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)
A B S T R A C T
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.
I N T R O D U C T I O N
The salt and b rack i sh m a r s h sites in n o r t h e r n E u r o p e are of ten small and o f a s imple g e o m o r p h o l o g i c a l s t ructure . H o w e v e r , the shore vege ta t ion as a
191
Biol. Conserv. 0006-3207/90/$03-50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
192 Kees S. Dijkema
substrate geology salt marsh type
ria bay - - 1 rocky shores loch/f jord head I i = . 1
beach head /
allocht honous barrier-connected
marine lagoonal sedementary shores foreland 1
estuarine i f luvial sedamentary deltaic shores
emerging f lat shore I
autochthonous 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 Venice classification coastal water
(%0 S)
Open Sea Norwegian Sea 33 30 euhalinicum North Sea 34 32 euhalinicum Skagerak 33-27 eu- and polyhalinicum
Transition Kattegat 30-18 polyhalinicum area Belt Sea 18-10 ~-mesohalinicum
Oresund 10-8 /%mesohalinicum
Baltic Sea Baltic Sea proper 10-5 /~-mesohalinicum Gulf of Riga Gulf of Finland 5~3 oligohalinicurn Gulf of Bothnia
Bothnian Sea 5-3 ~-oligohalinicum Bothnian Bay 3q3-3 /#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, Eleocharis palustris 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).
From 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 AND 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 non- halophytic 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
TA
BL
E 2
C
omm
on P
lant
Spe
cies
of
(Hal
ophy
tic)
Sho
re C
omm
unit
iesf
l So
uthe
rn N
orw
ay a
nd S
outh
wes
t an
d So
uthe
ast
Swed
en,
Bas
ed o
n L
iter
atur
e an
d O
wn
Obs
erva
tion
s
Sout
hern
Nor
way
So
uthw
est
Swed
en
Sout
heas
t Sw
eden
6%
00 S
(Tyl
er,
1969
b; W
alle
ntin
us,
1967
, 197
3)
Tele
mar
k 30
%0
S O
stoy
(O
s]of
jord
) Sk
ager
ak +
Kat
tega
t O
resu
nd 1
0-8%
o S
( Vev
le, 1
982,
198
5;
18%
o S
(Dah
l &
Had
ac,
3~20
%o
S (G
illne
r (D
ahlb
eck,
19
45;
Ope
n sh
ore
Shel
tere
d sh
ore
Hof
sten
& V
evle
, 198
2)
1941
) 19
60,
1965
) G
illne
r, 1
965)
Em
ergi
ng fl
ats
(Ele
ocha
ris
parv
ula)
(E
leoc
hari
s pa
rvul
a)
(Ele
ocha
ris
parv
ula)
Sa
licor
niet
um s
tric
tae
Suli
corn
ia do
licho
stac
hya
(Sal
icor
nia
dolie
host
aehy
a)
Salic
orni
a do
licho
stac
hya
Hal
o-Sc
irpe
tum
mar
itim
i (S
cirp
us m
ariti
mus
) Se
irpu
s m
ariti
mus
(S
cirp
us m
ariti
mus
) Se
irpu
s m
ariti
mus
Sc
irpu
s m
ariti
mus
(S
cirp
us m
ariti
mus
) (S
cirp
us t
aber
naem
onta
ni)
(Sci
rpus
tab
erna
emon
tani
) (S
eirp
us t
aber
naem
onta
ni)
Scir
pus
tabe
rnae
mon
tani
Ph
ragm
ition
(P
hrag
mite
s au
stra
lis*)
P
hrag
mite
s au
stra
lis*
(Phr
agm
ites
aust
ralis
*)
Phr
agm
ites
aust
ralis
* P
hrag
mite
s au
stra
lis*
Low
er m
arsh
(C
arex
pal
eace
a)
Car
e,,:
pale
acea
(C
arex
pal
eace
a)
Pucc
inel
liet
um m
arit
imae
P
ueci
nelli
a m
ariti
ma-
P
ucci
nelli
a m
ariti
ma-
P
ueci
nelli
a m
arit
ima
Puc
cine
llia
mar
itim
a-
Pla
nt.
mar
.-Ast
er t
rip.
P
lant
. m
ar.-
Ast
er t
rip.
A
ster
tri
p.-T
rigl
, m
ar.
Ele
ocha
ris
unig
lum
is
Ele
oeh.
uni
gl.-A
gr,
stol
. A
gros
tis s
tolo
n~fe
ra
Ele
ocha
ris
unig
lum
is
(Ele
oeha
ris
unig
lum
is)
Agr
. st
oL-T
rigl
, pa
l A
gr.
stol
.-Tri
gl,
pal.
(Car
ex m
aeke
nzie
i)
Car
ex m
aeke
nzie
i M
iddl
e m
arsh
A
rtem
isie
tum
mar
ltlm
ae
(Art
. m
ar.-L
im,
vulg
.)
Art
emis
ia m
arit
ima
J. g
er.-L
im,
vulg
are
Junc
etum
ger
ardi
i Ju
ncus
ger
ardi
i Ju
ncus
ger
ardi
i Ju
neus
ger
ardi
i Ju
neus
ger
ardi
i Ju
ncus
ger
ardi
i Sc
irpe
tum
ruf
f B
lysm
us r
ufus
B
lysm
us r
uJus
B
lysm
us r
t4[u
s J.
ger
.-Leo
nt,
aut.
J. g
er.-L
eont
, au
t Le
ont.
aut.-
Tr(f
. ]b
ag
. F
estu
ca r
ubra
F
estu
ca r
ubra
C
aric
etum
pul
lche
llae
C
arex
puk
'hel
lum
C
arex
ext
ensa
U
pper
mar
sh
Lol
io-P
oten
till
ion
ans.
E
lym
us r
epen
s E
lvm
us r
epen
s (E
lvm
us r
epen
s)
Fes
tuca
aru
ndin
acea
F
estu
ca a
rund
inae
ea
Fes
tuea
aru
ndin
acea
Fe
st u
ceto
-Car
icet
osum
C
arex
rec
ta
nigr
ae
Car
ex n
igra
* C
arex
nig
ra-F
est,
rubr
a W
oodl
and
com
mun
itie
s A
lnus
glu
tinos
a*
(Aln
us g
lutin
osa*
) A
lnus
glu
tinos
a*
Pru
nus
spin
osa*
A
lnus
glu
tinos
a*
Salt
patc
hes
Salic
orni
etum
Sa
licor
nia
euro
paea
Sa
licor
nia
euro
paea
Pu
cine
llie
tum
dis
tani
s Sp
ergu
lari
a sal
ina
Sper
g. s
al.-P
ucc,
dis
tans
Sp
erg.
sal
.-Puc
c, d
ista
ns
Sper
g. s
al.-P
ucc,
dis
tans
Pu
cc. m
ar.
Hal
imio
ne p
edun
eula
ta
Hal
. ped
.-Par
, st
rigo
sa
phar
apho
lieto
sum
Sa
g. m
ar.-C
och,
dan
ica
Sag.
mar
.-Coc
h, d
anic
a Sa
g. m
ar.-C
oeh,
dan
iea
Sag.
mar
.-Coc
h, d
anic
a
° *,
Non
-hal
ophy
tic
com
mun
itie
s; (
)
only
loc
ally
com
mon
.
TA
BL
E 3
C
omm
on P
lant
Spe
cies
of
Hal
ophy
tic
Shor
e C
omm
unit
ies,
Den
mar
k an
d th
e So
uthe
rn B
alti
c, B
ased
on
Lit
erat
ure
Den
mar
k G
erm
any
Bal
tic
Sea
Tipp
erne
(N
orth
Sea
) ls
e[]o
rd
Kat
tega
t)
Gra
ssw
arde
r D
arss
P
olan
d 7%
o S
Est
onia
n Lf
fand
s 7%
o S
5 8%
o S
19°/o
o S
14%
o S
10%
o S
(Fuk
arek
, 19
61)
[Pio
trow
ska,
197
4)
[Reb
asso
o, 1
975)
( S
ehm
eisk
v
(Gra
vese
n,
1972
) (M
ikke
lson
, 19
49)
1974
, 19
77)
Em
ergi
ng f
iats
E
leoc
hari
s pa
rvul
a
{Sal
icor
nia
euro
paea
) H
alo-
Sci
rpet
um m
arit
imi
Seir
pus
mar
itim
us
Scir
pus
mar
itim
us
Scir
pus
mar
itim
us
Scir
pus
mar
itir
nus
Scir
pus
mar
itim
us
(Sci
rpus
tab
erna
emon
tani
) Sc
irpu
s ta
bern
aem
onta
ni
Scir
pus
tabe
rnae
mon
tani
Sc
irpu
s ta
bern
aern
onta
ni
Phra
gmit
ion
Phr
agm
ites
aus
tral
is
Phr
agm
ites
aus
tral
i,s
Phr
agm
ites
aus
tral
i.s
Phr
agm
ites
aus
tral
is
Junc
us m
arit
imus
Ju
ncus
mar
itim
us
Low
er m
arsh
P
hrag
. au
str.
(un
graz
ed)
Phr
ag.
aust
r. (
ungr
azed
) Pu
ccin
ellie
tum
mar
it.
Trig
l. m
ar.-
Ast
er t
rip.
(P
ucci
nelli
a m
arit
ima)
P
ucci
nelli
a m
arit
ima
Puc
cine
llia
mar
itim
a [A
gros
tis
stol
on~[
bra)
A
gros
tis
stol
oni/
era
Ele
ocha
retu
m u
nigl
umis
[E
h,oc
hari
s un
iglu
mis
) E
qu
inqu
e[to
r ;-
Agr
.sto
l. E
leoc
hari
s un
iglu
rnis
M
agno
cari
cion
pal
eace
ae
('ar
e v
mac
kenz
iei
{she
lter
ed)
Mid
dle
mar
sh
Art
emis
ietu
rn m
arit
imae
A
rtem
isia
mar
itim
a .4
rt.
mar
. (u
ngra
zed)
(A
rtem
isia
mar
itim
a)
Lint
. ru
lg.
(ung
raze
d)
IPuc
cine
llia
mar
itim
a)
Junc
etum
ger
ardi
i J.
ger
.-Fes
t, ru
bra
Junc
us g
erar
dii
J. g
er.-
Agr
, st
ol.
Junc
us g
erar
dii
Sci
rpet
um r
utl
B(v
s.,m
ts r
u/us
B
lysm
us r
t(/u
s ('
arex
ext
ensa
(C
arex
ext
ensa
) ( C
are
v gl
areo
sa)
June
. ge
r. s
ubas
s.
.l.ge
r.-F
e.st
.r.-L
eo,t.
au
t. J.
ger.
-kk'
st.r
.-Leo
nt.
¢tll
l.
I'~'s
tt, a
ruh
ra
J. g
er.-
Leon
t, a
lll.
J.,z
er.-F
est.r
.-I,e
ont.
tttl
I.
Fcs
[. r
. + L
.-P
. sp
. w
ith
Leon
todo
n at
llum
ll.
Lol
io-P
oten
till
ion
ans.
E
ll'm
ils
repe
m (
ungr
aTcd
)
Salt
pat
ches
(,~
a/llo
rHttl
cl
lFop
~tea
) {~
'a/it
ornl
a ct
tro[
Taca
~ Sp
er~
~al.
-th,
~. d
ista
n.s
Sl?e
rg. .
sal.
-Pu~
. di
stan
s
I Onl
y lo
call>
com
mon
. N
o da
ta a
vail
able
.
TA
BL
E 4
C
omm
on P
lant
Spe
cies
of
(Hal
0phy
tic)
Sho
re C
omm
unit
ies
a in
the
Bot
hnia
n B
ay,
Bas
ed o
n L
iter
atur
e an
d O
wn
Obs
erva
tion
s
Jako
bsta
d ar
ea
Oul
u ar
ea
Nor
thea
ster
n A
rchi
pela
gos
Swed
ish
part
3"
6 4°
/oo S
2
3%o
S (M
iiki
rint
a, 1
970;
Sii
ra,
1970
) 1"
5 3°
oo S
0 3%
o S
(Eri
eson
& W
alle
ntin
us,
1979
) (S
ehw
ank,
197
7)
(Var
tiain
en,
1980
) O
pen
shor
e Sh
elte
red
shor
e O
pen
shor
e Sh
elte
red
shor
e
Em
ergi
ng fi
ats
(Ele
oeha
ris
acie
ular
is*)
E
leoe
hari
s ae
ieul
aris
* E
leoe
hari
s ae
ieul
aris
* E
leoe
hari
s ae
icul
aris
* {E
leoe
hari
s pa
lust
ris*
) E
leoc
hari
s pa
lust
ris*
(E
leoe
hari
s pa
lust
ris*
) E
leoc
hori
s pa
lust
ris*
(S
cirp
us t
aber
naem
onta
ni)
Scir
pus
tabe
rnae
mon
tani
(S
eirp
us
tahe
rnae
mon
tani
) Se
irpu
s ta
bern
aem
onta
ni
Phra
gmit
ion
Phr
agm
ites
aust
ralis
* P
hrag
mit
es a
ustr
alis
* P
hrag
mit
es a
ustr
a[is
* P
hrag
mit
es a
ustr
alis
* E
quis
etum
[tu
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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 common 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 non- halophytic 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 sub- association 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 sub- association with Leontodon autumnalis. The relation of the Carex nigra communit ies (Festuceto-Caricetosum nigrae, Tyler, 1969b) with the Juncetum gerardii association disappears northwards, where it is a non- halophytic 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 so- called 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 impoverish- ment 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
Baltic brackish marshes 201
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 well- developed aspect of the Baltic shore marshes are the natural transitions to woodland (mostly Alnus ) , 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 Nor th 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 Nor th 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
T A B L E 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 ~ Number qf Number ~!/ (in km 2} sites sites > 5 km 2
investigated
Sweden ~Baltic Sea c. 15 120 19 58 c. 11 Denmark 70 28 7J Norway c. 45 76 1
Denmark ~W J 81 t 12 t 7 ) Schlw.-Holst. FRG adden 71 22 354 74 6 28 Niedersachsen FRG[ Sea 117 25 The Netherlands 85 J 15 J
SW Netherlands 44 11 2 Belgium 4 3 0
Great Britain 371 69 17 Ireland (180) 53 c. 6
West France 148 56 6 Portugal 92 14 5 Spain (Dofiana) (400) 1 1
( ), Adjacent water bodies included.
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 AND 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 common 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,
Baltic brackish marshes 203
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.
204 Kees S. Dijkema
TABLE 6 Site Qualities Important for the Selection of Management Measures in Salt and Brackish
Marshes
Management Pre[erence for no Preference./br measure interferenee management
Grazing and/or Site with long period of no Natural richness of the site mowing interference exists due to grazing or mowing
Site exposed to extreme abiotic Low salinity area, where factors (outer skerries, young halophytes tend to disappear parts of barrier systems, arctic (Baltic) climate) Site with low plant production Site with high standing biomass (e.g. sandy soil, extreme abiotic and litter accumulation (e.g. factors) clayish soil, estuary,
eutrophication by plant litter or by pollution)
Protecting the Erosion occurs only locally, Erosion occurs generally or salt marsh edge temporarily, or is compensated concerns very valuable and/or
by accretion elsewhere small sites
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
Baltic brackish marshes 205
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.
206 Kees S. D~kema
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