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266 Mar Ecol Prog Ser 313: 261–270, 2006

enrichment gradient. The total contribution of fish regard to location, values were significantly different
farm waste to SOM also increased with increasing dis- among all locations for all 3 distance categories. For
tance from cages (pellet + ejection = 17%) to more dis- δ15NPOM (Fig. 5c), each location was significantly differ-
tant sites (pellet + ejection = 33%). ent from the others within each distance (ANOVA, p <
0.05), whereas δ15NSOM values at Capo D’Orlando Bay
HYDRO Control — Egadi Islands (CORL) and Gulf of Casetellammare (CAST) were sig-
nificantly different from each other (ANOVA, p < 0.05)
In the Egadi Islands (control location), δ13CPOM and only at the 0 and 1000 m sites, respectively (Fig. 5d).
δ15NPOM values showed significant differences
(ANOVA, p < 0.05) among the 3 distance categories, All systems together
except for N between the 500 and 1000 m distances
(Fig. 4a). In contrast, the isotopic composition of SOM The cluster analysis (Fig. 6) performed on the POM
was almost constant among distance categories, and mixing model percent data (converted to a similarity ma-
did not show significant differences (ANOVA, p > 0.05) trix using the Bray-Curtis similarity coefficient) isolated
for either δ13C or δ15N values (Fig. 4b). The mixing 3 clusters: a first cluster with control sites from Egadi Is-
model run with only 1 mixture and 2 sources indicated lands (EI) grouping close together; a second cluster
that the overall terrigenous contribution ranged on (hereafter referred to as cluster POM A) grouping Menfi
average between 23 and 41%, whereas the autochtho- Bay (MEN) HYDRO 2 (500 m), MEN HYDRO 2 (1000 m),
nous contribution (phytoplankton and sand microflora CORL HYDRO 3 (0 m) and CORL HYDRO 3 (500 m) with
for POM and SOM, respectively) ranged on average an average Bray Curtis similarity of 87%; and a third
between 59 and 69%. group (hereafter referred as cluster POM B) grouping
CAST HYDRO 1 (0 m), CAST HYDRO 1 (500 m), CAST
Comparison of δ13C and δ15N in POM and SOM HYDRO 1 (1000 m), MEN HYDRO 2 (0 m) and MEN
within distance categories HYDRO 3 (1000 m). The highest level of average Bray-
Curtis dissimilarity was recorded for comparisons
The comparison of δ13CPOM values from 0 m sites between EI control group sites and sites belonging to
among locations showed no significant differences POM B group. The POM A group exhibited an inter-
(ANOVA, p > 0.05), while values from the 500 and mediate position (Fig. 6). In all comparisons, the sources
1000 m sites were considerably more variable than contributing most to dissimilarities were phytoplankton
those from the 0 m sites, in particular because of the contribution (PHY) and pelletted food (PEL) (Fig. 6).
Gulf of Castellammore values (Fig. 5a). The origin of
δ13CSOM was markedly different (ANOVA, p < 0.05) The cluster analysis (Fig. 7) performed on SOM mix-
(Fig. 5b); while no clear trends were apparent with ing model percent data identified 3 main Bray-Curtis
groups. The first group (hereafter referred to as CTRL)
represented all control sites. The second group (here-

0m 500 m 1000 m (a)

C = –22.7 ± 0.2 C = –21.7 ± 0.6 C = –22.5 ± 0.1
N = 5.9 ± 0.3 N = 5.0 ± 0.2 N = 5.3 ± 0.1

C p < 0.05; N p < 0.05 C p < 0.05; N p > 0.05

PHY = 68.5 PHY = 65-5 PHY =63.5
TER = 31.5 TER = 34-5 TER = 36.5
PEL = ------ PEL = ------ PEL = ------
EJE = ------ EJE = ------ EJE = ------

C p < 0.05; N p < 0.05

0m 500 m 1000 m (b) Fig. 4. Mean isotopic values (‰)
± SE of carbon (δ13C) and nitrogen
C = –18.7 ± 0.2 C = –18.3 ± 0.1 C = –17.9 ± 0.5 (δ15N) measured at each distance at
Egadi Islands. Significant differ-
N = 4.2 ± 0.2 N = 4.4 ± 0.1 N = 4.6 ± 0.1 ences between distance categories
(0 vs. 500 m; 500 vs. 1000 m; 0 vs.
C p > 0.05; N p > 0.05 C p > 0.05; N p > 0.05 1000 m) indicated by p < 0.05
(ANOVA). Percent contribution of
SM = 59.0 SM = 62.5 SM = 67.0 each organic matter source also
TER = 41.0 TER = 37.5 TER = 23.0 reported for (a) POM and (b) SOM:
PEL = ------ PEL = ------ PEL = ------
EJE = ------ EJE = ------ EJE = ------ see Figs. 2 & 3 for abbreviations

C p > 0.05; N p > 0.05