increase in sedimentation effects. Indeed,
moving from the northern area (stations 1-3) to
the mid-basin (stations 4-6), total suspended
matter concentrations decrease approximately
two-fold. Such a decrease was also evident for
the POC concentrations, which decreased
approximately 2.3-tiroes moving from station 1
and approaching station 7. Here. the presence of
the Posidonia oceanica reef acts as a mechanical
barrier to the lateral drifting of seston, giving
rise to an increase in the sinking velocities of
particles (Pusceddu et a1. 1997a). To this regard,
other studies showed that seagrass meadows
strongly affect the sinking velocity of particles
(Pirc and Wollemweber 1988). In addition, in
this area, corresponding to stations Band C, SBPC concentrations in the sediments increase
up to 1.7-times with respect to concentrations at
station 1. Moving from station A to stations B
and C, the contribution of carbohydrates to SBPC increases, although not significantly. from
47% to about 52% against a decrease of the protein contribution from 45% to 37%. The carbohydrate content of sediments in this sector
(stations B and C) is probably caused by the
release of Posidonia fragments rich in structural
carbohydrates (Danovaro 1996). Since the gross
biochemical composition of Posidonia oceanica
is highly refractory (Lawrence et al. 1989), a
large fraction of organic matter accumulated in
the sediments of the southern area of the study
site may be not directly available for benthic
consumers. Kenworthy and Thayer (1984)
showed by in situ experiments, that 50-60% of
the organic carbon of the seagrass leaves was
lost within 170 days. Thus, although Posidonia
reefs provide an important input of organic
detritus to the sediments, only a small fraction
(Le. proteins, soluble carbohydrates and free
amino acids; Pirc and Wollemweber 1988) might
be potentially available to benthic consumers.
We are aware that further direct investigation of
particle sedimentation and sediment OM diagenesis rates is needed to clarify the depicted picture of the Marsala lagoon. However, results of
the present study suggest that quality. quantity
and spatial distribution of suspended and sediment organic matter in the study area are drastically controlled by the dynamic balance and
spatial repartition of sediment resuspension
and sedimentation processes, to a larger extent
than seasonal changes in temperature and salinity do.
Pelagic-benthic Coupling in a Semi-enclosed Marine System
443
Concluding Remarks on the Food Availability
of Organic Matter
Results of this study indicate that both quality
and quantity of particulate and sediment organic
matter in the Marsala lagoon are controlled by a
wide and complex array of variables. Although
the amount of primary organic matter is quite
low, as the low particulate and sediment chlorophyll-a concentrations suggest, relatively large
amounts of organic detritus are present throughout the year in the whole basin.
The biochemical composition of POM
(Particulate Organic Matter) revealed the dominance of large amounts of readily available compounds (e.g. proteins). Nevertheless, this pool of
suspended organic matter is highly diluted in a
dominant inorganic fraction, as the POM:TSM
ratio values suggest (annual average, 19%; range,
2.1-49.5%). The POM:TSM ratio, used as a food
index (Navarro et al.1993),is quite low if compared with that found in a Mediterranean shallow
coastal lagoon (average 50%; range 30-100%.
Pusceddu et al. 1996) and in an estuarine lagoon
(30-100%, Navarro et al. 1993). Thus, in terms of
food availability, we may conclude that a suspension feeder in the Marsala lagoon would mter a
sea water volume about 3-times higher than in
other coastal lagoons to reach the same amount
of particulate food. This could be considered a
partial confrrmation in the scarce filter feeding
mollusks community inhabiting the sediments
of the Marsala lagoon (Chemello and Riggio,
personal communication).
As in other oligotrophic environments (in
terms of phytopigment concentrations), in the
Marsala lagoon we might expect a large fraction
of the autochthonous organic matter production
to be rapidly canalised throughout planktonic
and sedimentary microbial loops. However, as
indicated by the value of the LOM (Labile
Organic Matter); TOM ratio (on average, about
14%. Pusceddu et al. 1999). most of the sediment
OM is unaccounted by labile organic compounds. This uncoupling between the large
amounts of sediment OM and its low nutritional
value suggests that this particular environment
probably behaves as a detrital "trap" in which the
large gross primary production of macroalgae
and seagrasses, not directly available to benthic
consumers. tends to accumulate. However,
further studies (now in progress) will have to
deal with dynamics and secondary production of
moving from the northern area (stations 1-3) to
the mid-basin (stations 4-6), total suspended
matter concentrations decrease approximately
two-fold. Such a decrease was also evident for
the POC concentrations, which decreased
approximately 2.3-tiroes moving from station 1
and approaching station 7. Here. the presence of
the Posidonia oceanica reef acts as a mechanical
barrier to the lateral drifting of seston, giving
rise to an increase in the sinking velocities of
particles (Pusceddu et a1. 1997a). To this regard,
other studies showed that seagrass meadows
strongly affect the sinking velocity of particles
(Pirc and Wollemweber 1988). In addition, in
this area, corresponding to stations Band C, SBPC concentrations in the sediments increase
up to 1.7-times with respect to concentrations at
station 1. Moving from station A to stations B
and C, the contribution of carbohydrates to SBPC increases, although not significantly. from
47% to about 52% against a decrease of the protein contribution from 45% to 37%. The carbohydrate content of sediments in this sector
(stations B and C) is probably caused by the
release of Posidonia fragments rich in structural
carbohydrates (Danovaro 1996). Since the gross
biochemical composition of Posidonia oceanica
is highly refractory (Lawrence et al. 1989), a
large fraction of organic matter accumulated in
the sediments of the southern area of the study
site may be not directly available for benthic
consumers. Kenworthy and Thayer (1984)
showed by in situ experiments, that 50-60% of
the organic carbon of the seagrass leaves was
lost within 170 days. Thus, although Posidonia
reefs provide an important input of organic
detritus to the sediments, only a small fraction
(Le. proteins, soluble carbohydrates and free
amino acids; Pirc and Wollemweber 1988) might
be potentially available to benthic consumers.
We are aware that further direct investigation of
particle sedimentation and sediment OM diagenesis rates is needed to clarify the depicted picture of the Marsala lagoon. However, results of
the present study suggest that quality. quantity
and spatial distribution of suspended and sediment organic matter in the study area are drastically controlled by the dynamic balance and
spatial repartition of sediment resuspension
and sedimentation processes, to a larger extent
than seasonal changes in temperature and salinity do.
Pelagic-benthic Coupling in a Semi-enclosed Marine System
443
Concluding Remarks on the Food Availability
of Organic Matter
Results of this study indicate that both quality
and quantity of particulate and sediment organic
matter in the Marsala lagoon are controlled by a
wide and complex array of variables. Although
the amount of primary organic matter is quite
low, as the low particulate and sediment chlorophyll-a concentrations suggest, relatively large
amounts of organic detritus are present throughout the year in the whole basin.
The biochemical composition of POM
(Particulate Organic Matter) revealed the dominance of large amounts of readily available compounds (e.g. proteins). Nevertheless, this pool of
suspended organic matter is highly diluted in a
dominant inorganic fraction, as the POM:TSM
ratio values suggest (annual average, 19%; range,
2.1-49.5%). The POM:TSM ratio, used as a food
index (Navarro et al.1993),is quite low if compared with that found in a Mediterranean shallow
coastal lagoon (average 50%; range 30-100%.
Pusceddu et al. 1996) and in an estuarine lagoon
(30-100%, Navarro et al. 1993). Thus, in terms of
food availability, we may conclude that a suspension feeder in the Marsala lagoon would mter a
sea water volume about 3-times higher than in
other coastal lagoons to reach the same amount
of particulate food. This could be considered a
partial confrrmation in the scarce filter feeding
mollusks community inhabiting the sediments
of the Marsala lagoon (Chemello and Riggio,
personal communication).
As in other oligotrophic environments (in
terms of phytopigment concentrations), in the
Marsala lagoon we might expect a large fraction
of the autochthonous organic matter production
to be rapidly canalised throughout planktonic
and sedimentary microbial loops. However, as
indicated by the value of the LOM (Labile
Organic Matter); TOM ratio (on average, about
14%. Pusceddu et al. 1999). most of the sediment
OM is unaccounted by labile organic compounds. This uncoupling between the large
amounts of sediment OM and its low nutritional
value suggests that this particular environment
probably behaves as a detrital "trap" in which the
large gross primary production of macroalgae
and seagrasses, not directly available to benthic
consumers. tends to accumulate. However,
further studies (now in progress) will have to
deal with dynamics and secondary production of