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In-Depth Information
hypogean taxa recorded also increased significantly
at the same time (Figure 13.1 c). Studies of benthic
faunal use of the HZ during low flows are now
under way in the River Lathkill and River Glen
in Central England; in common with studies in
other countries they indicate high spatial and
temporal variability in benthic faunal use of the
HZ (Stubbington
et al
., 2011). Future research in
this area needs to examine how vertical hydraulic
gradients (upwelling versus downwelling water)
and substratum characteristics influence benthic
faunal use of the HZ as a potential refugium.
more abundant (Dunscombe
et al
., unpublished
data). The hypogean amphipod
Niphargus
sp. only
occurred in parts of southern England consistent
with suggestions that the current distribution
of obligate groundwater organisms still reflect
the influence of past glaciations (Strayer, 1994;
Robertson
et al
., 2009; but see Proudlove
et al
.,
2003). However, further research is required to
unravel the significance both of geology and glacial
history on HZ community composition.
Nitrate pollution
Nitrate is the most common groundwater pollutant
in the UK and processes in the HZ are likely to
be important moderators of the flux of nitrate
between surface water and groundwater (Smith,
2007). However, the potential influence of nitrates
on hyporheic assemblages is less well known.
Studies in England have examined the hyporheic
assemblages of lowland chalk rivers which differed
in their degree of nitrate pollution: the catchments
of the rivers Piddle and Frome (Dorset) were
moderately polluted with nitrates (20-30 mg L
−
1
),
whereas the rivers Lee and Rib (London) were
highly polluted with nitrates (50-60 mg L
−
1
).
The Dorset rivers have been severely affected by
agricultural activities and dairy farming, with the
main source of nitrates coming from the use of
fertilizers. The London rivers are largely urban
and mainly influenced by sewage water treatment
works and septic tanks. Sites were sampled during
periods of high and low groundwater levels using
a Bou-Rouch pump, with piezometers inserted
to a depth of 20 cm and 50 cm into the river
bed. The abundance of hyporheic invertebrates
was low probably because of the small interstitial
space available and the influence of fine sediment
(clogging) (Pacioglu
et al
., unpublished data). Total
invertebrate abundance was significantly higher in
the shallow compared with the deep hyporheic
and did not differ significantly between the head
and tails of riffles, in agreement with Davy-Bowker
et al
. (2006). Results also indicate that hyporheic
assemblages may be used as bioindicators of nitrate
pollution and may be sensitive to clogging of
interstitial spaces by sediment (Pacioglu
et al
.,
unpublished data). Further research is needed to
Hydrogeological influences
on HZ ecology
The potential influence of hydrogeology on the
physical and hydrochemical characteristics of the
HZ and the distribution of fauna utilizing the HZ
habitat has been increasingly recognized (Hancock
et al
., 2005). Research in England is examining
the influence of hydrogeology on meiofaunal
assemblages living in the hyporheic zone in
three geological areas strongly associated with
groundwater aquifers. Thirty-two sites distributed
evenly across four geographical areas: Permo-
triassic sandstone (Devon); Cretaceous chalk
(Dorset); fluviokarst (Derbyshire) and glaciokarst
(North Yorkshire) have been sampled during
periods of high and low groundwater levels.
Bou-Rouch sampling, with piezometers inserted
to a depth of 20 cm and 50 cm into the river
bed, indicated that chalk and sandstone sites had
significantly lower abundances at both 20 cm and
50 cm compared with karst sites. The abundance of
fauna decreased significantly from 20 cm to 50 cm
at all sites in both upwelling and downwelling
areas except in the glaciokarst. Here, significantly
higher abundances of Chironomidae, Harpacticoida
and Oligochaeta were found in the deeper
samples in both upwelling and downwelling areas
(Dunscombe
et al
., in prep.). In this study,
changes in species assemblage amongst crustaceans
occurred between sites in northern and southern
England although the underlying causes have
yet to be examined. In northern England sites
were dominated by
Bryocamptus
sp. while in the
south
Nitocra
sp.
and
Canthocamptus
sp.
were
