Salt and brackish marshes around the Baltic Sea and adjacent parts of the North Sea: Their vegetation and management

Biological Conservation 51 (1990) 191-209

Salt and Brackish Marshes Around the Baltic Sea and Adjacent Parts of the North Sea: Their Vegetation and Management Kees S. Dijkema Research Institute for Nature Management, PO Box 59, 1790 AB Den Burg, Texel, The Netherlands (Received 26 January 1989; revised version received 20 April 1989; accepted 26 April 1989)

ABSTRACT European salt and brackish marshes are in urgent need of protection. This also holds for the many small marsh sites around the Baltic Sea, including the ad/acent parts of the North Sea. Although the individual Baltic shore marshes are small, together they form an important area of salt and brackish marshes in Europe. The Baltic' shore vegetation is very complicated because o.1"gradients in salinity, climate, exposure and water levelJtuctuations. There is a wide range of species and communities: from typical brackish communities around the Baltic Sea (with some arctic species in the Bothnian Bay) to more common central Atlantic communities in the western parts of Sweden and Denmark. A well-developed aspect are the natural transitions .from the shore marshes to woodland, bog and heath communities. The Baltic shore marshes have a long history of grazing and hay-making, which favour the occurrence of halophytes in brackish marshes. Nowadays, man), Baltic marshes are no longer usedJ~r these purposes, so that reed beds, tall grasses and woodland increase at the cost of halophytes. A general strategy for the management of salt and brackish marshes and proposalsjbr the management of the Baltic' marshes are presented.

INTRODUCTION T h e salt a n d b r a c k i s h m a r s h sites in n o r t h e r n E u r o p e are o f t e n small a n d o f a simple g e o m o r p h o l o g i c a l structure. H o w e v e r , the s h o r e v e g e t a t i o n as a 191 Biol. Conserv. 0006-3207/90/$03-50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

Kees S. Dijkema

192

substrate

geology

salt marsh type

rocky shores

loch/fjord head

-

ria bay

I

barrier-connected marine sedementary shores

lagoonal

1

foreland estuarine

fluvial sedamentary shores

autochthonous

1 /

beach head allocht honous

-

i = . 1

i

deltaic emerging flat shore

I

emerging skerries

I

peat type

Fig. I. Coastal salt marsh types in Europe, with the proportional distribution of sites per region, based on site numbers. Each column represents 100% (after Dijkema, 1987b).

whole is very complicated because of gradients in salinity, climate, exposure and water-level fluctuations. Salt marshes of sedimentary shores hardly occur (except for the southern Baltic Sea and Jutland, Denmark). Most of the shores are rocky, steep and without marshes at all, sometimes with small salt marsh sites in fjords or on skerries (Dijkema, 1987b; Fig. 1). On emerging flat shores near Oulu, Finland, extensive brackish salt marshes are formed. In former times salt marshes as a transitional belt between the sea and terrestrial habitats must have been very extensive in Europe, providing a continuous landscape at least along flat coasts of the sedimentary or deltaic type. Large areas of salt marshes have been reclaimed; first, for agriculture, later for other uses connected with human settlement (Dijkema, 1987c). The Council of Europe has published a list and map of more than 600 salt marsh sites in Europe including information on each site (Dijkema et al., 1984). An important conclusion is that to maintain the complete range of halophytic flora and fauna and to ensure the dispersal of halophytic species, all of the remaining salt marsh sites are in urgent need of protection. This also holds for the Baltic salt and brackish marshes. Isolated salt marsh and salt steppe areas (e.g. arctic islands, Gulf of Bothnia, northern Adriatic, inland saline areas of western, central and eastern Europe) have a special conservation value because of their potential for genetic deviation and halophyte dispersal (Beeftink, 1984). Apart from its isolation, the low salinity of the Baltic Sea is of evolutionary interest (Russell & Thomas, 1988). An initial proposal for the sites to be included in a European network of biogenetic reserves has been prepared (Dijkema, 1987a).

Baltic brackish marshes

193

As part of the Council of Europe study reviews have been made of the extensive literature on the salt marsh vegetation around the Baltic Sea and adjacent parts of the North Sea (e.g. Leivisk~, 1908; Dahl & Hadac, 1941; Dahlbeck, 1945; Mikkelsen, 1949; Gillner, 1960, 1965; Tyler, 1969a, 1969b; Siira, 1970; Gravesen, 1972; Ericson & Wallentinus, 1979; Vartiainen, 1980). In 1982 the marsh sites along the Baltic coasts were visited and data on area, vegetation, land use and protection status, collected by means of a questionnaire (Dijkema et al., 1984). The purpose of this compilation is to show the uniqueness of the Baltic shore marshes and the need to preserve them. ECOLOGICAL CONDITIONS The salinity of submerging water is an important factor in the distribution of halophytic species. Western and Southern Norway are bordered by the Norwegian Sea and the North Sea, which have normal saline conditions (Table 1). The Baltic Sea is a vast brackish sea, connected to the North Sea by a narrow transition area between Denmark and southwestern Sweden. The transition area has strong gradients and fluctuations in salinity. The Baltic Sea can be divided into several basins with different ecological conditions (Table 1): the Baltic Sea proper and the Gulfs of Riga, Finland and Bothnia, the last including the Bothnian Sea and the Bothnian Bay. Compared to the TABLE 1

Salinities in the Basins of the Baltic and the Adjacent North Sea [after Tyler, 1969a and Ericson & Wallentinus, 1979). Salinity o]open coastal water

Venice classification

(%0 S)

Open Sea

NorwegianSea North Sea Skagerak

33 30 34 32 33-27

euhalinicum euhalinicum eu- and polyhalinicum

Transition area

Kattegat Belt Sea Oresund

30-18 18-10 10-8

polyhalinicum ~-mesohalinicum /%mesohalinicum

Baltic Sea

Baltic Sea proper Gulf of Riga Gulf of Finland Gulf of Bothnia Bothnian Sea Bothnian Bay

10-5

/~-mesohalinicum

5~3

oligohalinicurn

5-3 3q3-3

~-oligohalinicum /#oligohalinicum

194

Kees S. Dijkema

transition area the open water of the Baltic Sea has a stable salinity, but at the heads of the Bothnian Bay and the Gulf of Finland, and in bays, fjords and near river mouths, large fluctuations occur. Tidal fluctuations are small, from about 30 cm in the Skagerak, decreasing to 15 cm in the Kattegat and 10cm in the Belt Sea. In the Baltic Sea and the Gulf of Bothnia tides scarcely exist (Gillner, 1965; Ericson & Wallentinus, 1979). In the Baltic Sea and the Gulfs, water-level fluctuations are mainly determined by the season--by wind direction and force, air pressure and discharge from rivers--and are lowest in late winter and spring and highest in autumn and early winter. The extremes in fluctuation for these periods are 70 and 150 cm, respectively, and reach up to 180 and 300 cm, respectively, at the heads of the Bothnian Bay and the Gulf of Finland, and even 370 cm at The Polish-German border (Gillner, 1965; Ericson & Wallentinus, 1979). The vertical extent of the marshes increases accordingly from 50-60 cm on the Swedish west coast to 50-130cm around Oulu off the Bothnian Bay (Gillner, 1960; Siira, 1970). The average water levels between spring and summer vary about 15 cm for the transition area (Gillner, 1960) and 23 cm for the central Baltic Sea (Tyler, 1969a). A low water level in spring is of particular importance for the vegetation of the low marsh zones. For example, seedlings of Scirpus tabernaemontani, Eleocharispalustris and Eleocharis uniglumis make a rapid appearance during years with comparatively low spring water levels (Ericson & Wallentinus, 1979). Year-to-year changes are considerable and show a rather regular shift between low- and high-water years (Ericson, 1981). An important ecological condition is the uplift of northern Europe, which increases from zero in southern Sweden, southern Norway and northern Denmark to more than 0"9 cm year-1 in the Bothnian Bay (Devoy, 1987). Land-uplift causes the vegetation zones to move seaward in a long-term process. Annual fluctuations in water level are more important in the short term ( < 10 years) (Schwanck, 1974; Ericson & Wallentinus, 1979; Ericson, 1980; Vartiainen, 1980; Cramer & Hytteborn, 1987). The expected worldwide rise in sea level (20-165 cm for the next century; Robin, 1986) might reach higher values than the present rate of land-uplift. From the southern Baltic Sea to the northern Bothnian Bay the length of the growth period is reduced by about a third, and the duration and thickness of the ice cover increase strongly. In the Bothnian Bay ice may have severe damaging effects on the vegetation and thus prevent the occurrence of reeds (Siira, 1970; Ericson & Wallentinus, 1979). F r o m the open sea into the archipelagos there is a decreasing gradient in salinity, wave action and sunshine, whereas sedimentation, duration and thickness of ice, and precipitation increase (Ericson & Wallentinus, 1979).

Baltic brackish marshes

195

VEGETATION A N D DISTRIBUTION In Tables 2, 3 and 4 the vegetation communities of the Baltic shore marshes and the salt marshes of the adjacent parts of the North Sea are compared by listing the common plant species for some important regions arranged according to the gradients in climate and salinity. The communities have been arranged in zones related to the mean water level, comparable to the zones used for North Sea salt marshes (Dijkema et al., 1984; cf. Beeftink, 1977a). Due to the small vertical range in the zonation of the Baltic shore marshes the zones are close together and often difficult to distinguish from each other.

Communities of emerging flats In this pioneer zone below mean water level a striking shift from halophytic to freshwater communities occurs. Due to decreasing salinity the Salicornia dolichostachya community on Danish, German and western Swedish sites is replaced by a brackish reed belt (Scirpus maritima and S. tabernaemontani) in the inner transition area and the Baltic Sea and Gulfs. Aster tripolium is an important species in this zone. In the southern Baltic Sea area a Juncus maritimus community may occur (Voderberg, 1955; Fukarek, 1961; Gillner, 1965; Piotrowska, 1974). Towards the least saline Gulf of Bothnia the nonhalophytic Phragmites australis and Eleocharis palustris communities increase. Scirpus tabernaemontani still occurs in brackish water; in freshwater an Equisetumfluviatile community appears. Northwards the reed belts may disappear on exposed sites due to ice erosion. Around the Baltic Sea the halophytic Eleocharis parvula community may occupy bare emerging places, in the Gulf of Bothnia and the Gulf of Finland they are replaced by a non-halophytic Eleocharis acicularis community.

Communities of lower marshes Above mean water level the lower marsh forms a distinct zone, mostly belonging to the Puccinellietum maritimae association in marine conditions and to the Eleocharetum uniglumis association in brackish conditions. However, in areas without grazing, reed beds may extend upwards (e.g. Kauppi, 1967). On the shores of Norway, southwestern Sweden, Denmark and the German Baltic Sea Puccinellia maritima dominates the lower marsh (Puccinellion maritimae alliance). It is accompanied by halophytic species like Aster tripolium, Plantago maritima, Triglochin maritima, Spergularia media, Salicornia europaea and sometimes Limonium humile. With

Sag. mar.-Coch, danica

Salicornia europaea Spergulariasalina

Alnus glutinosa*

Juncus gerardii Blysmus rufus J. ger.-Leont, aut

Festuca arundinacea

Juncus gerardii

Care,,: paleacea Puccinellia maritimaPlant. mar.-Aster trip.

(Carex paleacea) Puecinellia maritimaPlant. mar.-Aster trip.

Sag. mar.-Coeh, daniea

Salicornia europaea Sperg. sal.-Pucc, distans Halimione peduneulata

Carex recta Carex nigra* (Alnus glutinosa*)

Elymus repens

Juneus gerardii Blysmus ruJus Leont. aut.-Tr(f. ] b a g .

(Art. mar.-Lim, v u l g . )

(Carex maekenziei)

Eleoeh. unigl.-Agr, stol.

(Carex paleacea) Puecinellia maritima

(Eleocharis parvula) (Salicornia doliehostaehya) Salicornia dolichostachya Seirpus maritimus (Scirpus maritimus) (Scirpus tabernaemontani) Phragmites australis* (Phragmites australis*)

Sag. mar.-Coch, danica

Hal. ped.-Par, strigosa

Alnus glutinosa*

Elvmus repens Festuca arundinaeea

Festuca rubra

Artemisia maritima J. ger.-Lim, vulgare Juneus gerardii

Puccinellia maritimaAster trip.-Trigl, mar. Agrostis stolon~fera

Seirpus maritimus (Seirpus tabernaemontani) Phragmites australis*

(Eleocharis parvula)

Oresund 10-8%o S (Dahlbeck, 1945; Gillner, 1965)

Southwest Sweden Skagerak + Kattegat 3~20%o S (Gillner 1960, 1965)

Sulicorniadolichostachya (Scirpus maritimus) (Scirpus tabernaemontani) (Phragmites australis*)

Ostoy (Os]ofjord) 18%o S (Dahl & Hadac, 1941)

° *, Non-halophytic communities; ( ) only locally c o m m o n .

pharapholietosum Sag. mar.-Coch, danica

Pucinellietum distanis Pucc. mar.

Salt patches Salicornietum

W o o d l a n d communities

nigrae

Fest uceto-Caricetosum

Lolio-Potentillion ans.

Upper marsh

Caricetum pullchellae

J. ger.-Leont, aut.

Juncetum gerardii Scirpetum ruff

Artemisietum marltlmae

Middle marsh

Eleocharis uniglumis

Puccinellietum maritimae

Phragmition Lower marsh

Halo-Scirpetum maritimi

Salicornietum strictae

Emerging flats

Telemark 30%0 S ( Vevle, 1982, 1985; Hofsten & Vevle, 1982)

Southern Norway

Sperg. sal.-Pucc, distans

Prunus spinosa*

(Elvmus repens) Festuea arundinacea

Carex puk'hellum Carex extensa

Blysmus rt4[us

Eleocharis uniglumis Agr. stoL-Trigl, pal

Scirpus maritimus

(Eleocharis parvula)

Open shore

Sperg. sal.-Pucc, distans

Carex nigra-Fest, rubra Alnus glutinosa*

Festuca rubra

Juncus gerardii

(Eleoeharis uniglumis) Agr. stol.-Trigl, pal. Carex maekenziei

(Scirpus maritimus) Scirpus tabernaemontani Phragmites australis*

Sheltered shore

Southeast Sweden 6%00S (Tyler, 1969b; Wallentinus, 1967, 1973)

TABLE 2 Common Plant Species of (Halophytic) Shore Communitiesfl Southern Norway and Southwest and Southeast Sweden, Based on Literature and Own Observations

I Only locall> common. No data available.

Salt patches

J.ger.-kk'st.r.-Leont.

June. ger. subass. .l.ger.-Fe.st.r.-Leo,t. with Leontodon atllumll. Lolio-Potentillion ans.

Artemisia maritima

¢tlll.

Puccinellia maritima

Scirpus maritimus Scirpus tabernaemontani Phragmites australi,s

J. ger.-Fest, rubra

aut.

(Puccinellia maritima) [Agrostis stolon~[bra) [Eh,ocharis uniglumis)

Phrag. austr. (ungrazed)

{Salicornia europaea) Seirpus maritimus (Scirpus tabernaemontani) Phragmites australis

(Mikkelson, 1949)

(Gravesen, 1972)

Juncetum gerardii Scirpetum rutl

Middle marsh Artemisieturn maritimae

Eleocharetum uniglumis Magnocaricion paleaceae

Puccinellietum marit.

Lower marsh

Phragmition

Halo-Scirpetum maritimi

Emerging fiats

lse[]ord Kattegat) 19°/oo S

Tipperne (North Sea) 5 8%o S

Denmark

Ell'mils repem (ungraTcd)

I'~'stt, a ruhra

.4rt. mar. (ungrazed) Lint. rulg. (ungrazed) Juncus gerardii

Puccinellia maritima

Phrag. austr. (ungrazed)

Grasswarder 14%o S (Sehmeisk v 1974, 1977)

J. ger.-Leont, alll.

B(vs.,mts ru/us ('arex extensa

E quinque[tor ;-Agr.stol.

Phragmites australi.s Juncus maritimus

Scirpus maritimus

Darss 10%o S (Fukarek, 1961)

Germany

Eleocharis parvula

Estonian Lffands 7%o S [Rebassoo, 1975)

(,~a/llorHttl cllFop~tea) {~'a/itornla cttro[Taca~ Sper~ ~al.-th,~. distan.s Sl?erg. .sal.-Pu~. distans

(Artemisia maritima) IPuccinellia maritima) J. ger.-Agr, stol. Juncus gerardii Blysmus rt(/us (Carex extensa) (Care v glareosa) J.,zer.-Fest.r.-I,eont. tttlI. Fcs[. r. + L.-P. sp.

Agrostis stoloni/era Eleocharis uniglurnis ('are v mackenziei {sheltered)

Trigl. mar.-Aster trip.

Scirpus maritirnus Scirpus maritimus Scirpus tabernaemontani Scirpus tabernaernontani Phragmites australis Juncus maritimus

Poland 7%o S [Piotrowska, 1974)

Baltic Sea

TABLE 3 Common Plant Species of Halophytic Shore Communities, Denmark and the Southern Baltic, Based on Literature

Lower marsh Eleocharetum uniglumis

Festuea rubra

(Blysmus ru/us)

Juncus gerardii

Eleocharis uniglumis

Eleoeharis aeieularis*

Open shore

Salicornia europaea Spergularia salina Pueinellia retroflexa

Salieornia europaea Spergularia salina Puceinellia retroflexa

Spergularia salina

peatland*

Carex nigra Cal stricta* Salix phylicifolia*

Cal. strict.-Erioph, ang.

Carex nigra* Carex mackenziei Carex halophila Care.,: paleaeea

Eleoeharis aeieularis* Eleocharis palustris* Scirpus tabernaemontani Phragmites australis* Equisetum [tu~'iatile Carex aquatilis*

Sheltered shore

Oulu area 2 3%o S (Miikirinta, 1970; Siira, 1970)

Salix phylie(['olia* Alnus ineana & A. glutinosa* Alnus incana*

Festuea rubra

*, Non-haolphytic species; ( ), only focally common.

Salt patches Salicornietum Puccinellio-Sperg. salinae

Woodland communities

Scirpetum ruff Upper marsh

Middle marsh Juncetum gerardii

Juneus gerardii

Eleoeharis uniglumis

Phragmition

Magnocaricion paleaceae

(Eleoeharis acieularis*) {Eleoeharis palustris*) (Scirpus tabernaemontani) Phragmites australis*

Emerging fiats

Jakobstad area 3"6 4°/ooS (Sehwank, 1977)

Alnus ineana*

Festuca rubra

Agr. stol.-Juneus ger. Cal. strieta-Juneus ger.

(Carex rnackenziei) (Carex halophila) Carex paleacea

Eleoeharis uniglumis (Desehampsia bottniea)

(Eleoeharis palustris*) (Seirpus tahernaemontani) Phragmites austra[is*

Northeastern Archipelagos 1"5 3°oo S (Vartiainen, 1980)

Spergularia salina

Carex nigra-F, rubra* Salix phyliciJolia* Alnus ineana*

Cal. stricta-J, ger.

(Eleocharis un~lumis) Deschampsia bottniea ( × eaespitosa)

Open shore

Spergularia salina

Carex nigra-Cal, strieta* Salix phylie(['olia*

Cal. strieta-Erioph, ang.

Carex nigra*

(Eleoeharis uniglumis)

Eleoeharis aeicularis* Eleochoris palustris* Seirpus tabernaemontani Phragmites australis* Equisetum [tutTiatile Carex aquatilis*

Sheltered shore

Swedish part 0 3%o S (Erieson & Wallentinus, 1979)

TABLE 4 a in the Bothnian Bay, Based on Literature and Own Observations

Common Plant Species of (Hal0phytic) Shore Communities

Baltic" brackish marshes

199

decreasing salinity towards the Baltic Sea first Agrostis stolonifera and then Eleocharis uniglumis increase. The E. uniglumis community (Eleocharion uniglumis alliance) dominates most lower marshes in the Baltic Sea and Gulfs from southeastern Sweden and Estonia northward. It is accompanied by reed, Agrostis stolonifera, Triglochin maritima, T. palustre, Juncus gerardii, Glaux maritima, etc. and is still a really halophytic community. In western Norway E. uniglumis communities are c o m m o n in brackish areas such as estuaries, head areas of deep fjords, and with freshwater seepage (Reider Elven, pers. comm.). On sheltered marshes in the Baltic Sea and Gulfs the Eleocharetum uniglumis association is replaced by communities dominated by halophytic Carex species (Magnocaricion paleaceae alliance, Gillner, 1960, including the Caricion alliance, Siira, 1970). In the northern Bothnian Bay E. uniglumis may be replaced by communities with the endemic shore species Deschampsia bottnica on exposed shores (Willers, 1987) and the nonhalophytic Carex nigra on sheltered shores. Communities of middle marshes

The most important shore community of northern Europe is the Juncetum gerardii association (Armerion maritimae alliance). It is rich in species and includes about ten halophytes, although without grazing it may be replaced by non-halophytic reed beds. The lowest zone is dominated by Juncus gerardii, Agrostis stolonifera, Triglochin maritima, Plantago maritima and Glaux maritima. Communities with Limonium eulgare and Artemisia maritima are found in Denmark, along the Oresund and the German Baltic. Locally in the Baltic Sea and Gulfs closely related halophytic communities occur with Blysmus rufus and on exposed shores Carex serotina ssp. pulchella. In the Bothnian Bay are to be found floristically related communities with Calamagrostis stricta, Parnassia palustris, the arctic halophytes Potentilla anserina ssp. egedii, Carex glareosa and Primula siberica ssp. finnmarchica, on sheltered sites accompanied by Eriophorum angust([blium. Without grazing or mowing it is replaced by woodland (Kauppi, 1967). A Juncus gerardii-Agrostis stolonifera community persists on exposed marshes in the Finnish Bothnian Bay. On the western Swedish and the Baltic Sea shores the higher zone of the Juncetum gerardii association is distinguished by Festuca rubra, Leontodon autumnalis, Poa pratensis ssp. irrigata, Trifolium repens, etc. and is included in the subassociation with Leontodon autumnalis, Raabe, 1950 (=Festucetosum rubrae association, Tyler, 1969b). It is transitional to the Lolio-Potentillion anserinae alliance of upper marshes. Little is known about a (secondary?) community of the boreal species AIopecurus arundinaceus (Tyler, 1969b; Rebassoo, 1975; Schamin6e et al., 1982).

200

Kees S. Dijkema

Communities of upper marshes In the upper marsh zone some halophytes of the Juncetum gerardii association still occur. The Festuca rubra communities on exposed shores in the Bothnian Bay still have relationships with the above-mentioned subassociation with Leontodon autumnalis. The relation of the Carex nigra communities (Festuceto-Caricetosum nigrae, Tyler, 1969b) with the Juncetum gerardii association disappears northwards, where it is a nonhalophytic community. On the upper marshes of southwestern Sweden and the southern Baltic Sea communities of the Lolio-Potentillion anserinae alliance occur.

Communities of salt patches On the Baltic shore marshes important sites for halophytes are found on socalled salt patches, where chlorides and sulphates accumulate in the topsoil. They are found on a high level in the zonation and more inland mainly on heavily grazed marshes, although natural salt patches may occur. Important species are Salicornia europaea, Spergularia salina, Puccinellia distans, P. retroflexa (mainly Bothnian Bay), and P. phryganodes (arctic halophyte, found near Oulu; Siira & Haapala, 1969). Comparable sites are found inland near Oulu (Siira, 1971).

Range and distribution The shore marshes of the Baltic Sea and the adjacent parts of the North Sea include a wide range of species and communities: from brackish Baltic communities (north and central Atlantic, containing some arctic species in the Bothnian Bay) to central Atlantic communities in western Sweden, southern Norway and Denmark (Beeftink, 1984; Thannheiser, 1986, 1987). There ale many phytogeographical similarities between the salt marshes of western Sweden, southern Norway and Denmark and those of the British Isles (Adam, 1981), the Wadden Sea (Dijkema & Heydemann, 1984) and central Norway (Thannheiser, 1987). However, due to floristic impoverishment of the communities northwards (the same holds for Scotland) there are differences (Raabe, 1950; Siira, 1970; Thannheiser, 1986). Communities with Spartina anglica and Halimione portulacoides do not occur in the investigated area; those with Limonium vulgare, Artemisia maritima and Parapholis strigosa are only found in the transition area. In the Baltic Sea, due to the brackish conditions, the pioneer and lower marsh associations Salicornietum strictae and Puccinellietum maritimae are replaced by reed beds or the Eleocharis uniglumis association. Dry

201

Baltic brackish marshes

conditions in the growing season cause an increase of soil salinity upwards in the zonation (also favoured by grazing and mowing), the gradient being reversed compared to Atlantic salt marshes (M/ikirinta, 1970; Siira, 1970). That is why there exists a large similarity in the middle marsh association Juncetum gerardii on the Atlantic and Baltic salt marshes, which similarity does not hold true for the lower marsh zone. An exceptionally welldeveloped aspect of the Baltic shore marshes are the natural transitions to woodland (mostly A l n u s ) , bog and heath communities. These transitions have almost been lost in most European salt marshes (Dijkema et al., 1984). The salt and brackish marshes of the Baltic Sea and the adjacent parts of the N o r t h Sea are found in association with several geomorphological forms (Dijkema, 1987b; Fig. 1). Sedimentary shores more or less comparable to the salt marshes around the N o r t h Sea are only found in Denmark ('fjords' of Jutland, Alborg Bugt, Laes6, Saltholm), in southern Sweden (extensive foreland salt marshes along the Oresund, the barrier-connected green beach near Malta6, the Tullgarnsn~set peninsula south of Stockholm) and along

TABLE 5

Areas and Numbers of Shore Marsh Sites Around the Baltic Sea and Transitions to the North Sea, Compared to the Salt Marshes of the North Sea and the Atlantic coasts of Europe (after Dijkema et al., 1984) Country

Area ~ (in km 2}

Number qf sites investigated

Number ~!/ sites > 5 km 2

Sweden ~Baltic Sea Denmark

c. 15 120 70

19 58 28

Norway

c. 45

76

1

12 t 22 74 25 15 J

7) 6 28

44 4

11 3

2 0

Great Britain Ireland

371 (180)

69 53

17 c. 6

West France Portugal Spain (Dofiana)

148 92 (400)

56 14 1

6 5 1

Denmark J ~W Schlw.-Holst. FRG adden Niedersachsen FRG[ Sea The Netherlands SW Netherlands Belgium

( ), Adjacent water bodies included.

81 t 71 354 117 85 J

c.

7J

11

202

Kees S. D~]kema

the southern Baltic Sea (some barrier-connected salt marshes on the German and Polish shore). Other favourable conditions for salt marshes occur on flat and skerry coasts in combination with land-uplift. Flat coasts with extensive marshes and salt patches are found in the Bothnian Bay near Oulu (Siira, 1970) and in some inlets (e.g. Stor6 hamn, Kallax, Kage). Skerry coasts with extensive archipelagos occur in southern Norway, southern Finland, between the Baltic Sea and the Bothnian Sea (S6dra Kvarken and Aland archipelago), between the Bothnian Sea and Bothnian Bay (Norra Kvarken, archipelagos of Vaasa and Jakobstad) and in the northern part of the Bothnian Bay. There are many small salt marsh sites, often only some hectares in size. Small salt marshes also occur locally in the Norwegian and Swedish fjords. Steep coasts without marshes are found along the Swedish Bothnian Sea and the southern part of the Swedish Bothnian Bay. In Table 5 the areas and numbers of sites have been compared to those of the North Sea and the Atlantic coasts of Europe. The importance of Great Britain and the Wadden Sea is evident, especially since these regions include many large sites in a relatively concentrated area. Although most shore marsh sites around the Baltic Sea are small, together they form more than 10% of the area of shore marshes in northwest Europe north of the English Channel. Moreover, the well-developed gradient in salinity and in other ecological conditions gives them a special interest.

USE A N D M A N A G E M E N T

Grazing Shore marshes in northern Europe have a long history of grazing by domestic animals and hay-making. A period of at least 500 years has been mentioned by Thannheiser (1982) for northern Norway, by Siira (1970) for the Bothnian Bay near Oulu, by Tyler (1969b) for southeastern Sweden and by Dahlbeck (1945) for the Swedish salt marshes along the Oresund. Along the west coast of Sweden (Gillner, 1960) and the southern Baltic Sea from Germany to Estonia (Christiansen, 1937; Fukarek, 1961; Piotrowska, 1974; Schmeisky, 1974, 1977; Rebassoo, 1975; Jeschke, 1983) agricultural use was a c o m m o n practice on most salt marshes. Siira (1970) mentions extensive hay-making for the first half of this century on the shore marshes near Oulu, gradually decreasing and becoming quite rare by the 1960s. Nowadays grazing is the only agricultural land-use of Baltic shore marshes, although during the last 25 years the pressure of domestic animals has decreased and many marshes are no longer used (Tyler, 1969b; Schwanck, 1977; Willers,

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1987; Pehrsson, 1988). From a questionnaire it appears that in 1982 about 50% of the Baltic shore marsh sites were grazed. A reduction in grazing happens on many salt marshes in Europe (Dijkema et al., 1984; Bakker, 1989). Grazing and hay-making favour the occurrence of halophytic vegetation on brackish marshes: directly by eliminating the competition for light from tall-growing glycophytes (e.g. reeds, Elymus repens, trees) and indirectly by compacting the soil which hinders leaching of salts (Kauppi, 1967: Siira, 1970; Schmeisky, 1974, 1977). After cessation of grazing and hay-making on Baltic shore marshes, reed beds, tall grasses, and woodland increase at the cost of the halophytic vegetation. Examples have been described by Fukarek (1961), Tyler (1969b), Siira (1970), Gravesen (1972), Piotrowska (1974), Schmeisky (1974, 1977), Rebassoo (1975), Ericson & Wallentinus (1979), Willers (1987), and Pehrsson (1988), Only on wave-exposed sites can halophytic communities survive without interference (e.g. outer skerriesl.

Management strategy for salt and brackish marshes Management measures deal with the minimum human interference needed to preserve the natural values of a site. In a purely natural ecosystem management may be unnecessary. The management measures may be related to traditional land-use or replace former natural grazing. As salt marshes mainly consist of natural grasslands, grazing should simulate as closely as possible the natural grazing by herbivores which occurred before human occupation (Beeftink, 1977b). Examples of less natural measures that can be used for marsh management include mowing, draining and cutting sods. Each of these management measures can become a threat when applied at the wrong rate or in the wrong place (for example, too heavy grazing). Table 6 gives examples of attributes of sites which should be considered in the selection of management measures. Such a selection has to be based on knowledge of the effects of various forms of management measures on the marsh ecosystems. A general selection of management measures, including no interference, is best done on a country or regional basis. For example, in The Netherlands Wadden Sea a general management scheme has been accepted, aiming at no interference for the dynamic parts of the barrier islands (about 25% of the total salt marsh areal. After the general selection, management measures have to be drawn up for each site. It is recommended that the selection of methods and intensities of management should express the potential diversity of vegetation types according to natural patterns in geomorphology and salinity (for example, light grazing). Guidelines are presented in the next section.

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TABLE 6 Site Qualities Important for the Selection of Management Measures in Salt and Brackish Marshes Management measure Grazing and/or mowing

Protecting the salt marsh edge

Pre[erence for no interferenee

Preference./br

management

Site with long period of no interference Site exposed to extreme abiotic factors (outer skerries, young parts of barrier systems, arctic climate) Site with low plant production (e.g. sandy soil, extreme abiotic factors)

Natural richness of the site exists due to grazing or mowing Low salinity area, where halophytes tend to disappear (Baltic)

Erosion occurs only locally, temporarily, or is compensated by accretion elsewhere

Erosion occurs generally or concerns very valuable and/or small sites

Site with high standing biomass and litter accumulation (e.g. clayish soil, estuary, eutrophication by plant litter or by pollution)

Conservation, in contrast to management, measures deal with the maximum allowable human impact (e.g. changes in estuarine hydrography, embankment, pollution, recreation, introduced species). Control of human impact is a precondition for the success of management measures. From the 1982 questionnaire it appears that the marshes in the Baltic Sea are not badly affected when compared with the situation in Europe (except for reclamations in the land-uplift areas near Oulu, which threaten 50% of the area of shore marshes in Finland). Disturbances such as dumping of rubbish, excavation and filling, a normal practice on about half of the salt marsh sites in Europe south of the English Channel, hardly occurs (Dijkema et al., 1984). Although the degree of statutory protection is not as high as for the salt marshes of Great Britain and the Wadden Sea (both almost 100%), the greater part of the sites have some kind of protection (in 1982 in Finland, 8%; in Sweden, 80%; and in Denmark 60%; statutory protection is also in preparation for most sites in Norway). Management guidelines The general selection of management measures should be made according to the strategy described above (Table 6). Spatial differences in management

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measures (including no interference) will contribute to the diversity of vegetation types, invertebrate fauna and birds, including halophytic and brackish communities and their natural transitions to woodland, bog, heath and sand-dune communities. Introduction of features which do not naturally occur in the area should be strongly resisted (e.g. artificial islands, artificial ponds, dumping of dredge soil). Reintroduction of management measures in the Baltic shore marshes should be accompanied by ecological investigations and monitoring, as few quantitative data on the effects of different animals and stocking rates are known. When a wrong management is introduced, it will be difficult to counteract harmful effects, once they are detected (Beeftink, 1977b). Therefore the effects of new management measures should be studied before they are undertaken (Ranwell, 1961; Beeftink, 1977b; Beeftink et al., 1978~ Basset, 1980; Dijkema & Dankers, 1983; Bakker, 1985, 1989; Vestergaard, 1985; Pehrsson, 1988). The principal forms of management for the halophytic communities on the Baltic marshes should be grazing and hay-making. Abandonment of these old land-use practices on salt marshes has caused the loss of halophytic communities, a specific feature of ecological value. It is not possible to preserve halophytic communities in this brackish environment without grazing and/or hay-making, except on exposed sites (e.g. on the outer archipelagos). Grazing is a better form of management than hay-making (Schmeisky, 1977). On large sites (> c. 50 ha) the development of gradients in the grazing intensity should be promoted, e.g. by open-range grazing which contributes to the diversity (Looyen & Bakker, 1987; Bakker, 1989). Additional management measures may be needed for the invertebrate fauna and for breeding and grazing birds, e.g. by spatial differences in management measures (cf. Pehrsson, 1988). Guidelines for the choice of which animal(s) should be used have been discussed by Van Wieren (1987). For natural grazers there are some restrictions in the sense that they need a large area (much larger than most Baltic shore marshes), including higher feeding grounds for the winter. Of the domestic animals cattle are most suitable to prevent the development to domination of tall-growing glycophytes and litter accumulation. A well-balanced management of the Baltic shore marshes is worthwhile, also in the broader context of European salt marshes. Although most marsh sites summarized in this paper are small, together they form a unique chain with well-developed ecological gradients. The isolation of the Baltic Sea and its brackish character give the Baltic shore marshes even an evolutionary interest.

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