Helgol Mar Res (2005) 59: 9–21
DOI 10.1007/s10152-004-0202-6

O R I GI N A L A R T IC L E



Karsten Reise

Coast of change: habitat loss and transformations
in the Wadden Sea


Received: 31 July 2004 / Revised: 10 October 2004 / Accepted: 20 October 2004 / Published online: 18 January 2005
Ó Springer-Verlag and AWI 2005

Abstract In the southern North Sea, coastal people
commenced with habitat conversions 1,000 years ago.
                                  Introduction
Partly interrupted in late medieval times by large-scale
                                  Sea level rise and sediment supply have been the main
inundations of marshland, progressive embankments
                                  drivers of change in coastal morphology of the south-
transformed the landward half of the amphibic transi-
                                  ern North Sea until diking commenced about
tion zone between a limno-terrestric and a brackish-
                                  1,000 years ago (Behre 2002; Flemming and Davis
marine ecosystem into arable land and freshwater lakes.
                                  1994; Pethick 2001; Rippon 2000; Wolff 1992a). Here,
Sea walls rigidly separated the land from the sea. Dy-
                                  an attempt is made to reconstruct natural states before
namic transitional habitats have vanished. Areal loss has
                                  diking became the key process affecting habitat diver-
diminished the capacity of the Wadden Sea to dissipate
                                  sity in the Wadden Sea region. Similar developments
wave and tidal energy. A coastal ecosystem once rich in
                                  took place at the east coast of Britain (Burbridge and
marsh plants, seagrass and diatoms on mud flats became
                                  Pethick 2003), in China (Wang et al. 2000) and else-
transformed into one with less autochthonous photo-
                                  where at sedimentary coasts. However, the tradition of
troph production, dominated by sandy tidal flats, and
                                  gaining and separating land from the Wadden Sea has
dependent primarily on allochthonous plankton supply.
                                  come to an end (Wolff 1992b). During the 1980s, nat-
The large estuaries have been dredged to serve as ship-
                                  ural wetlands came to be valued more highly than a
ping canals, and have lost most of their former retention
                                  potential gain from turning them into arable land.
and filter capacity. Riverine loads are now flushed right
                                  Nevertheless, further embankments and storm surge
into the North Sea. Symptoms of a syndromatic coastal
                                  barriers were employed for improving water manage-
habitat degradation are diagnosed, leading to a decline
                                  ment in the hinterland and for the sake of coastal de-
in natural habitat diversity. The conventional on-line
                                  fence. The present coastal defence policy is to hold the
coastal protection may not achieve a sustainable coastal
                                  line. The Netherlands, Germany and Denmark have
habitat configuration. At sedimentary coasts immobi-
                                  adopted a common management plan for integrating
lised by dikes and petrified shores, a more flexible re-
                                  nature conservation and human use in the Wadden Sea
sponse to sea level rise is recommended.
                                  (Stade Declaration 1998).
Keywords Coastal dynamics Æ Estuary Æ Habitat loss Æ          What is the outcome of coastal transformations on
Sea level rise Æ Tidal flats                    habitat diversity and natural habitat dynamics? The
                                  large areal extent of embankments also poses the ques-
                                  tion whether the tidal area of the Wadden Sea, if left to
                                  itself, will remain as it is or is there a legacy of past
                                  interventions by which habitats will continue to change?
                                  This is to be expected if present coastal morphology is
                                  not in line with hydrodynamic conditions. Further, sea
Communicated by H.K. Lotze                     level is assumed to continue to rise, presumably with
                                  some acceleration (IPCC 2001). Will this cause tidal flats
K. Reise                              to fade away and muddy sediments to become more
Alfred Wegener Institute for Polar and Marine Research,      sandy? Habitats also change when habitat generating
Wadden Sea Station Sylt, 25992, List, Germany
E-mail: kreise@awi-bremerhaven.de                 species (ecosystem engineers, sensu Jones et al. 1994),
Tel.: +49-4651-956110                       such as seagrasses, oysters and mussels, are affected
Fax: +49-4651-956200                        (Lotze 2005; Wolff 2005).
10

   Abrahamse (1976), Bantelmann (1966), Behre (2001,
2002), van Eerden et al. (1997) and Knottnerus (2001)
provide reconstructions of historical Wadden Sea land-
scapes. First surveys on habitats in the tidal area were
conducted in the 1920s and 1930s (i.e., Hagmeier and
Kandler 1927; Nienburg 1927; Wohlenberg 1937; Linke
  ¨
1939; Plath 1943). Comprehensive habitat maps for the
1970s have been provided by Dijkema (1983, 1991; Di-
jkema et al. 1989) and a status list of biotopes and
biotope complexes has been compiled by Ssymank and
Dankers (1996). Habitats are here defined as conspicu-      Fig. 2 Areal share of land- and sea-scape components in the
ous, recurrent areal components with a characteristic      Wadden Sea region, including the Zuiderzee area
cluster of hydrographic, morphological, sedimentary
and biotic features.                       100 km2 on the islands, 4,000 km2 of intertidal sand and
   I will focus on the tidal area and shoreline habitats,   mud flats, and also 4,000 km2 of a sandy subtidal zone
consider effects of past sea level change, and speculate     dissected by deep tidal channels (gulleys) and inlets
on the functioning of the primordial Wadden Sea coastal     (Fig. 2). About 6,000 km2 of the adjacent North Sea
ecosystem. In a second part, I describe transformations     down to a depth of À15 m may be added to include the
towards modern coastal architecture and reflect on a       immediate sharing system of sediment and water.
syndromatic development that has arisen. I conclude         A thousand years ago, before people started to
with a few suggestions for future habitat management in     transform this coastal landscape, salt marshes, brackish
the Wadden Sea.                         reed marshes, lagoons, bogs and lakes comprised
                                 approximately 14,650 km2. These are now embanked,
                                 including some tidal areas. In the northern Wadden Sea,
Areal overview                          the tidal area used to be smaller than it is now, but may
                                 have extended further seaward to an unknown extent.
In former times, the Wadden Sea region continued into      Today, the tidal area of the entire Wadden Sea is pre-
the Rhine delta and beyond, but at present it is under-     sumably close to its size before 1000 AD.
stood as the 500-km long coastal zone between the          In medieval times, areal proportions were changed
                  ˚
Marsdiep in the west and Gradyb in the north (Fig.1;       intermittently by storm surges inundating embanked
Anonymous 1991; De Jong et al. 1993, 1999; Wolff         land which had been artificially subsided by cultivation
1983). It is still one of the largest coherent tidal flat areas  and peat mining (Table 1). These inundations occurred
in the world. Nowadays, a mainland coastal plain of       at different times between 1000 and 1634 AD, earliest in
about 24,000 km2 comprises 15,000 km2 of embanked        the southwest and latest in the north of the Wadden Sea.
marshes and embayments (including the former Zuid-        Subsequent re-embankments followed the same pattern.
erzee), 1,000 km2 of islands and high sandy shoals,       The largest embayment which came into existence was
200 km2 of salt marshes along the mainland coast and       the brackish Zuiderzee (3,600 km2). When it was dam-
                                 med in 1932, almost one-third of the entire tidal area of
                                 the Wadden Sea had been cut off, and was subsequently
                                 converted into arable land and a freshwater lake.


                                 Rise and fall of sea level in a pristine Wadden Sea

                                 Since the end of last glaciation in 16,000 BP, relative sea
                                 level has risen by 120 m in the North Sea region (Behre
                                 2002; Rippon 2000; Streif 1989). During a fast rise until

                                 Table 1 Estimated areal sizes of medieval inundations of mostly
                                 cultivated marsh and peatland (IMP) between 1000 and 1634 AD,
                                 of their partial re-embankment, of the still inundated parts now
                                 belonging to the tidal area, and that part of the tidal area which
                                 became embanked, but never had been medieval marshland. Given
                                 as percentages of all former marshes and bogs (14,650 km2)

                                 Type of area                  km2      %

                                 Inundated marsh and peatland (IMP)       6,200     42
                                 Re-embanked IMP                 5,180     35
Fig. 1 The Wadden Sea between the 15-m depth contour and the   Remaining IMP                  1,020     7
embanked marshland, with the former Zuiderzee area and part of  Embanked primordial tidal area         1,370     –
the Elbe estuary cut off
                                                               11

7000 BP, tundra and boreal forest in the southern North     Below neap high tide level, salt and reed marshes gave
Sea became flooded. When the shoreline approached the     way to extensive mud flats. The waters above may have
region of the present Wadden Sea, sea level was still     been relatively transparent most of the time, because of:
>10 m lower than today and the rise decelerated. Short    (1) the riverine filters, (2) a high energy dissipation and
phases of stasis or fall occurred intermittently. A switch  deposition capacity of the wide flood plain, and (3) vast
from transgression to 200 years of regression occurred    oyster beds between Britain and the Wadden Sea (Olsen
around the beginning of the Christian calendar. This     1883). Their filtering capacity may have caused clearer
interval was followed by a rise of roughly 2 m until to-   waters than nowadays, when the waters are without
day (Rippon 2000).                      oysters but with anthropogenic eutrophication. There-
  When sea level rise slowed down, barrier spits with    fore, seagrass beds may have been very extensive (Fig. 3)
sand dunes developed. These were eventually breached     and would have served as an additional sink for fine
and cut into barrier islands as sea level continued to rise  particles. A modern analogue may be the pristine tidal
and tidal range increased. In the southern Wadden Sea,    area of the Banc d‘ Arguin in Mauretania mostly cov-
this happened between 7500 and 6000 BP and consti-      ered by seagrasses (Wolff et al. 1993). Not only the
tutes the birth of the coastal configuration of the Wad-    intertidal zone but also the subtidal was suitable for
den Sea (Flemming 2002). In the central Wadden Sea,      extensive seagrass beds. These also stabilised sediment,
marsh and tidal deposits accumulated in front of pleis-    were rich in associated species, and served as spawning
tocene elevations, and no permanent seaward barrier      grounds for herring (De Jonge and de Jong 1992; Phil-
developed. In the northern part, a tidal area extended    ippart et al. 1992; van Katwijk 2003; Wohlenberg 1935).
behind protruding pleistocene islands which developed      Conspicuous biogenic habitats were mussel beds
long sandy spits, forming a barrier similar to the islands  (Mytilus edulis) in the lower intertidal to upper subtidal
in the south (Bantelmann 1966).                zone, oyster beds (Ostrea edulis) in the shallow subtidal
  The ratio between external sediment supply and the    and along deep channels, and reefs 0.5 m high were built
sediment deficits entailed by sea level rise differed be-    by the colonial polychaete Sabellaria spinulosa (Fig. 3;
tween subregions. Together with tidal range, this deter-   Hagmeier and Kandler 1927; Wolff 2005). These sus-
                                         ¨
mined long-term coastal developments (Ehlers 1988;      pension feeder assemblages provided a substrate for
Louters and Gerritsen 1994; Oost and de Boer 1994). The    macroalgae and other sessile invertebrates. Thus, in spite
tidal area behind barrier islands gradually enlarged with   of being a sedimentary coast, the primordial Wadden
sea level rise. When sea level rise stopped intermittently  Sea was rich in epibenthic habitat structures, particu-
(i.e., 100 BC to 100 AD), the tidal area probably de-     larly in its subtidal zone, mitigating sediment mobility
creased, and then increased again when sea level rise     and increasing biodiversity.
continued. Landward of the tidal area, a marsh of a       The extensive mud flats imply a high carrying
similar areal extent provided a wide episodically flooded   capacity for higher trophic levels. Salt marshes were not
plain, consisting of saltmarsh vegetation and brackish to   only large, but also rich in residual water bodies, creeks,
limnic reed marshes, including raised bogs. Particularly   ponds and puddles, and along the mainland showed
at the edge of pleistocene elevations, extensive freshwater  various transitions over brackish to limnic waters with
peat bogs developed. Along major rivers, gallery forests   the corresponding sequences of biota. Waterbirds would
occurred at the levees (Behre 2002). Otherwise the marsh   have found a wide and diverse habitat. With extensive
was a treeless plain kept open by episodic flooding.
  The total area from the barrier islands over the tidal
area and the open marsh plain to the glacial moraines
remained about the same from 4000 BC to 1000 AD,
while the position of the shoreline between tidal area and
salt marsh was highly dynamic and shifted back and
forth with sea level and sediment supply.


Functioning of the primordial coastal ecosystem

Episodic marine intrusions transported fine sediment
particles and organic carbon of marine origin far inland.
A dense vegetation and elevated berms at the seaward
edges of salt and river marshes retained this material.
Similarly, rivers deposited particle loads in the reed    Fig. 3 Schematic cross section through a tidal channel with
marshes behind the levees. Rivers had ramifications and    adjacent tidal area in primordial (left) and modern time. H and L
inner deltas, which functioned as an effective filter for    high and low tide level, respectively. Note that H but not L has
                               risen over time. Cordgrass S. anglica; short seagrass includes
riverine waters before entering the Wadden Sea. Most of    intertidal Zostera noltii and Z. marina; tall seagrass subtidal Z.
what was carried downstream by rivers was probably      marina; mussels M. edulis; native oysters O. edulis; Pacific oysters
retained in the wide estuaries and their flood plains.     C. gigas; a polychaete worm S. spinulosa
12

salt marshes, seagrass beds, and mud flats rich in            for every 1 mm of sea level rise, as calculated by Pethick
microphytobenthos, the primordial Wadden Sea               (2001) for the Humber estuary, the present shoreline
ecosystem had a high share of phototrophs, entailing           would have been 15 km further inland than it was at
assemblages of grazers and surface deposit feeders. It is        1000 AD. However, diking has in many cases moved the
suggested that the primordial ecosystem of the Wadden          modern shoreline further seaward instead.
Sea may have relied primarily on its autochthonous              Crucial for an understanding of long-term trends in
production, subsidised by a low to moderate import of          shoreline development is the relation between sea level
phytoplankton from the North Sea.                    change, sediment availability under natural conditions
                                     and when dikes protect the land against flooding (CPSL
                                     2001; Louters and Gerritsen 1994; Oost and de Boer
Transformations towards modern coastal architecture           1994). The development of tidal areas is largely deter-
and habitat composition                         mined by the ratio between the increase in water volume
                                     associated with sea level rise and the sediment volume
When large scale human impacts on habitats com-             supplied from external sources. With an excess of sedi-
menced about 1,000 years ago, these generally pro-            ment supply, as at the Dithmarschen coast just north of
gressed from the land towards the open sea, and             the Elbe estuary, shores are accreting in spite of sea level
diversified and intensified over time (Fig. 4). A mainly          rise. Where sediment supply is lower, however, tidal
autochthonous brackish to marine coastal ecosystem            areas are expanding by shoreline erosion. This serves as
with an extensive flood plain became transformed into a          a source of sediment, raising the level of the nearshore
marine fringe of the North Sea, relying on allochthonous         zone to keep pace with sea level rise (Fig. 5; see Bruun
supply.                                 rule: French 2001; Pugh 2004). A dike in tidal areas with
                                     sediment deficiency will cause a squeeze of upper shore
Sea level rise and coastal defence                    habitats until these are lost. Increasing hydrodynamics
                                     prevent fine sediment particles from settling and even-
When comparing shorelines 1,000 years apart one needs          tually tidal flats will fade away. This process is exacer-
to consider the relative change in sea level. From his-         bated where dikes have been built below high tide
torical documents since 1570 AD (i.e., marks on build-          level. The confrontation between land and sea becomes
ings), Rohde (1977) infers an increase in the peak levels        focussed at the foot of the dike which then needs to
of storm surges by 0.2–0.3 m per century. This is similar        be defended with hard structures. These replace an
to recent gauge measurements which indicate an increase         entire sequence of natural shoreline habitats. Such a
of 0.20–0.25 m in high tide level over the last century
(CPSL 2001), and is also similar to global sea level rise
of 1.5–2.0 mm per year obtained from gauge records or
of $2.5 mm per year from ongoing satellite altimeters
for the period 1993–2003 (Miller and Douglas 2004).
Since 1500 AD, a cumulative rise by 1.0–1.5 m in high
tide level has occurred, and for the time since 1000 AD
the upper estimate might be the best guess. In the ab-
sence of sea defences, such an increase could cause a
conspicuous retreat of the shoreline and a concomitant
rise in marsh level by natural deposition. Assuming a
rate of transgression in the order of 10 m horizontally




                                     Fig. 5 Effects of sea level rise (SLR) on soft tidal shores lacking an
                                     external sediment supply. Above: when undefended, shoreline
                                     retreat is associated with sediment redistributions to build a new
                                     berm at high water level and to raise the surface of the tidal flats.
Fig. 4 Selected trends in the fate of relative length, depth or extent  Below: when provided with a dike to prevent flooding and ersosion,
of habitats in the Wadden Sea over the past 1,000 years. Time AD     sediment hunger arising from SLR is directed downshore and
on log scale (increase darken, decrease lighten)             offshore by stone revetments
                                                                13

developmental spiral is irreversible as long as sea level is
rising. It can only be mitigated by shifting dikes inland
or by artificial sediment supplies taken from an external
source. Hard structural defences merely transfer the
hunger for sediment downshore and thus constitute no
sustainable solution.
  Barrier islands may absorb most of an external sed-
iment supply or may partly serve as a source of sediment
for the back-barrier zone. A size reduction in tidal area
by embankments causes a corresponding reduction in
the sediment volume of ebb tidal deltas. Part of this
volume is needed to adjust tidal basins to sea level rise,
while the remainder may contribute to the development
of island dunes and high sandy shoals. On the other
hand, a decrease in the size of ebb tidal deltas entails
erosion of islands in a downstream direction because less
wave energy is dissipated. In such cases, islands have
been defended with revetments or artificial beach re-
charge (CPSL 2001).


Embankments and reclamations at the mainland

A physical, ecological and societal regime shift occurred
in the Wadden Sea once a coherent line of earthen sea
walls prevented flooding of the marsh. A rigid separa-
tion was achieved between land and sea. Diking began
slowly after 1000 AD with small, isolated polders, which
finally coalesced into a coherent ‘‘Golden Ring’’ between
1200 and 1300 AD (Kramer and Rohde 1992; Wolff
1992b). In a first phase, existing salt marsh pastures were
embanked. After 1200 AD, this defensive strategy
gradually moved on to foster mud accretion seaward of
the dikes to gain new salt marsh area which then in turn
could be embanked. One polder was created in front of
the other, extending the outer dikeline into the tidal area
(Fig. 6; Bantelmann 1966; Behre 1999; Prange 1986).
  At the same time, peatland in front of pleistocene
elevations was reclaimed and pumped dry. This caused
bio-oxidation and compaction of the peaty soil. Peat
layers were excavated, dried and then burnt. Peat was
also mined where it occurred underneath a layer of       Fig. 6 Progressive re-embankment following medieval storm
marine clay. After burning, the salt was extracted for     surges at 20 km of coastal length in Nordfriesland with Dagebull ¨
                                (D) and Fahretoft islands in the middle surrounded by low
trading. Diked areas had to be drained because of heavy    summerdikes in 1500. Pleistocene elevations (Geest) are hatched,
precipitation on an almost impermeable clay or on peaty    salt marshes stippled, tidal flats dotted, former creeks narrowly
soil which retained the water. The result of draining and   dotted. Years of diking are indicated. A causeway for lorries to the
peat mining was an overall subsidence by 2–5 m at       island of Oland was first built in 1899 and renewed in 1927. Arrows
                                point to sites of shore erosion. Modified from Bantelmann (1966)
former mires, which thus came to be lower than the       with data from Prange (1986). After 1965 only 1 km2 of foreland
surface of new polders closer to the sea, resulting in an   became embanked when reinforcing of the outer dikeline
inverted wedge-shaped coastal plain.
  This vulnerable configuration contributed to the
formation of large coastal inlets when dikes were brea-    1362 and 1509, Leybucht in 1219, 1362 and 1374,
ched during storm surges: Zuiderzee, Middelzee,        Nordfriesland in $1250, 1338, 1354 and 1362. Marsh
Lauwerszee and Harlebucht formed between 1000 and       islands such as Bant and Strand became inundated and
1200 AD (Behre 1999; Oost 1995; Oost and de Boer        eroded away. In all these cases man-made subsidence
1994; Flemming and Davis 1994). Later, more coastal      aggravated the effects of flooding.
inlets formed or were enlarged in the course of storm       The eroded material was deposited elsewhere along
tides (Bantelmann 1966; Behre 1999; Prange 1986): Ja-     the coast where embankments started anew. During an
debusen in 1164, 1334, 1362 and 1511, Dollard in 1287,     exceptional storm surge in 1634, the last major losses of
14

Table 2 Large areas separated from the Wadden Sea and con-    The net result of this long history of successive
verted to land or lakes in the 20th century         embankments and land reclamations is a strict and
Area                Year       km2   straight separation between land and sea. Almost all of
                               the episodically flooded marsh, many embayments and
Zuiderzee              1932       3,600  brackish areas have been transformed into land or lakes.
Lauwerszee             1969       90   The Wadden Sea area became reduced to nearly half of
Lower Elbe             1969–1979    150   its primordial size.
Meldorfer Bucht           1973–1978    48
Nordstrander Bucht         1987       33

                               Saltmarsh works
land occurred. Dike building technology has improved
since then, and land reclamation works in front of      Compared to pristine salt marshes at the mainland coast,
existing dikes have been intensified (Esselink 2000;     modern ones are small, fragmented, truncated at the
Kramer and Rohde 1992). The outer dikeline was shifted    landward side, artificially created and of a simplified
stepwise seaward and the shore became straight (Ta-     structure (Fig. 7). The present saltmarsh area of 200 km2
ble 2; Fig. 6). In the west, the large brackish coastal   along the mainland, is probably only one-tenth of pri-
inlets, Zuiderzee and Lauwerszee, had been cut off in     mordial saltmarsh size. The exact area salt marshes once
1932 and 1969, respectively. In Lower Saxony, several    covered is uncertain because of the gradual transition
small embayments became embanked, while two larger      and intricate mosaic with brackish reed marshes and
coastal inlets remained open (Dollard and Jadebusen).    limnic mires in former times.
Both the lower Weser and Elbe became narrower by         Saltmarsh works in front of mainland dikes have
successive embankments. The Dithmarschen coast has      replaced almost all natural saltmarsh development (Di-
been accretional and here the outer dikeline moved      jkema 1987; Esselink 2000; Probst 1996). Sea walls have
seaward by about 14 km on average. The Eider estuary     stopped any landward extension of salt marshes in re-
became protected by a storm surge barrier in 1972, and    sponse to sea level rise, and have separated salt marshes
has been narrowed by embankments. In Nordfriesland,     from their brackish and limnic hinterland. The habitat
about half of the previously lost marsh became reem-     for brackish water organisms has been eliminated in the
banked until today. A few undiked marsh islands       marsh (Michaelis et al. 1992).
(Halligen) remain scattered over the tidal area. Cause-     Artificial forelands dissipate wave energy and thus
ways have been built to the islands of Sylt (1927) and    protect dikes. Since the 1960s, this became the primary
Rømø (1949), separating the List tidal basin from      function and rational for continued saltmarsh works.
adjacent ones. In the Danish Wadden Sea, diking was     Without this activity, many salt marshes would erode
mainly defensive, confined to existing salt marshes, ex-   away. Natural saltmarsh development is limited to a few
         ˚
cept for the Vida estuary which has been embanked      sheltered embayments (i.e. Jadebusen, Leybucht) and to
completely.                         islands where dune development provides new shelter

Fig. 7 Morphology of a natural
salt marsh (left) and a
reclamation field to create a
foreland fronting a dike (right).
Adopted from Reents (1995)
and schematically drawn from
aerial photographs
                                                           15

(Dijkema 1987; Bunje and Ringot 2003). The halophytic     sand dwelling subsurface deposit feeders with poly-
vegetation is similar between natural and artificial      chaetes and agile peracarid crustaceans adapted to live
marshes, while domestic grazing pressure may decrease     in rippled sandy shoals (Lackschewitz and Reise 1998).
vegetation height from 0.5 to 0.05 m and dominance       The List tidal basin behind the islands of Sylt and Rømø
shifts between grasses from Elymus athericus to Puccin-    lost tidal flat area during the last century. Profiles
ellia maritima (Bakker et al. 2003). Recent policies seek a  adopted a concave shape with a steep upper slope and a
compromise between salt marshes functioning as dike-      gentle tapering towards low tide line (Reise 1998). In
foreland for coastal defence and salt marshes as natural    such a case, species of the nearshore zone become
areas. Artificial draining and lifestock grazing have been   squeezed out.
reduced (Dijkema et al. 2001; CPSL 2001).             Subtidal seagrass beds never recovered from a wast-
  In contrast to American salt marshes, those in the     ing phenomenon in 1932–1934 which affected seagrass
Wadden Sea lacked the tall grasses of the genus Spar-     beds all along northern Atlantic coasts (Den Hartog
tina. However, a hybrid originating from a natural cross    1987; Vergeer et al. 1995). At the two sites with recorded
between S. maritima and the introduced American S.       subtidal beds, large scale damming happened concur-
alterniflora was planted in the 1920s at several sites in the  rently (Hindenburgdamm at Sylt in 1927; Afsluitdijk at
Wadden Sea in order to enhance mud accretion (Reise      Zuiderzee in 1932), which may have contributed to a
1994). The subsequent spread of the new S. anglica       lack of recovery. Increased turbidity or sediment
conspicuously changed the lower salt marsh vegetation.     mobility adjacent to the dams could have been the causal
Where Salicornia stricta and P. maritima once domi-      mechanisms. Intertidal seagrass beds declined in the
nated, this cordgrass took over to form a coherent belt.    southern and central Wadden Sea between the 1970s and
Tussocks often grow as pioneers far seaward in the up-     the early 1990s (De Jonge et al. 1993; Kastler and
per tidal zone. S. anglica is now the major pioneer plant   Michaelis 1999; Philippart and Dijkema 1995). Eutro-
of many salt marshes in the Wadden Sea providing a       phication has been suggested as a main driver (Philip-
new habitat type.                       part 1995; van Katwijk et al. 1999). Increasing
                                turbulence is harmful, too, and may aggravate eutro-
                                phication effects (van Katwijk and Hermus 2000; Schanz
Habitat changes in the tidal area               and Asmus 2003; Schanz et al. 2002).
                                 Overharvesting of natural oyster beds in the Wadden
A consequence of an increasing sea level and a smaller     Sea commenced in the 19th century when railways of-
flood plain is a steeper slope in bottom topography       fered swift transports of oysters to distant inland mar-
(Figs. 3, 5). The closure of coastal inlets such as Zuid-   kets (Mobius 1877). The same happened to the vast
                                     ¨
erzee and Lauwerszee, partial closure of the Eider       oyster beds offshore in the North Sea. The final demise
estuary, size reductions of Dollard, Leybucht, Jadebu-     of profitable beds occurred in the 1920s, and the Wad-
sen, Meldorfer Bucht and Nordfriesland tidal area, as     den Sea population of O. edulis became extinct in the
well as embankments elsewhere along the coast, short-     1950s (Reise et al. 1989). At about that time, Sabellaria-
ened the distance between barrier islands and mainland.    reefs were systematically destroyed with heavy anchor
After closure of the Zuiderzee, the tidal range in the     chains because these reefs interfered with bottom
Marsdiep tidal basin increased up to 0.5 m (De Jonge      trawling for shrimp (Reise and Schubert 1987).
et al. 1993). Generally, high water levels went up while     Mussel fishing proliferated in the 1940s, and subtidal
low water levels hardly changed. A shortened width,      bottom cultures were established since the 1950s
steeper slope and higher high water levels enhanced      (Fig. 3). The area reserved for cultures comprises about
hydrodynamics. Therefore, exemplified at the East Fri-     100 km2 (De Jong et al. 1999). Plots with wild beds and
sian coast, the amount of fine particle deposition is      former oyster beds were raked free of epibenthos and
decreasing and muddy sediments are losing ground        shell gravel. Then seed mussels dredged from wild beds
(Dellwig et al. 2000; Flemming and Nyandwi 1994;        were spread on the plots (Dankers and Zuidema 1995).
Flemming and Bartholoma 1997; Mai and Bartholoma
              ¨              ¨   In the Dutch Wadden Sea, wild and cultured mussel
2000). The fining gradient of sediment particle compo-     stocks are of similar size, and presumably the total
sition became truncated at its landward side by        population has been increased by the cultures. On the
embankments. Embanking muddy coastal inlets and the      other hand, regular harvesting and managing of culture
nearshore mud belt removed fine particles from the       plots is detrimental to most associated organisms,
sediment budget. Calculations for two tidal basins sug-    including birds feeding on mussels (Beukema and Cadee  ´
gest that the amount of mud lost by land reclamation      1996; Camphuysen et al. 2002).
exceeds the amount still present by a factor of 4–5        Mussel cultures and the loss of seagrass beds, oysters
(Delafontaine et al. 2000).                  and Sabellaria-reefs have together caused a conspicuous
  More studies are needed to explore the generality of    habitat change in the subtidal zone (Fig. 3; Lotze 2005).
this trend from muddy flats to sandy flats. Where this      Comparisons of the subtidal occurrence of epibenthos
gradual process occurs, the composition of the benthos     between the 1920–1930s and the 1980–1990s revealed a
is changing from a dominance of mud dwelling surface      decline of sessile species abundances (Buhs and Reise
deposit feeders and diatom grazers to a dominance of      1997; Reise et al. 1989; Reise and Buhs 1999). It has been
16

suggested that intensive bottom trawling for shrimp also         quired to maintain the desired depths. Submersed
contributed to this decline. Recently, a new habitat-          vegetation is mostly gone. The loss of sidearms dimin-
forming oyster (Crassostrea gigas) has been introduced          ished the fauna. However, as pointed out by Witt (2004)
and is spreading throughout the Wadden Sea (Dankers           for the Weser estuary, in spite of a low point diversity
et al. 2004; Diederich et al., unpublished data). These         the total number of benthic species is still rather high.
Pacific oysters are about to take over on intertidal           This is explained by the salinity gradient plus a variety of
mussel beds and also gradually colonise ambient sedi-          substrates at various depth zones which together
ment and subtidal bottoms. The emerging reefs will            amount to a high habitat diversity. Nevertheless, most of
constitute a new type of habitat in the Wadden Sea.           the former filtering and retention capacity of the estu-
                                     aries is lost. The deep canals flush riverine loads right
                                     through into the North Sea.
Lost and converted estuaries
           ˚
Except for Varde A in the north, all of the many small          Wadden islands and adjacent offshore areas
estuaries have been replaced by sluices or barred against
storm surges. Still open but heavily modified are the           Uninhabited small islands and sand bars still undergo
three large estuaries of Elbe, Weser and Ems. Although          considerable changes in size and shape (Bunje and
the transformations in the mainland marsh and tidal           Ringot 2003; Ehlers 1988; CPSL 2001). This was also the
areas described above also apply to estuaries, these are a        case with larger islands. Villages often had to be given
special case (Kausch 1996; Schirmer 1994; Schuchardt           up and were then rebuilt at new locations. Since the last
et al. 1999). Major ports in upstream position have           century, however, attempts to fix island positions against
prompted the conversion of inner estuaries into deep           natural dynamics have become the rule. In the long run,
shipping canals lined by groynes and sidewalls to            this strategy may exceed the costs of moving or
accommodate the ever larger vessels (Table 3). The            rebuilding infrastructures. Dunes were occasionally
largest amounts of dredged sediments are taken from the         breached by storm surges. This gave rise to complex
shipping lanes in the Ems, Jade and Elbe (De Jong et al.         habitat successions. However, in most cases gaps were
1999). Most of the material is removed from harbours           artificially closed to stabilise the situation. Also, almost
and inner estuaries and dumped in outer estuarine sec-          all mobile dunes have been planted with marram grass
tions with deleterious effects on the benthos. This            (Ammophila arenaria) for stabilisation.
dredging commenced at the mid-19th century. The depth            Dune islands were scarcely populated until a massive
of the Elbe estuary has been dredged from 4 to 14 m and         tourist invasion in the 20th century. Since then, beach or
the Weser from 3 to 9 m and 12 m in the outer part. This         cliff erosion has been partly stopped by hard defences, i.e.,
is a threefold increase in depth. As a consequence, the         stone walls, revetments and groynes at Texel, Vlieland,
tidal flow accelerated. At Hamburg, the tidal range in-          Borkum, Norderney, Baltrum, Spiekeroog, Wangerooge,
creased from 1.8 to 3.5 m, and at Bremen from 0.3 to           Amrum, Sylt and Skallingen peninsula. In the last two
4.1 m during the last century. While higher high waters         decades, regular sand replenishments have been employed
require higher dikes, lower low waters caused a more           to compensate losses at the beach. Marsh islands in the
effective draining of sidearms and adjacent marshes. The         northern Wadden Sea have been diked or protected by
enhanced currents, together with wave action from the          stone revetments against the prevailing erosion. The sit-
passing vessels, are aggrevating erosion along the banks.        uation is unstable and requires intensive maintenance.
To prevent this, shores have been petrified; about 60%            The shallow bottoms seaward of barrier islands are
in the Weser between Bremen and Bremerhaven . Up-            subject to disturbance by towed bottom fishing gears,
stream weirs prevent riverine sediment flux into the           aiming primarily at flatfish, shrimp and shellfish. Lasting
estuary, while the deepened canals trigger sediment im-         effects on the habitat are probably subtle because of the
ports from the North Sea. Continuous dredging is re-           high natural dynamics of these sandy shallow bottoms
                                     close to the coast (Kaiser et al. 2003). Sand extractions
Table 3 Areal decrease of habitats in the 20th century in the Elbe    for beach nourishment are taken at specified sites within
estuary between Hamburg and Cuxhaven (from data in Schirmer       a few kilometres offshore, and in the Dutch part from
1994). Note that the intertidal area decreased in spite of an increase  below a depth of À20 m. The overall habitat disturbance
in tidal range, indicating the growing steepness of the slope towards
the deep shipping lane                          by human activities in this zone adjacent to the barrier
                                     islands may be still regarded as low at least until now.
Habitats            Habitats (Km2)              This may change with the prospect of wind energy farms
                1896–1905   1981–1982   Loss%
                                     next to this nearshore zone.

Foreland between        214.3     72.7      66
dike and river (marsh)                         An introduced habitat: artificial hard substrates
Intertidal flats         216.7     192.9     11
Shallow subtidal between    78.2      57.7      26
mean low tide and À2 m
                                     The primordial Wadden Sea was free of rock. There are
                                     only a few boulders and some shingle where pleistocene
                                                            17

deposits came under erosion (i.e., at cliffs of Fohr, Sylt,
                         ¨       atmosphere and oceans, and may cause higher storm
Emmerlev Klit and at great depths in estuaries and tidal    surge probabilities.
inlets). This scarcity of hard substrates changed with the     The syndrome gradually came into existence as sea
beginning of coastal defence and harbour piers. These      level has risen, as the surface level of reclaimed land has
were wooden structures in earlier times, but, since the     gone down, and with a dikeline that was moved further
20th century, breakwaters, groynes, revetments, sea       into the sea. It may soon enter a critical and malignant
walls and harbour walls are built primarily out of stone    stage, and has locked coastal management into a spiral
or concrete. Occasionally, iron or other materials are     of increasing investments for coastal defence, the coastal
employed as buoys, ships, groynes and piers. The fouling    inhabitants into increasing risks from storm surges,
biota generally resemble those of natural rocky shores     while recreational value, richness of living resources and
and biogenic reefs (Buschbaum 2000; Luther 1976; Witt      habitat diversity are in decline.
2004).                               The nine major symptoms of modern coastal archi-
  As a rough estimate, the approximate length of pet-     tecture are:
rified shorelines below mean high tide is about 730 km
                                1. Inverted coastal wedge: ongoing subsidence of em-
(based on maps in Dijkema et al. 1989; and own
                                  banked and reclaimed marsh surfaces caused by
unpublished data). Multiplied by an average width of 3–
                                  drainage and soil compaction, with the oldest marsh
5 m, an artificial reef area of 2–4 km2 of hard bottoms
                                  sinking below sea level, locally aggravated by former
may be assumed, stretching almost continuously
                                  peat mining and recent gas extraction. This entails a
through the entire Wadden Sea. With increasing
                                  risk for inhabitants of the old marsh and requires
hydrodynamics and shore erosion in the tidal basins of
                                  highly reliable sea defences.
the Wadden Sea in the wake of sea level rise and with
                                2. Impermeable dikeline: with a few exceptions, sluices
more infrastructures, these artificial hard substrates are
                                  let freshwater out but no seawater in. Together with
expected to propagate. They account for the widespread
                                  dikes, this effectively eliminates brackish transitional
occurrence of species otherwise rare or absent in a sed-
                                  habitats and blocks migrations of aquatic organisms
imentary environment, ranging from the isopod Ligia
                                  between marine and limnic waters.
oceanica in the supratidal to the kelp Laminaria sac-
                                3. Straight outer dikeline: this is dictated by cost effi-
charina in the subtidal zone.
                                  ciency and eliminates sheltered embayments in which
                                  salt marsh development by natural accretion could
                                  proceed.
Syndromatic coastal degradation                 4. Foreland displaces natural saltmarsh development:
                                  artificially created and maintained dike forelands of
The term syndrome is here used in anology to medicine as      simplified structure tend to be narrow and are iso-
a typical cluster of symptoms indicating an evident mis-      lated from brackish and limnic habitats as well as
development or problematic people–environment inter-        from other saltmarsh areas.
action (Schellnhuber et al. 1997; Ludeke et al. 2004),
                    ¨            5. Sprawl of hard coastal defences: revetments and
applied by Meybeck (2003) to rivers and in this paper to a     groynes to stop shore erosion do not mitigate sedi-
coastal region. In the Wadden Sea, the disposition factor     ment deficiency caused by sea level rise, but initiate
is a low-lying coastal area rich in resources and readily     downstream erosion at undefended shores. The arti-
accessible from land and sea. Therefore this region was      ficial rock habitat often replaces an upper sequence of
populated since it came into existence, and has been        sedimentary habitats.
increasingly converted to human needs since the last      6. Storm surge barriers: prevent flooding of estuarine
1,000 years. The kernel of the underlying functional        habitats during extreme high water levels and thus
pattern of the Wadden Sea syndrome is the growing         delimit space for storm flood waters in the fronting
discrepancy between the rising level of the sea and the      embayments. This discloses the marsh from deposits
position of an artificially fixed and defended shoreline. A     to rise with sea level and outside the barrier habitat
trajectory of non-sustainable coastal development results     erosion is accelerated.
from attempts to hold the line in the face of rapid sea     7. Estuarine conversion into shipping canals: this re-
level rise (Charlier 2003). Coastal defence structures have    quires continuous dredging and causes increased
to be strengthened and natural habitats between land and      sediment mobility, increased tidal range and current
sea become squeezed out or deteriorate. In estuaries, this     speeds. This entails more shore erosion and a lack of
is exacerbated by the maintenance of deep shipping ca-       slackwater habitats at sidearms. Riverine loads are
nals. Embedded in these grand processes are the effects of     flushed right through.
fisheries, introduced species, eutrophication and pollu-     8. Land reclamation, embankments and damming have
tion, all contributing to a decline or change in coastal      delimited the tidal area, increased storm surge levels
habitat diversity. The process of relative sea level rise has   and hydrodynamic energy input per unit area
anthropogenic components in itself, locally and region-      resulting in a loss of fine particulate mud flats.
ally in the modified coastal shape and man-made subsi-      9. Island stabilisations prevent islands from adjusting
dence, globally in greenhouse gas emissions which warm       their position to changing sea level and sediment
18

   supply. Hard defence structures and continuous sand   Wadden Sea which has not yet been strongly modified
   recharges are employed.                 (i.e., by excluding the large estuaries, the former Zuid-
                               erzee, the embanked marshes), is a conceptual retreat to
  These symptoms of the anthropogenic coastal archi-
                               a truncated coastal ecosystem which may not be able to
tecture are interlinked and may interact with other
                               persist or to be maintained in the face of sea level rise.
anthropogenic changes in the Wadden Sea. Effects of
                                 Over the last three decades the attitude in coastal
pollution and eutrophication are partly aggravated by
                               management has been changing. Most of the tidal area is
the lack of extensive coastal and riverine marshes acting
                               under nature protection, and the idea of long-term
as filter and retention areas for allochthonous sub-
                               coastal sustainability is literally gaining ground (CPSL
stances. On the other hand, these substances are now
                               2001). Low summer dikes become perforated to convert
flushed through a simplified and a hydrographically
                               pastures back into salt marshes. Land reclamation
more dynamic coastal zone. The effects are diverted to
                               works proceed only in front of those dikes which lack
other regions downstream. The global dissemination of
                               sufficient foreland. Grazing pressure and drainage has
allochthonous coastal species has resulted in the intro-
                               been reduced to restore more natural saltmarsh vegeta-
duction of the cordgrass S. anglica and the Pacific oyster
                               tion. In some reclaimed polders, artificial lagoons are
C. gigas, both of which have established new habitat
                               managed to sustain semi-marine conditions. On barrier
types displacing native ones. The former may extend salt
                               islands, eroding beaches are recharged with sand taken
marshes, and reefs of the latter may stabilise sediments
                               from offshore, instead of employing more and more
to some extent. The coastal fishery has removed high-
                               hard defences.
diversity habitats (oyster beds and Sabellaria-reefs), and
                                 However, there is still a long way to go until coastal
added low-diversity mussel culture plots. Besides many
                               sustainability can be achieved in a sense, that the ability of
local deviations from general trends and some com-
                               future generations to live at and enjoy the coast is not
pensating processes, a spiral of habitat loss and degra-
                               compromised. The recognition of the long temporal scales
dation has been the prevailing course of change in
                               over which coastal morphodynamics unfold is still in its
historical time.
                               infancy with regard to coastal policies. The strategy to
                               cope with accelerating sea level rise continues to rely on
                               on-line defences rather than adopting a strategy of coastal
Conclusions and perspectives                 realignment as is being attempted in Britain (Burbridge
                               and Pethick 2003; French 2001). In particular, harbour
This historical approach highlights that massive inter-   locations and nuclear power plants in estuaries, and urban
ventions in the Wadden Sea coastal system over the last   developments on barrier islands, are still regarded as
1,000 years have increasingly distracted coastal mor-    permanent and to be defended against all risks at all times.
phology and habitats from their functions to dissipate    On the other hand, there are options worthy of consid-
energy from waves and tides, to retain imports of      eration which could develop the Wadden Sea region more
deposits and substances, and to provide enough space     into the direction of habitat sustainability:
and dynamic heterogeneity for the persistence of biodi-
versity (Fig. 5). The intensity of change has been highest  1. The method of sand replenishments to balance the
in the marsh and estuaries, moderate in the tidal area     sediment hunger of sea level rise could be extended to
and relatively minor on the barrier islands. The offshore    sheltered shores, where lost transitional habitats of
area is the least modified, but will probably have to      the upper shore could be restored with sand deposits
accommodate wind energy farms in the near future.        in the form of islets, bars and spits (Reise 2003; Reise
  Human effects on extent, diversity and composition      and Lackschewitz 2003).
of habitats were confined to the marsh until about 1200    2. The inverted wedge-shaped profile of the embanked
AD. Then, major storm surges enlarged the tidal area by     marsh could be employed to convert polders which
flooding the reclaimed and subsiding land. Most of these     are below mean sea level into reservoirs for flood
losses were subsequently regained (Table 1). Since the     waters during storm surges by using sluices and
mid-17th century, embankments became permanent.         drainage canals in reversed direction (Reise 1996).
During the last century particularly, large tidal areas     Where necessary, houses have to be placed upon
were embanked or dammed (Table 2), brackish transi-       mounds as was practised before diking. Such reser-
tions vanished and estuaries were turned into shipping     voirs could alleviate pressure on the outer dikeline,
canals (Table 3). Inhabited parts of barrier islands were    and also improve the touristic reputation of the
stabilised. Shoreline petrifications sprawled and habitat    marsh.
dynamics at the upper shore almost ceased. Fisheries     3. Estuaries are bound to function as shipping canals
and eutrophication contributed to the loss of high-       with ever greater depth as long as ports remain in
diversity habitats.                       upstream positions. Shifting harbour facilities to a
  The Wadden Sea region not only became physically       central platform in the North Sea from where small
smaller by the separation of landward habitats from the     feeder ships commute to ports along rivers could al-
sea. Confining the trilateral management plan, the bio-     low estuaries to regain former retention properties
sphere reserves and national parks to that part of the     and habitat diversity. These options need to be ad-
                                                                   19

  dressed by interdisciplinary research to find cost-          Marine science frontiers of Europe. Springer, Berlin Heidelberg
  effective solutions with multiple benefits, and to be          New York, pp 217–228
                                   Buschbaum C (2000) Siedlungsmuster und Wechselbeziehungen
  developed together with stakeholders. The coastal           von Seepocken (Cirripedia) on mussel beds (Mytilus edulis L.)
  syndrome diagnosed above may be viewed on a glo-           in the Wadden Sea. Ber Polar-Meeresforsch, Alfred-Wegener-
  bal scale. Similar coastal wetland degradations have         Institut fur Polar- und Meeresforschungen 408, Bremerhaven
                                          ¨
  occurred elsewhere in Europe and overseas, and all         Camphuysen CJ, Berrevoets CM, Cremers HJWM, Dekinga A,
                                     Dekker R, Ens BJ, van der Have TM, Kats RKH, Kuiken T,
  are subject to continued sea level rise with a limited        Leopold MF, van der Meer J, Piersma T (2002) Mass mortality
  potential for natural transgressions to unfold. The          of common eiders (Somateria mollissima) in the Dutch Wadden
  habitat history of the Wadden Sea offers lessons            Sea, winter 1999/2000: starvation in a commercially exploited
  which could help to avoid repeating failures of the          wetland of international importance. Biol Conserv 106:303–317
  past. Concerning the habitat history of the Wadden         Charlier RH (2003) Hold the sea back—is it sustainable? Retro-
                                     spective and projection. J Coast Res 19:875–883
  Sea, there are still gaps in knowledge which in par-        CPSL (2001) Final report of the trilateral working group on coastal
  ticular could benefit from paleoecological research          protection and sea level rise. Wadden Sea ecosystem 13. Com-
  and attempts to model habitat proportions.              mon Wadden Sea Secretariat, Wilhelmshaven, Germany
                                   Dankers N, Zuidema DR (1995) The role of the mussel (Mytilus
                                     edulis L.) and mussel culture in the Dutch Wadden Sea. Estu-
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