Geoscience Reference
In-Depth Information
A. Study Sites—The Seven Food Webs
1. Afon Hirnant
The study was carried out at three sites within the Afon Hirnant, in North
Wales, UK (52
52
0
N03
34
0
W). Mean annual discharge varied between 2.08
and 7.26 m
3
/s and pH from 5.5 to 7 (see
Figueroa, 2007 and Woodward et al.,
2010
for full details).
Invertebrates were collected using a Hess sampler (sampling area:
0.028 m
2
; mesh aperture 80
m), with 15 sample-units taken randomly at
each site and season, providing a total of 180 samples for the whole sampling
period. The invertebrate fauna was preserved immediately in 100% ethanol
and sorted in the laboratory. Individuals were identified to species wherever
possible using published taxonomic keys (listed in
Woodward et al., 2010
).
Gut contents analysis was performed to establish feeding interactions by
removing predators' foreguts, which were mounted in Euparal medium and
examined at 400
m
magnification. Prey were identified from reference slides of
taxa collected in the Afon Hirnant, after
Schmid (1993) and Schmid-Araya
et al. (2002)
.
Linear dimensions (e.g. body length) of every predator and individual prey
item found within every predator's foregut were measured and subsequently
converted to body mass values using published regression equations for each
taxon (listed in
Woodward et al., 2010
). However, if the prey items were
highly digested, mean body length of individuals from the same family
(Ephemeroptera, Plecoptera or Trichoptera) in the environment was used.
Chironomidae (Diptera) body lengths were estimated from previously estab-
lished species-specific regressions between head capsule width and body
length (
Figueroa, 2007
).
2. Broadstone Stream
Broadstone Stream is a tributary of the River Medway in south-east England
(see
Hildrew, 2009
for a detailed site description). The stream was acid (pH
4.7-6.6) and fishless at the time of sampling in 1996-1997, as it had been since
at least the early 1970s prior to a more recent invasion by brown trout in the
2000s (
Layer et al., 2011
). The macroinvertebrate food web contains about
31 common species, including six dominant predators and a suite of detriti-
vorous stoneflies and chironomids (
Woodward et al., 2005b
).
Thirty randomly dispersed benthic Surber sample-units (25 cm
25 cm
quadrat; mesh aperture 330
m) were taken every two months between June
1996 and April 1997 and preserved in 5% formalin. All invertebrate taxa were
described to species where possible, and the few that could not be
m
