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invertebrates and vertebrates and which therefore support the high species
richness observed (e.g. see photographs in
Figure 1
A and B). Most of the
Weddell Sea food web consumers are benthic invertebrates and fish species,
with four trophic entities (phytodetritus, sediment, bacteria and particulate
organic matter (POM)), forming important basal resources (
Hall and
Raffaelli, 1991; Warren, 1989
).
We compiled a species list that encompasses 489 consumer and resource
species from the high Antarctic Weddell Sea (over 500 publications were
analyzed and standardized: for a full description of the methods used and a
full list of these publications see
Jacob, 2005
). This marine food web, which
includes all the food web data available for the high Antarctic Weddell Sea
collected since 1983, is one of the most highly resolved marine food webs
documented to date, although it is a summary web that ignores seasonal
changes. Diet composition of each species was observed from a combination
of field observations and stomach content analyses performed between 2001
and 2004 (see
Jacob, 2005
). Expert ecologists specialized in different species
assisted with identification and sample provision. A list of taxonomic keys
used can be found in
Jacob (2005)
. In some species, that is, benthic grazers
and suspension feeders, poor taxonomic resolution of prey items would have
biased estimates. Here, we used information obtained in the laboratory
about these species' size, behaviour and stable isotope signatures (
Brose
et al., 2005a; Jacob et al., 2005
) to deduce their feeding habits. Stable isotope
analysis was performed for
3100 individuals in total and
included mainly sponges, fishes and benthic invertebrates. In brief, stable
isotope (
d
13
C and
d
15
N) signatures serve as proxies of the trophic distance of
an organism from the primary food source of the corresponding food chain
(
Fry, 1988
).
d
13
C signatures are commonly used as carbon source tracers,
whereas
d
15
N values are a useful tool for detecting the trophic position and
therefore the trophic hierarchy of the system (
Post, 2002
). Samples were
lyophilisated for 24 h in a Finn-Aqua Lyovac GT2E and then ground to a
fine powder. Each sample was acidified to remove CaCO
3
in accordance with
Fry (1988) and Jacob et al. (2005)
. Stable isotope analysis and concentration
measurements of nitrogen and carbon were performed simultaneously with a
THERMO/Finnigan MAT Delta plus isotope ratio mass spectrometer, cou-
pled to a THERMO NA2500 elemental analyzer via a THERMO/Finnigan
Conflo II-interface. Stable isotope ratios are given in the conventional delta
notation (d
13
C; d
15
N) relative to atmospheric nitrogen and PDB (PeeDee
Belemnite standard).
For the food web construction, following the approach of
Martinez
(1991)
, a directional feeding link was assigned to any pair of species A and
B whenever an investigator reports that A consumes B. Species were not
divided further into larvae, juveniles or adults but treated as 'adults': conse-
quently, with the data used here, we cannot address ontogenetic diet shifts.
600 species and
