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Development of Former Dredging
Holes
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Elisabeth Götting, Niedersächsisches Landesamt
für Ökologie - Forschungsstelle Küste,
NLÖ - FSK, Arbeitsgruppe Wilhelmshaven, FRG
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INTRODUCTION
The raising and reinforcing of sea dikes to carry out the
higher demands of the "General Plan Coastal Defense" is
actually planned or already going on in large parts of the
Lower Saxon coast. For this purpose large amounts of clay
soil are necessary. In some areas, this clay was
traditionally taken from the salt marshes which, under
economical aspects, were considered less valuable than
embanked marsh land. The result of these former activities
are many dredging holes (so-called clay pits) within the
salt marshes, which, today, are refilled with sediment and
recolonized with plants and animals. In the Jadebusen area,
the still discernible clay pits have been laid out from 1955
to 1986 (mostly in the sixties). They have a 9 % share in
the total salt marsh area of the western Jadebusen, in the
southern and eastern parts even 14 %.
During the actual proceedings, it became a decisive question
if the necessary clay should further be taken from salt
marshes. For the proper authorities, a lack of information
about the ongoing processes in former dredging holes made
decisions more difficult, so that research was necessary. A
question to answer before any decision was taken was how
clay pits get refilled and recolonized and what is their
ecological value within the salt marsh ecosystem.
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INVESTIGATIONS
Since 1990, NLÖ - FSK has been investigating former
dredging holes in the Jadebusen salt marshes. For an
overview over different development stages, several clay
pits at the age of 2 to 35 years in various parts of the
salt marshes (different soil level, hydromorphological
conditions) as well as old salt
marshes with different agricultural treatment were selected
for comparison.
The development of the clay pits was investigated regarding
the following parameters:
- soil level, several physical and chemical soil
parameters;
- vegetation;
- benthic macrofauna;
- activity density of elements of the terrestrial
evertebrate fauna: carabids and amphipoda, additionally
in 1990/91 spiders, bugs and cicads;
- density of breeding and resting birds.
Some results have already been described in several
reports.
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MORPHOLOGY
After their connection to the Wadden Sea, most clay pits
grow up to a high eulitoral level within 2 - 3 years. With
rising level, the process is slowing down because of less
inundation and decreasing sediment input. Within 10 years
large parts of a clay pit can reach the level of a low salt
marsh, and 25 years after clay removal, the area can almost
reach its former level. The duration of this process differs
locally and depends on several factors, mainly
hydrodynamical conditions, sediment input and soil level of
the surroundings.
The morphological structures of former clay pits contrast
sharply to the anthropic ditch structure of most of the old
salt marshes. They are characterized by naturally developed,
branched tideway systems (Fig. 1). In the deeper
regions between the tideways mud flats are often
developing.
The soil texture of clay pits is finer than in old salt
marshes because the sediment input consists mostly of
fine-grained particles. Soil classes determined in former
dredging holes were clay or silty clay, in old salt marshes,
the soil often contains more sand. Bulk density is clearly
lower than in salt marshes with agricultural use. Soil
development proceeds relatively fast. In 25 year old clay
pits a well-defined A-horizon of 6 to 44 cm depth could be
observed which is more than in most old utilized salt
marshes.
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Fig. 1: Clay pits and surrounding old salt
marshes (Augustgroden, eastern Jadebusen). Aerial picture:
GTP, Stade, 1990.
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VEGETATION
A first sparse vegetation of Salicornia sp. can appear
within 2 - 3 years, and after 5 years often a dense
Salicornia- and Spartina-vegetation has spread out. 10 years
after clay removal, large parts of a clay pit are covered by
a lower Puccinellietum maritimae typicum spread out. 10
years after clay removal, large parts of a clay pit are
covered by a lower Puccinellietum maritimae typicum with a
high share of Spartina anglica. On the banks of tideways,
the typical plant community of the tidal drift zone is an
Atriplicetum littoralis. Around the mud flats which are
often covered by a Salicornia- and Spartina- vegetation, a
pioneer zone of Aster tripolium can mark the transition to
the Puccinellietum.
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Most salt marsh types, up to a high salt marsh, can exist in
a 25 - 30 year old clay pit, dependent on location and size,
the soil level of the surroundings, inundation frequency and
material input. Within adjacent higher salt marshes, parts
of a clay pit can also develop up to this level, mainly
higher banks of tideways. However, investigations have shown
that, concerning the number of species, a former dredging
hole stays below adjacent salt marshes for decades, and some
plant species, e.g. Armeria maritima, Limonium vulgare,
Atriplex portulacoides and Artemisia maritima, will not, or
rarely, recolonize a clay pit within 30 years. Some possible
reasons for this are the finer soil texture and - for some
species - the lack of agricultural use.
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EVERTEBRATE FAUNA
During the first years after clay removal, the typical
benthic fauna of eulitoral mud flats is found in the
refilled area. Frequent species are the molluscs Macoma
balthica and Hydrobia ulvae, polychaets like Nereis
diversicolor and Heteromastus filiformis, oligochaets
(Tubificoides benedii, Tubifex costatus) and crustacea. With
rising soil level and immigration of plants, this community
is replaced by terrestrial species and can only survive in
tideways and mud flats.
The first carabid community to recolonize a clay pit - here
described exemplary for the epigaeic fauna - consists of few
typical inhabitants of lower salt marshes, halobiontic and
adapted to inundation. Dominant species are Pogonus chalceus
and Dicheirotrichus gustavi. They are accompanied by the
halobiontic Bembidion normannum and the halophilic Bembidion
minimum, a species that avoids very deep regions. Several
other salt marsh and inland species are found occasionally,
so that the typical species inventory of lower salt marshes
can already be found after 12 years. Total abundance can be
relatively high, but species numbers are still low.
About 25 years after clay removal, the species density of a
clay pit is comparable to the surroundings.
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Bembidion minimum is the dominant species in those parts
grown up to upper salt marshes, accompanied by species of
the lower salt marsh as well as by Bembidion aeneum, a
halophilic species which is not resistant against submersion
and therefore inhabits upper salt marshes and summer
polders. The total number of species and specimens is
usually higher than in low salt marshes.
The investigations have shown that the carabid fauna of the
clay pits develops similar to that of a salt marsh without
agricultural use, and on the level of a middle to upper salt
marsh, the total abundance is much higher than in utilized
areas. An indicator species for soil quality is the
terrestrial amphipod Orchestia gammarellus which lives in
ground burrows and is severely impaired by agricultural use.
In older clay pits, its density is much higher than in mowed
areas and comparable to old salt marshes without any
utilization.
The phytophagous insect fauna immigrates into a clay pit
corresponding to the vegetation. Abundances can be high
after several years, when the host plants of specialized
salt marsh species (e.g. Aster tripolium) cover large areas
of the former dredging hole.
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AVIFAUNA
Already after a few years, a refilled dredging hole is
accepted by several birds as a valuable resting and feeding
area. Breeding occurs when the soil level gives a certain
shelter from tidal inundation which would destroy nests.
Habitat diversity and natural tideways for leading the
juveniles provide an attracting area for breeding birds, so
that territory density 10 - 12 years after clay removal is
usually higher than in adjacent old salt marshes.
The most frequent species are redshank (Tringa totanus),
skylark (Alauda arvensis), meadow pipit (Anthus pratensis),
blue-headed wagtail (Motacilla flava) and reed bunting
(Emberiza schoeniclus).
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In several clay pits in the southern and eastern Jadebusen,
avocets (Recurvirostra avosetta) are breeding near mud
flats.
Occasionally Oystercatchers (Haematopus ostralegus) were
registered breeding here. However, black-tailed godwit
(Limosa limosa) and lapwing (Vanellus vanellus), which
prefer areas with agricultural treatment, will hardly accept
clay pits for breeding.
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CONCLUSIONS
The investigations have shown that a clay pit, for a few
years, is withdrawn from its function as a biosphere for
plants and animals, but in the long term, valuable salt
marshes can develop again. The diverse clay pit structures
provide a large number of different habitat types for
vegetation and fauna, and in most older clay pits even
relatively high abundances of several specialized salt marsh
evertebrates and breeding birds are found. Important factors
for the development of clay pits are the location, the size
and the hydrodynamical conditions. The investigations have
shown that parts of a clay pit can reach their former level
and salt marsh type within 25 - 30 years, relatively
independent of the absolute soil level of the surroundings.
Nevertheless, the dynamics in most clay pits is still higher
than in old salt marshes.
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To assess the relative value of former clay pits within the
salt marsh ecosystem, the state and development potential of
the old salt marshes are fundamental reference data, which
must be evaluated as locally quite different.
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Authors address:
NLÖ Forschungsstelle
Küste, FSK
Arbeitsgruppe Wilhelmshaven
Fliegerdeich 1
D - 26382 Wilhelmshaven
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