110
A. Mazzola et al.
Discussion and Conclusion
Several studies have focused on identifying the
origin of the organic matter entering food webs
and quantifying the fraction actually available to
consumers (Fry and Sherr 1984; Mann 1988;
Michener and Shell 1994). Seagrass ecosystems
(Posidonia oceanica) are highly productive,
although Posidonia produced organic carbon is
highly refractory (Pollard and Kogure 1993). It
has been recently demonstrated that the main
transfer route for seagrass carbon into the benthic food web is through bacterial decomposition
(Pollard and Kogure 1993). Seagrass meadows
function as "detrital traps", gathering detrital
carbon from all sources (Dauby 1989).
In the study area, five main primary producers were identified. Posidonia oceanica and
Cymodocea nodosa appear to be slightly
enriched compared to the literature data
(McMillan et al.1980; Dauby 1989; Jennings et al.
1997). Such an enrichment could be the result of
higher temperatures and PAR (Wieneke and
Fisher 1990) due to the shallowness of the studied area compared to other investigated systems
(Dauby 1989; Tufano 1991; Jennings et al. 1997),
In shallow environments, sediments accumulate
organic carbon from different sources, including
the overlying function of water column.
Exchange between sediments and the overlying
water column are a function of water movement
intensity, which depends on the pulsing of wind
and tidal energy (Sara et al.1999). Thus, variability in the sedimentary and particulate organic
matter content and composition can be more
strongly affected by these physical phenomena
than by seasonality of the biological life cycle of
each primary producer. In this way, most of the
sedimentary carbon can be resuspended. to settle again rapidly within a short time. Such
processes can lead to continuous changes in
organic carbon composition, affecting the overall availability of organic matter. In the Stagnone
di Marsala a coexistence of two different zones
has been demonstrated. These two systems (the
northern and the southern basin) are constrained by different hydrodynamic regimes
depending upon wind exposure (Mazzola et al.
1999; Pusceddu et al. 1999; Sara et al.1999). It has
been estimated that the occurrence of resuspension phenomena as a function of depth. effective
fetch and wind speed (Smaal and Haas 1997), is
about double in the northern basin of the
Stagnone (Sara et al. 1999) in which sedimentation is prevalent.
Based on Dauby model calculations (Dauby
1989), our isotopic data indicate that in the
northern basin, sediments are mainly affected
by macro algae carbon (46%), while carbon from
Cymodocea. microphytobenthos and phytoplankton represented about 14.5% each one,
whilst Cymodocea detritus contributed for about
10%. In the water column, phytoplankton (60%)
and microphytobenthos (30%) represent the
main sources of suspended organic carbon,
while the contribution of carbon from
Cymodocea, macro algae and detritus is negligible. If we correlate the monthly occurrence of
resuspension at this site (Sara et al. 1999), it can
be inferred that organic carbon derived from
Cymodocea and macro algae accumulated in sediments (resuspension factor YS Cymodocea r. =
0.65; P<0.05j n = 12; resuspension factor vs
macro algae " = 0.75; P<0.05; n = 12). Carbon
from these two sources is not transferred to the
water column as suggested by the absence of a
correlation with the occurrence of resuspension
events. In contrast, when resuspension intensity
increases carbon from microphytobenthos and
Cymodocea detritus is largely transferred to the
water column. However, the absence of a correlation between the isotopic signature of these
two producers and resuspension could suggest
that much of this carbon is probably transported outside.
In Fig. 6a and b, the monthly carbon contributions are reported in comparison with the
monthly trend of sedimentary and particulate
biopolymeric organic carbon. A significant correlation between sedimentary BPC and microphytobenthic carbon was observed (S-BPC vs
microphytobenthos r, = 0.8; P
Between March and July, when the microphytobenthic signature in sedimentary carbon was
negligible (Fig. 6a), the biopolymeric organic
carbon was quite low, while from August to
February, when the microphytobenthic signature
was present, S-BPC increased. Although no significant correlation was shown with S-BPC,
enriched Cymodocea detritus also seems to play
a definite role.
In the southern basin, the frequency of
resuspension is quite low and tidal movements
(Sara et al.1999) continuously mix the water col-
A. Mazzola et al.
Discussion and Conclusion
Several studies have focused on identifying the
origin of the organic matter entering food webs
and quantifying the fraction actually available to
consumers (Fry and Sherr 1984; Mann 1988;
Michener and Shell 1994). Seagrass ecosystems
(Posidonia oceanica) are highly productive,
although Posidonia produced organic carbon is
highly refractory (Pollard and Kogure 1993). It
has been recently demonstrated that the main
transfer route for seagrass carbon into the benthic food web is through bacterial decomposition
(Pollard and Kogure 1993). Seagrass meadows
function as "detrital traps", gathering detrital
carbon from all sources (Dauby 1989).
In the study area, five main primary producers were identified. Posidonia oceanica and
Cymodocea nodosa appear to be slightly
enriched compared to the literature data
(McMillan et al.1980; Dauby 1989; Jennings et al.
1997). Such an enrichment could be the result of
higher temperatures and PAR (Wieneke and
Fisher 1990) due to the shallowness of the studied area compared to other investigated systems
(Dauby 1989; Tufano 1991; Jennings et al. 1997),
In shallow environments, sediments accumulate
organic carbon from different sources, including
the overlying function of water column.
Exchange between sediments and the overlying
water column are a function of water movement
intensity, which depends on the pulsing of wind
and tidal energy (Sara et al.1999). Thus, variability in the sedimentary and particulate organic
matter content and composition can be more
strongly affected by these physical phenomena
than by seasonality of the biological life cycle of
each primary producer. In this way, most of the
sedimentary carbon can be resuspended. to settle again rapidly within a short time. Such
processes can lead to continuous changes in
organic carbon composition, affecting the overall availability of organic matter. In the Stagnone
di Marsala a coexistence of two different zones
has been demonstrated. These two systems (the
northern and the southern basin) are constrained by different hydrodynamic regimes
depending upon wind exposure (Mazzola et al.
1999; Pusceddu et al. 1999; Sara et al.1999). It has
been estimated that the occurrence of resuspension phenomena as a function of depth. effective
fetch and wind speed (Smaal and Haas 1997), is
about double in the northern basin of the
Stagnone (Sara et al. 1999) in which sedimentation is prevalent.
Based on Dauby model calculations (Dauby
1989), our isotopic data indicate that in the
northern basin, sediments are mainly affected
by macro algae carbon (46%), while carbon from
Cymodocea. microphytobenthos and phytoplankton represented about 14.5% each one,
whilst Cymodocea detritus contributed for about
10%. In the water column, phytoplankton (60%)
and microphytobenthos (30%) represent the
main sources of suspended organic carbon,
while the contribution of carbon from
Cymodocea, macro algae and detritus is negligible. If we correlate the monthly occurrence of
resuspension at this site (Sara et al. 1999), it can
be inferred that organic carbon derived from
Cymodocea and macro algae accumulated in sediments (resuspension factor YS Cymodocea r. =
0.65; P<0.05j n = 12; resuspension factor vs
macro algae " = 0.75; P<0.05; n = 12). Carbon
from these two sources is not transferred to the
water column as suggested by the absence of a
correlation with the occurrence of resuspension
events. In contrast, when resuspension intensity
increases carbon from microphytobenthos and
Cymodocea detritus is largely transferred to the
water column. However, the absence of a correlation between the isotopic signature of these
two producers and resuspension could suggest
that much of this carbon is probably transported outside.
In Fig. 6a and b, the monthly carbon contributions are reported in comparison with the
monthly trend of sedimentary and particulate
biopolymeric organic carbon. A significant correlation between sedimentary BPC and microphytobenthic carbon was observed (S-BPC vs
microphytobenthos r, = 0.8; P
Between March and July, when the microphytobenthic signature in sedimentary carbon was
negligible (Fig. 6a), the biopolymeric organic
carbon was quite low, while from August to
February, when the microphytobenthic signature
was present, S-BPC increased. Although no significant correlation was shown with S-BPC,
enriched Cymodocea detritus also seems to play
a definite role.
In the southern basin, the frequency of
resuspension is quite low and tidal movements
(Sara et al.1999) continuously mix the water col-