Taken From The Knepp Castle Estate Baseline Ecological Survey




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Taken From
The Knepp Castle Estate Baseline Ecological Survey

Theresa E. Greenaway
2005
3.5 Wetland Mollusca
3.5.1 Survey Brief
A wetland mollusc survey was required to contribute to the baseline data required by the River Restoration Centre and the Environment Agency prior to river restoration work to be carried out on the River Adur corridor. Dr Martin Willing was commissioned to carry out a survey of aquatic and wetland Mollusca in the river and on the floodplain and banks of the River Adur (Area B, Map 1).
3.5.2 Methodology

Fieldwork was undertaken on 23rd October 2005 along the length of the River Adur corridor between approximately TQ 164207 (southeast) and TQ 149217 (northwest). Mollusca were sampled at 7 sites; 5 aquatic in the River Adur and two terrestrial in damp ditches / hollows on the river floodplain. In addition, one extra (un-commissioned) site at the head of Knepp Mill Pond was sampled.


Survey sites: The survey focussed on taking aquatic samples at regular intervals along the River Adur and terrestrial samples from the only two pockets of wetland located. Fieldwork was undertaken on 23rd October 2005 with bulk sample processing, specimen extraction and identification taking a further day immediately following this field visit.
Aquatic sampling: River waters were sampled using a standard (EA approved) FBA-pattern extendable-handled freshwater sampling net with a 0.5mm mesh, supplied by GB Nets. This was used to sweep areas of water near to the surface, in mid-water and close to the bottom. A total of about three - four 'sweeps' were taken at three different locations at each site separated by about 10m. The samples removed were amalgamated before processing. This procedure allowed approximately quantitative comparisons of mollusc populations to be made between sites. In order to recover all small species and juveniles, 'bulk samples' of aquatic vegetation and ditch sediments were removed from each site for later laboratory sieve extraction. Before samples were bagged, larger snails (e.g. Planorbarius corneus, adult Lymnaea peregra) were identified, counted and returned to the sample site. Sample sites were digitally photographed. Laboratory processing involved washing vegetation and sediments through a sieve-tier to retain all molluscan remains > 0.5mm. Residues were examined on gridded white trays and smaller samples inspected microscopically using a x5 - x56 binocular microscope. A selection of Pisidium spp. from all sites were disarticulated in bleach and shell hinge characteristics studied microscopically to confirm identifications.
Species recovered from aquatic samples have been recorded in abundance classes thus:

R = rare (1 – 2 specimens recovered)

F = frequent (3 - 30 specimens recovered)

A = abundant (> 30 specimens recovered)



X = dead shell only found
Terrestrial sampling: Areas of fen and river bank grassland were surveyed by the field examination of vegetation litter and soil surfaces together with the field sieving of vegetation litter and mosses to inspect remains >0.5mm. Bulk samples of moss/vegetation litter were removed from a selection of sites in each of the survey locations in order to record the smallest species, which can easily be under-recorded in the field. The bulk samples were air dried in muslin bags for two weeks and then dry sieved to remove all remains >0.5mm. Residues were examined on gridded white trays and smaller samples inspected microscopically using a x5 - x56 stereoscopic binocular microscope.
3.5.4 Results

A total of 23 aquatic and 7 terrestrial species were recorded (Table 3.5.a). No Red Data species or Biodiversity Steering Group (BAP) Priority Species were found on the survey. Brief descriptions of sites together with GPS derived national grid references are given in Table 3.5.c.


Table 3.5.a.: Samples – River Adur corridor – Knepp Castle Estate
Site number

1 (A*)

2 (A*)

3 (A*)

4 (A*)

5 (T*)

6 (A*)

7 (T*)
Aquatic species (A*)




Valvata piscinalis

F

R

R

F




X




Valvata cristata

F

R




R










Bithynia tentaculata

F

F




F




F




Physella acuta

F

F




F










Lymnaea stagnalis









X




X




Lymnaea peregra

R

R




F




R




Lymnaea palustris / fuscus (agg)

R










A







Planorbis planorbis










X










Planorbis carinatus

R



















Anisus vortex

R













Bathyomphalus contortus

R













Gyraulus crista
















Gyraulus albus

R

R

X

R


X



Hippeutis complanatus

F

F

X













Planorbarius corneus
















F




Acroloxus lacustris

R

R















Spaerium corneum


F

F

X

F




F



Musculium lacustre


R

R
















Pisidium henslowanum




F




F




R




Pisidium milium
















F




Pisidium subtruncatum

F

F




F




F



Pisidium nitidum


F

R

X

R


A


Pisidium personatum













F


F


Terrestrial species (T*)





















Carychium minimum














F




F

Succinea putris



















R

Vertigo pygmaea



















R

Zonitoides nitidus













F




R

Nesovitrea hammonis



















R

Deroceras reticulatum













R







Monacha cantiana













R







*naming follows Kerney 1999

Table 3.5.b Samples – fen / carr (head of Kneppmill Pond – Knepp Castle Estate)



Site number

8 (T*)
Aquatic species (A*)




Lymnaea palustris / fuscus (agg)

R

Pisidium personatum


F

Terrestrial species (T*)



Carychium minimum


F

Succinea putris

F

Vertigo antivertigo

F

Zonitoides nitidus

F

Nesovitrea hammonis

R

Aegopinella nitidula

F

Oxychilus alliarius

R

Euconulus fulvus (agg)

X

Table 3.5.c. Habitat descriptions.



Site:

Brief Habitat Description (Freshwater & aquatic)

Grid Reference.

(central position of site)

1

Steep banked vegetated predominately with nettles and Phalaris arundinacea ; very slow flowing; little aquatic vegetation except filamentous algae; water sampled from margins to central channel (approx 3m depth); river bed predominately hard clay with a small amount of over-lying silt.

TQ 16026 20746

2

Steep bank to south, more gently sloping to north, apart from some Lemna spp. no aquatic vegetation; bordered by Scirpus lacustris; water sampled from margins to central channel (approx 1m depth)

TQ 15538 20964

3

Steep banks immediately below Environment Agency Weir, little aquatic vegetation except filamentous algae; water sampled from margins to central channel (approx 0.7m depth), ); river bed predominantly soft clay with no over-lying silt.

TQ 15389 20971

4

Steep banks leading into relatively shallow, slow flowing water > 1m depth; little aquatic vegetation, soft clay bottom to channel.

TQ 15217 21235

5

Damp Juncus lined ditch on flood plain adjacent to site 4.

TQ 15217 21235

6

Steep banks leading into slow flowing water up to 1.2m depth; soft clay bottom overlain with organic silt; water surface partly covered with Lemna spp, channel bordered by Phalaris arundinacea and Sparganum sp.

TQ 15013 21793

7

Damp Carex and Juncus lined ditch lined ditches on flood plain close to Site 2.

TQ 15622 20947

8

Alder / willow carr and associated marginal fen (Carex spp, Typha, Sparganium sp)

TQ 15909 22065



3.5.6. Discussion

The River Adur supports small numbers of a relatively few common mollusc species found frequently elsewhere in Sussex. The improved and semi-improved grassland that borders the River Adur contains only a few remnant pockets of wetland vegetation. These support a low diversity community of common wetland and catholic species.


Freshwater: A total of 23 aquatic molluscan species were found during the survey. All of these species are common and widespread elsewhere in Sussex and southeast England (Kerney 1999, personal observations at many freshwater sites in Sussex and Hampshire). In overall terms the freshwater molluscan community present in the survey area is one of low diversity and common taxa dominated by the five species; Lymnaea peregra, Physella acuta, Bithynia tentaculata, Sphaerium corneum and Pisidium nitidum. These five species are all tolerant of a wide range of different freshwater habitats including stagnant and slightly polluted ones. P. acuta is an introduced species (Anderson 2003). Surprisingly no living representatives of the large unionid freshwater bivalves were located on the survey although a small shell fragment believed to be from such a mussel was found in sediments at site 5. The Adur in the survey corridor appears to occupy an artificially deepened channel. The lack of extensive shallow marginal areas may have reduced molluscan diversity as might eutrophication, which may also account for the relative lack of macrophytic vegetation.
Terrestrial faunas: Field searches and bulk litter samples (of rush / sedge debris) from a two sites reveals an impoverished wetland fauna, which may have been more extensive in the past. All species recovered are common and widespread elsewhere in Sussex and southeast England. Lymnaea palustris (agg.) Succinea putris, Carychium minimum, Zonitoides nitidus and Pisidium personatum are typical wetland species. The remaining species found at the two terrestrial sites are all rather ‘catholic’, being found in a wide variety of both open habitats.
Two bags of moss, sedge and alder leaf litter were collected at the head of Kneppmill Pond. These produced a wetland fauna typical of rather neutral / acidic conditions. Most of the species were also recorded from the small wetland ‘relict’ sites adjacent to the River Adur. One species of note is Vertigo antivertigo. This species, which is local but widespread in the county, has been used as one of a suite of species typical of old wetland sites (Kerney & Stubbs 1980).

References

ANDERSON, R. 2003. Physella (Costatella) acuta Draparnaud in Britain and Ireland – Its taxonomy, origins and relationships to other introduced Physidae. Journal of Conchology

38: 7 – 21.


KERNEY, M.P. 1999. Atlas of the Land and Freshwater Molluscs of Britain and Ireland. Colchester: Harley Books.
KERNEY, M.P. & STUBBS, A, 1980. The conservation of snails, slugs and freshwater mussels. Nature Conservancy Council. Shrewsbury.





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