Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
δ 15 N
16
15
14
13
12
11
10
0 10 20 30 40
Cephalothorax length (mm)
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
232
δ 13 C
-14
-15
-16
-17
-18
-19
0 10 20 30 40
Cephalothorax length (mm)
Figure 6 (a): Relation between δ 15 N muscle and size length of M. rugosa (n = 22) (b): Relation between
δ 13 C muscle and size length of M. rugosa (n = 22)
15 N
16
14
12
10
0 10 20 30 40
Length (cm)
13 C
-14
-15
-16
-17
-18
-19
0 10 20 30 40
Length (cm)
Figure 7 (a): Relation between δ 15 N muscle and size length of M. poutassou (black rhombus) (n = 10),
T. minutus (black triangle) (n = 12) and T. trachurus (empty square) (n = 13) (b): Relation between δ 13 C
muscle and size length of M poutassou (n = 10), T. minutus (n = 12) and T. trachurus (n = 13)
Discussion
Trophic level 1 was attributed to POM, which was considered the primary producer. No
significant differences appeared in the stable isotopic composition of surface and bottom POM. This
similarity would indicate that during the September period POM reached the benthic ecosystem
without isotopic changes and concurs with the observations of Hobson et al. (1995). Our POM δ 15 N
values (mean = 4.18, SD = 0.95) are in agreement with those reported by Fry (1988) for the George
Bank, Hobson et al. (1995) for living phytoplankton of New Polynya, Kang et al. (1999), Riera et al.
(1999) for the coastal ecosystem of the Bay of Biscay, and by Savoye (2001) for the Bay of Brest.
These similar values obtained in different ecosystems at different periods of the year support the
notion that POM δ 15 N isotopic values remain quite stable in any given ecosystem throughout the year,
particularly in offshore areas as the Great Mud Bank which is subjected to fewer terrestrial inputs than
coastal areas. Consequently the difference in the sampling periods for POM and fauna should not
induce error interpretation on the organization of the trophic system. Goericke et al. (1994) showed
that the δ 13 C values of POM in surface waters of temperate marine ecosystems (between 45 and 50° in
latitude) range between -25 and -18‰. On the continental shelf of the Bay of Biscay, Fontugne and
Jouanneau (1987) measured δ 13 C POM values ranging between -24‰ and -21‰. Our results are close
to these values (mean δ 13 C POM = -23.3‰ SD = 0.95). As in the case of δ 15 N, there was no significant
difference between surface and bottom δ 13 C POM values, indicating that sedimentation occurs without
qualitative transformation.