Worcestershire Record No. 24 April 2008 pp. 23-24
The Bow Brook rises in the Himbleton area of Worcestershire and runs for some 35 km before reaching the River Avon at Defford. It drains the Jurassic clay solid geology and also the permeable Quaternary sediments derived from the activity of the River Avon throughout that time.
If one accepts that the impacts of modern agricultural regimes on fluvial biotas began in 1960, it is reasonable to assume that the evident subsequent impoverishment of those biotas is due in large part to that. That process includes the radical canalisation and dredging of watercourses with its dramatic impacts on entire systems and processes.
There is plenty of available evidence for the structure and composition of fossil aquatic mollusc faunas in Britain extending over several hundred thousand years of time. This has been gained from study of fluvial sediments long since abandoned by their rivers, but it is much more difficult to gather knowledge of recent faunal change in riverine systems due to the difficulties of dating submerged sediments.
On 3 March 1974 a major dredging operation took place on the Bow Brook in the Defford area of Worcestershire (SO9243, 14 m O.D. the site is given as Defford but the Bow Brook forms the physical boundary of Defford and Birlingham parishes). Although this practice has been repeated since, the 1974 dredging was more aggressive than anything preceding it. Features removed included ancient rock-built fords, and at one spot quantities of intensely black peaty silt, the organic basal deposits of the brook, were exposed. A bulk sample of 1.9 kg was collected and passed through successional sieves, the molluscs and ostracods being retained for identification (Fig. 1).
The age of the sample
Fortunately, it proved possible to date the sediment fairly precisely by reference to:
a) fragments of distinctive forms of glass bottles in use between 1780 and 1860
b) the presence of four very large examples, 6.5 mm long, of the antipodean gastropod Potamopyrgus antipodarum, which was unknown in Britain before 1852. This is significant because the presence of so few examples of what is now a numerous species, in such a diverse fauna, is indicative of their earliest colonisation of the region, probably about 1870
c) the presence of the orb mussel Musculium transversum, a Nearctic species first observed in Britain in 1856, and which subsequently spread rapidly but is now rare and little-known in the region
The sample represents bed sediments likely to have developed over some time, in an episode of stability, up to at least 1890. Changes in bed sedimentology at this location after that are likely to have resulted from a variety of anthropogenic impacts, initially minor, but accelerating after 1960. To forestall any notions that the sample sediments are modern and that the four P. antipodarum represent a species subject to negative selection pressure (e.g. absence of exposed pebbles) in the Bow Brook now, it is observed that the extant fauna of the Bow Brook has P. antipodarum in plenty, and that the composition of the mollusc fauna in it is now different and relatively impoverished.
Forty two species of molluscs were identified, represented by 2169 individuals, the bivalve count being given as the number of valves divided by two. The shells are well preserved, with many juvenile shells, and the assemblage is autochthonous. The two species of the genus Bithynia are lumped together for juvenile shells <0.6mm in length, and their opercula, which were fewer in number than their shells, have been excluded from the counts. Two species of ostracod were identified. Only one shell of the large bivalve, Unio pictorum, was noted, that being a juvenile nepionic shell. The molluscs represent both the aquatic habitat and terrestrial edge habitat which supported Vertigo pygmaea, Vallonia pulchella, Trichia hispida sensu lato and Zonitoides nitidus. The bank sides were evidently largely stable and well vegetated; Galba truncatula, an amphibious marginal species was present in good numbers. Vertigo pygmaea can be mildly hygrophilous, occurring for example in Typha litter, and there is no evidence of either inwashed xeric species or shade tolerant terrestrial species, such as Clausilia bidentata (Ström) and Aegopinella nitidula (Draparnaud).
The aquatic mollusc fauna clearly demonstrates the quality of the fluvial habitat over 150 years ago, with the occurrence in numbers of fastidious species, some of which are now scarce or absent in Worcestershire. These include Bithynia leachii, which accepts pristine habitats with plenty of vegetation in optimal condition. It is further attested to by high numbers of the planorbids Gyraulus albus and Gyraulus crista, and by the presence of Valvata cristata and Stagnicola palustris in numbers. The impression is of a well-vegetated, well-oxygenated, clean, clear system, with an organic bed, free of negative impacts, and with a slow flow. The two ostracods Candona neglecta and Eucypris ornata confirm these conditions. The system had probably remained more or less stable for much of post-glacial time. There is a single example of the planorbid Gyraulus laevis which, although sometimes appearing in pioneer communities, seems to abandon sites that have passed through a number of seral successions. This species could usefully be researched in the region, especially in old gravel pits, since there are few regional records of it. Gyraulus laevis and the fastidious Bithynia leachii and Pisidium moitessierianum occurred in the Bronze Age River Avon at Pensham, Worcestershire (Whitehead, 2006) so they earn their place in the primary post-glacial fluvial mollusc fauna of the regional drainage. Both are likely to be sensitive to anthropo-factors leading to light-reduction or increased phytoplankton following eutrophication.
In 1820 the adjacent landscape is likely to have been much as it now. Plant macrofossils recovered from the sample indicate that willow Salix sp., and hawthorn Crataegus sp., grew along the valley as they still do on alluvium, and that there was an abundance of Creeping Buttercup Ranunculus repens L. on the nearby floodplain. Oenanthe fistulosa L. grew along the bank sides with Apium nodiflorum (L.) Lag. The Nineteenth Century brook contained Potamogeton sp. and Yellow Water Lily Nuphar lutea (L.) Smith, along with abundant Bulrush Schoenoplectus lacustris (L.) Palla.
The extant biota
I returned to the find site at Defford on 30 April 1981 and although I did not conduct a meticulous survey, I could find no evidence of some of the Nineteenth Century species in the modern mollusc fauna. The biologically impoverished watercourse supported Physa fontinalis, Galba truncatula, Radix peregra, Gyraulus albus and Potamopyrgus antipodarum, a somewhat featureless association of widespread species that might occur in any minor lowland watercourse in England today. Sphaerium corneum persists at the present time and Vertigo pygmaea remains numerous amongst Phragmites litter on upstream alluvium protected from adjacent agricultural practice. Changes in the aquatic vegetation have been marked. There has been a particular reduction in Ranunculus subgenus Batrachium in the past 40 years (P.F. Whitehead, pers. obs.), to the point of obliteration. Potamogeton is now absent and Schoenoplectus lacustris also no longer occurs upstream near the find-site.
Molluscan evidence indicates that the Bow Brook supported an exceptional assemblage of aquatic molluscs as recently as 150 years ago and that the diverse fauna included what are now temporal relicts, some of which occurred in high numbers. Sometime after 1880, human interference, both close by and from a distance, began to impact negatively on the biota and habitat, and progressively reduced the quality of the aquatic environment to what now remains. It should not be assumed that aquatic biotas of this richness survived for so long in the region wherever they once existed, or even that they existed widely or uniformly (Shotton, 1972). Particular factors, such as, for example, the damming effect of the pre-industrial, pre-navigable, nearby River Avon on the hydrosphere of its tributaries, needs to be considered.
Dr Katherine Courtledge, during her time at the University of Sheffield, kindly helped with difficult Pisidium spp., some of which were also checked by Dr M. P. Kerney. Dr Eric Robinson identified the ostracods.
|SHOTTON, F. W. 1972. A comparison of modern and Bronze Age mollusc faunas from the Warwickshire-Worcestershire Avon. Proceedings of the Coventry and District Natural History and Scientific Society, 4:6:173-182.|
WHITEHEAD, P. F. 2006. The alluvial archaeobiota of the Worcestershire River Avon. Worcestershire Record, 20:34-42.
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