332
L. Rossi and G. Mancinelli
effects of epifaunal macrodetritivores on phosphorus dynamics in coastal marine systems in
relation to both macrodetritus processing and
superficial layer turbation. The trophic activity
of this group of organisms has been demonstrated to affect plant detritus breakdown, microbial colonisation and decomposition rates
(Fenchel 1970; Harrison 1977; Robertson and
Mann 1980).
In our study, orthophosphate 32p was used in
simplified water-sediment systems to analyse
both phosphorus distribution among compartments and trophic linkages between detritivores
and other compartments. Two main questions
were addressed: (1) What is the role of macrodetritus in phosphorus fluxes across the water-sediment interface~ (2) Does the trophic interaction
detritus-macrodetritivores cause significant
variation in sediment 32p adsorption and release?
Materials and Methods
Samples were collected from a coastal site in
Northern Adriatic Sea (45°08.96' N, 12°23.21' E;
depth: 26 m), located 4.28 nautical miles offshore. Previous investigations (e.g. Mancinelli et
ala 1998) showed that the clay - silt sediments of
the area were characterised by the occurrence of
coarse plant detritus of allochthonous origins
(fragments of the phanerogams Zostera Spa and
Cymodocea nodosa Ascherson). The detritivorous amphipod Gammarus insensibilis Stock
characterised the benthic epifauna all year
round, reaching maximal densities in summer
(ca. 1400 individuals m- 2 ).
In July 1995, sediment cores (5 replicates)
were collected using a Reineck box-corer (sampling area: 170 cm 2 ). The sampling device permitted to retrieve undisturbed cores; on board,
the oxidised layer (uppermost 5 cm) was cut and
placed in plastic containers. After returning to
the laboratory, sediment cores were wet-sieved
(0.5 mm mesh size) to remove coarse particles
and macrofauna; living Gammarus insensibilis
specimens were simultaneously collected.
The freshly sieved sediment was homogenised and frozen at -BO°C for 12 h. The procedure eliminated phytobenthos and meiofauna.
without inactivation of the microbial component
(Kristensen and Blackburn 1987). After complete
defrosting at room temperature, sediment
aliquots (30 ± 0.005 g wet weight, 19.66 ± 0.04 g
dry weight) were transferred into 18 sterilised
microcosms (Pyrex jars, 500 ml total volume, 73.9
cm 2 basal area) containing 220 ± 0.5 m1 of pre-filtered (Corningl\ll cellulose acetate filters, 0.45 pm
pore size) sea water. Each microcosm was provided with an aeration system (flow was set under
sediment resuspension level) and acclimated in a
controlled temperature room for 60 h at 22°C.
Pre-weighted aliquots of Cymodocea nodosa
fragments ("leaf packs»: 200 ± 5 mg dry wt) were
conditioned in an aerated suspension of fresh
sediment for 72 h at 22 °e. At the end of the conditioning phase, Cymodocea leaf packs were carefully introduced in nine microcosms (1 leaf
pack/microcosm; "sediment + detritus" condition); the remaining microcosms were left undisturbed ("sediment only" condition). After 12 h,
three treatments were set for each condition: (1)
"low density" animal treatment: 4 dimensionally
similar Gammarus insensibilis specimens were
added (corresponding to a spatial density of 541
individuals· m- 2 ); (2) "high density" animal treatment: 12 amphipods were added (1624 individuals· m- 2 ); (3) defaunated control. For each treatment three replicated microcosms were provided.
Immediately after amphipod introduction,S
ml of a sterile solution containing 16.85 ± 0.37
mCi of orthophosphate 32p were added to each
microcosm. An identical amount of radiotracer
was added to microcosms (3 replicates) containing 200 ml of sterile seawater and 10 amphipods
to assess 32p physical adsorption.
During the experiment, filtered (pre-weighted
Sartorius® cellulose acetate filters, 0.2 pm pore
size) water samples (50 pI, 3 replicas/microcosm)
were taken on an exponential time scale until the
192 th hour. Filters were oven dried (72 h, 60°C),
and weighed.
The experiment was terminated after 192 h.
Amphipods were collected and enumerated.
They were consequently washed, oven-dried at
60°C for 72 h, weighed, and analysed for 32p content. Detritus subsamples (4 replicas/microcosm) were taken from "sediment + detritus"
microcosms and subjected to the same procedure; sediment cores (i.d. 0.5 em) were collected
(4 replicate/microcosm; coring operations in
"sediment + detritus" microcosms resulted
infeasible due to the large detritus particles
interspersed in the sediment), frozen (48 h,
-30°C), and cut into three layers (0-3, 3-6, 6-9
mm), For each layer, sediment was separated
from interstitial water by squeezing under N2 on
L. Rossi and G. Mancinelli
effects of epifaunal macrodetritivores on phosphorus dynamics in coastal marine systems in
relation to both macrodetritus processing and
superficial layer turbation. The trophic activity
of this group of organisms has been demonstrated to affect plant detritus breakdown, microbial colonisation and decomposition rates
(Fenchel 1970; Harrison 1977; Robertson and
Mann 1980).
In our study, orthophosphate 32p was used in
simplified water-sediment systems to analyse
both phosphorus distribution among compartments and trophic linkages between detritivores
and other compartments. Two main questions
were addressed: (1) What is the role of macrodetritus in phosphorus fluxes across the water-sediment interface~ (2) Does the trophic interaction
detritus-macrodetritivores cause significant
variation in sediment 32p adsorption and release?
Materials and Methods
Samples were collected from a coastal site in
Northern Adriatic Sea (45°08.96' N, 12°23.21' E;
depth: 26 m), located 4.28 nautical miles offshore. Previous investigations (e.g. Mancinelli et
ala 1998) showed that the clay - silt sediments of
the area were characterised by the occurrence of
coarse plant detritus of allochthonous origins
(fragments of the phanerogams Zostera Spa and
Cymodocea nodosa Ascherson). The detritivorous amphipod Gammarus insensibilis Stock
characterised the benthic epifauna all year
round, reaching maximal densities in summer
(ca. 1400 individuals m- 2 ).
In July 1995, sediment cores (5 replicates)
were collected using a Reineck box-corer (sampling area: 170 cm 2 ). The sampling device permitted to retrieve undisturbed cores; on board,
the oxidised layer (uppermost 5 cm) was cut and
placed in plastic containers. After returning to
the laboratory, sediment cores were wet-sieved
(0.5 mm mesh size) to remove coarse particles
and macrofauna; living Gammarus insensibilis
specimens were simultaneously collected.
The freshly sieved sediment was homogenised and frozen at -BO°C for 12 h. The procedure eliminated phytobenthos and meiofauna.
without inactivation of the microbial component
(Kristensen and Blackburn 1987). After complete
defrosting at room temperature, sediment
aliquots (30 ± 0.005 g wet weight, 19.66 ± 0.04 g
dry weight) were transferred into 18 sterilised
microcosms (Pyrex jars, 500 ml total volume, 73.9
cm 2 basal area) containing 220 ± 0.5 m1 of pre-filtered (Corningl\ll cellulose acetate filters, 0.45 pm
pore size) sea water. Each microcosm was provided with an aeration system (flow was set under
sediment resuspension level) and acclimated in a
controlled temperature room for 60 h at 22°C.
Pre-weighted aliquots of Cymodocea nodosa
fragments ("leaf packs»: 200 ± 5 mg dry wt) were
conditioned in an aerated suspension of fresh
sediment for 72 h at 22 °e. At the end of the conditioning phase, Cymodocea leaf packs were carefully introduced in nine microcosms (1 leaf
pack/microcosm; "sediment + detritus" condition); the remaining microcosms were left undisturbed ("sediment only" condition). After 12 h,
three treatments were set for each condition: (1)
"low density" animal treatment: 4 dimensionally
similar Gammarus insensibilis specimens were
added (corresponding to a spatial density of 541
individuals· m- 2 ); (2) "high density" animal treatment: 12 amphipods were added (1624 individuals· m- 2 ); (3) defaunated control. For each treatment three replicated microcosms were provided.
Immediately after amphipod introduction,S
ml of a sterile solution containing 16.85 ± 0.37
mCi of orthophosphate 32p were added to each
microcosm. An identical amount of radiotracer
was added to microcosms (3 replicates) containing 200 ml of sterile seawater and 10 amphipods
to assess 32p physical adsorption.
During the experiment, filtered (pre-weighted
Sartorius® cellulose acetate filters, 0.2 pm pore
size) water samples (50 pI, 3 replicas/microcosm)
were taken on an exponential time scale until the
192 th hour. Filters were oven dried (72 h, 60°C),
and weighed.
The experiment was terminated after 192 h.
Amphipods were collected and enumerated.
They were consequently washed, oven-dried at
60°C for 72 h, weighed, and analysed for 32p content. Detritus subsamples (4 replicas/microcosm) were taken from "sediment + detritus"
microcosms and subjected to the same procedure; sediment cores (i.d. 0.5 em) were collected
(4 replicate/microcosm; coring operations in
"sediment + detritus" microcosms resulted
infeasible due to the large detritus particles
interspersed in the sediment), frozen (48 h,
-30°C), and cut into three layers (0-3, 3-6, 6-9
mm), For each layer, sediment was separated
from interstitial water by squeezing under N2 on