 1977-1991(a), and the subsequent deterioriation of the zoobenthiccommunities in the area at shallow «10 m) (b) and deep(>20 m) (c) bottoms. A, total abundance (individuals m - 2,

hatched bars); B, total biomass (g wet weight m - 2, solidbars), S, total number of species (_).

been analysed in the Finnish archipelago (Elmgrenet al., 1984). As meiofauna community responses toorganic enrichment are small (or suppressed by themacrofauna) in comparison to those of the macro­fauna (Widbom & Frithsen, 1995), macrozoobenthosseems to be more important as a tool in monitoring.

The benthic biota correlate with the environment

In connection with analysing the benthic community,several environmental parameters have been analysed,and various methods are employed in the Alandarchipelago. Hence, the following have beenmeasured: the hydrography and nutrients in thebottom water, the organic content (loss on ignition) ofthe surface sediment and (through Sediment ProfileImaging, as described in Rosenberg & Diaz, 1993 )sediment type; surface relief of the sediment; softness(as penetration of the gear used), the redox potentialdiscontinuity layer (RPD) in the sediment; and loca­tion of possible dark (anoxic) layer in the sediment.These were correlated with the number of macroben­thic species present, total community abundance, andtotal community biomass. The number of speciesrecorded has a prime significant positive correlationwith oxygen saturation of the bottom water (P<O'05 ;r=0 ·40). Total abundance correlates best with organiccontent of the sediment (P<0 '001; r=0 ·69), sedimenttype (P<O'001; r=O'68), and RPD layer (P<O'OOl ;r= O· 61). Biomass, again, correlates best with organiccontent of the sediment (P<0'05; r=O·44). Biomassand species number also correlate significantly(P<O·OI; r=O'59), and the present authors haveshown previously that the benthic community par­ameters correlate significantly with oxygen, organiccontent and nutrients (primarily nitrogen content)in the bottom water of the same area (Bonsdorffet al., 1991) . Thus, it is evident that the changesrecorded in the environment as a consequence ofincreased nutrient and organic input, are directlyreflected in the sediment [as sh own by Jonsson &Carman (1994 ) in their sediment studies] andfurther in the zoobenthos (pearson & Rosenberg,1978, 1987) . With the increasing trends in overalleutrophication of the archipelago ecosystem, theproblems for monitoring increase, and the questionof reliable control sites becomes crucial (Chapmanet al., 1995).

Concluding remarks

Eutrophication in the marine environment hasattracted much attention lately, and several con­ceptual models have been put forward in order to

Coastal eutrophication in the Baltic Sea 69

facilitate the understanding of the process, includinganalysis of multiple factors (Karydis & Coccossis,1990) and couplings to pollutants and contaminants(Gunnarsson et al., 1995) . The evident processesof eutrophication of Baltic archipelago waters issummarized in Figure 8 and Table 1. The model(Figure 8) is limited to published information (refer­ences given in the graph), with the purpose ofillustrating the complexity of the couplings and effectsarising from the seemingly simple impact of increasednutrient input to the aquatic environment. Theincrease of nutrient concentrations in the sea stemsfrom numerous sources, and it is important to separ­ate local and regional aspects from a basin-wideanalysis. In the archipelago waters of SW Finland,local input is highly important, although the totalinput to the Baltic Sea from this region may be only afraction of the entire nutrient load in the Baltic Sea.Due to topography and water movements, localsources primarily affect local ecosystems. In order totackle the problems caused by eutrophication, man­agement measures are vital. Marine ecologists canparticipate with adequate monitoring and interpret­ation of long-term data (or data involving spatialvariability due to environmental gradients), but thefinal measures will have to be decided upon bypoliticians with financial considerations. Legal andeconomic feedback loops to environmental monitor­ing seem to offer the best way to improve the state ofthe sea (Gray, 1994; Hilden, 1995). In this context,monitoring must not just be a statistical tool, butalso be a tool for giving (early) warning signals(Gray, 1990), which should be taken seriously, as nosafe limits of environmental stress can be defined(Leppanen et al., 1995; MacGarvin, 1995). Warningsmust be given in time, however, as future ecosystemresponse may already be determined by the time thesignals are recorded. Thus, as an example, Barica(1993) classified algal community responses at earlywarning levels as 'sustainable', at serious warninglevels as 'reaching the limits', and at late warninglevels as ' unsustainable'. This is also true of the riskof potentially toxic algal blooms and their interactionswith the soft-bottom biota (Olsgaard, 1993; Pearl,1995). As the topography of the archipelago waters inthe northern Baltic Sea is highly complex, modellingof the water exchange and nutrient dynamics is verydifficult in comparison to open coastal systems, suchas the Gulf of Riga (Yurkovskis et al., 1993) . Asepisodic events, such as hypoxia or anoxia, occurfrequently, modelling is even more speculative(Chapelle et al., 1994), and it seems that pinpointingsources and effects of eutrophication to facilitatemanagement and measures is of prime importance

70 E. Bonsdorff et al.

Measures (legal/financial) - -- ---- --- - -- - ---- ----- - --- ---------------------- ---- ----I,,------, " " II ,, , (38-40) " (1,17) II ,

''4. Airborne II I

Traffic, industry, heating, agriculture,

I , ,, I ,,+ +,,

I,Direct loading in archipelago watersI Riverine input and runoff from land,, Agriculture, forestry, industry Aquaculture, agriculture, wastewater,

II

• (1-3)I

0.3,6, 18) ~IIII, ~ I Reduced dilution potentialI

Upwelling from ... .. .deep B,I"c basins Increasing input of limiting nutrients /

(2)

(3,16)

< Amount of non-limitingJ

Increasing amount of nutrients in archipelago waterselements (Si)

i .>: j~ t (4,5) (1,2, 20)

> Pelagic primary production > Production of annual benthic algae> Frequency oftoxic algal blooms < Survival of perennial (brown) algae t--

Altered succession and ssp composition > Amounts of drifting algal mats

(1,2,7,22)

~~ ~(1, 2, 8-10, 14, 15)

(2,3,23-25,27-30,33,35-37)Altered production > Turbidity; < light and < transparency

> Sedimentation of organic matter r------- Altered zoobenthic communitiesand community of > Oxygen consumption in water mass and atzooplankton > Zoobenthic biomassbottom -> Community collapse-> Hypoxia/anoxia and hydrogen sulphide(11,21)

(1,2, 12, 13, 19) 11f Altered fish production Effects on birdsI > Increased standing stockI and marine mammalsI < Spawning groundsI

~~I Altered benthic/pelagic ratioI

lI -> Ultimately community collapse (1)I

I

II

II (1,2,25,26,31,32,34)

< Condition; < recruitment------ 'Management': Monitoring and time/space analysis < Recovery potential

J,,,,,,,r.,

(38-40) (1,32,41)

FIGURE 8. A generalized flow model of the eutrophication processes in the coastal and archipelago areas of the Baltic Sea.The pathway from sources (causes) of nutrients through the nutrient pool in the sea to the primary effect parameters (thicklines around boxes) and secondary effects at various trophic and functional levels (solid arrows) are listed. Measures andmanagement are indicated with broken arrows. The references to the various steps in the diagram, numbered in the graph,are: (1) jumppanen & Mattila, 1994; (2) Bonsdorff et al.• 1997; (3) Cederwall & Elmgren, 1990; (4) Wulff et al., 1994; (5)Wulff et al., 1990; (6) Carlsson & Bergstrom. 1993; (7) Gronlund & Leppanen, 1990; (8) Bonsdorff, 1992; (9) Norkko &Bonsdorff, 1996a,b; (10) Kautsky, 1991; (11) Heerkloss et al.• 1991; (12) Babdenerd, 1991; (13) Jonsson & Carman, 1994;(14) Kautsky & Kautsky, 1995; (15) Makinen & Aulio, 1986; (16) Nehring & Matthaus, 1991; (17) Pearl, 1995; (18)Pitkanen, 1994; (19) Rosenberg & Diaz, 1993; (20) Smith, 1984; (21) Viitasalo, 1994; (22) Olsgard, 1993; (23) Andersin &Sandler. 1989; (24) Andersin & Sandler. 1991; (25) Baden et al., 1990; (26) Bonsdorff & Blomqvist, 1992; (27) Bonsdorffet al.• 1991; (28) Brey, 1986; (29) Cederwall & Elmgren, 1980; (30) Everett, 1994; (31) Hansson & Rudstam, 1990; (32)Aneer, 1987; (33) Iergensen, 1990; (34) Kaiiria et ai.• 1988; (35) Prena, 1995; (36) Rosenberg & Loo, 1988; (37) Weigelt.1991; (38) Hilden. 1995; (39) Gray. 1994; (40) HELCOM, 1993a; (41) Bonsdorff & Blomqvist, 1993.

(Figure 8). A further complication is the immediatethreat for the complexity of the biota of the sea.Biodiversity may be altered, and functional groups lostfrom the ecosystem (Suchanek, 1994). On the otherhand, possible positive effects of any measures takenwill take years to show in nature, and as the postglacialsuccession of the northern Baltic Sea is still active,the outcome must be related to the changing con­ditions. Thus, even on a geological time scale, humanactivities significantly affect this water body, andfuture use of the Baltic basin largely depends onmeasures taken today.

Acknowledgements

The authors thank the Academy of Finland forfinancial support, and the water authorities in Finlandfor access to data on nutrients and water quality.

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sulphide and low oxygen concentrations in the Baltic deep basins.Proceedings of the 16th Conference of the Baltic Oceanographers 1,102-111.

Andersin, A.-B. & Sandler, H. 1991 Macrobenthic fauna andoxygen deficiency in the Gulf of Finland. Memoranda SocietasFauna Flora Fennica 67,3-10.

Aneer, G. 1987 High natural mortality of Baltic herring (Clupeaharengus) eggs caused by algal exudates? Marine Biology 94,163-169.

Anonymous 1994 Eutrophication of soil, fresh water and the sea.Swedish Environmental Protection Agency Report 4244, 207 pp.

Babdenerd, B. 1991 Increasing oxygen deficiency in Kiel Bay(Western Baltic). A paradigm of progressing coastal eutrophi­cation. Meeresforschungen 33, 121-140.

Baden, S. P., Loo, L.-O., Phil, L. & Rosenberg, R 1990 Effects ofeutrophication on benthic communities including fish: Swedishwest coast. Ambia 19, 113-122.

Barica, J. 1993 Ecosystem stability and sustainability: A lesson fromalgae. Verhandlungen der Intemationalen Verein filr Limnologie 25,307-311.

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