P . Sobral, J. Widdows J. Exp. Mar. Biol. Ecol. 245 2000 111 –125
115
At 30-min intervals over a period of 1 h 30 min, one 20-ml sample was taken from each beaker with a syringe and the volume of particles measured by means of a Coulter
Counter Multisizer fitted with a 100-mm orifice tube. At the two highest seston concentrations dilutions 1:10 and 1:20 were made to avoid coincidence counts and
blockage of the orifice tube. Samples were always stirred prior to counting.
Clearance rates, minus control values, were calculated over two consecutive time intervals and weight standardised to a 0.3 g dry weight animal.
2.3. Particle size selection The suspended particle-size selection experiments were performed using the flume
21
and 13 animals buried in sediment. Two SPM concentrations 10 and 100 mg l were
21
studied. These concentrations were obtained and algal cells 10 000 cells ml were
added in the same manner as for the turbidity experiments. Samples were taken from two heights in the water column, measured by volume,
over a wide range of channels particle sizes 2.4 to 9.6 mm diameter with a Coulter Counter Multisizer fitted with a 100-mm orifice tube and averaged. Dilution 1:10 was
needed at the higher seston concentration to avoid coincidence counts. Size selection was quantified by examining the relative depletion of different particle
sizes over a period of 1 h 30 min. The retention efficiency was calculated per channel using the formula 1 2 V V , V , and V being the volumes measured at two different
2 1
1 2
sampling times i.e. at the beginning and after 1 h 30 min, and then expressed as the percentage of the larger channels particle sizes i.e. 9.6-mm equivalent spherical
diameter were maximum volumes registered. For this calculation only particles of diameters between 2.4 mm to 9.6 mm were considered because larger particles introduce
considerable variability into the counts, due to their relative high volumes and relatively low numbers.
3. Results
3.1. Effect of current velocity on clearance rate and sediment movement
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Fig. 2 shows that the maximum clearance rate c. 2.5 l h ind
occurs at the lower
21
current velocities i.e. below 8 cm s and declines with increasing velocities,
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particularly above 17 cm s i.e. shear stresses . 0.9 Pa. At velocities of 24 cm s
the clearance rate declines to c. 50. A significant linear relation between clearance
21 21
rates l h and current velocity cm s
is explained by the equation y 5 2 0.066x 1 2.828, with r 5 0.98 n 5 6.
The current velocity and the shear stress needed to induce bed movement are clearly
21
related to sediment granulometry. At free stream current velocities , 12 cm s , or bed
shear stresses of less than c. 0.4 Pa there was no movement of surface sediment
21
particles. At 17 cm s 0.89 Pa some resuspension of faeces was observed, together
21
with some rolling of sand grains and smaller particles on the bed. At 24 cm s 1.6 Pa
resuspension of faeces and smaller particles was apparent and there was rolling and
116 P
. Sobral, J. Widdows J. Exp. Mar. Biol. Ecol. 245 2000 111 –125
21 21
Fig. 2. Variation of clearance rate l h ind
6S.E. of Ruditapes decussatus 0.3 g dw with current velocity
21
cm s in the annular flume.
saltation of sand grains on the bottom leading to the formation of small slowly moving sand ripples. At this current velocity the siphons were being bombarded by particles and
the clams responded by occasional ‘coughing’ or ejection of material from the mantle
21
cavity via the inhalant siphon. At a free stream velocity of 36 cm s 3.8 Pa, there was
continuous resuspension of sediment, migrating sand ripples and movement of the bed sediment, which kept covering and uncovering the clams. Two individuals could not stay
in position and were transported to the next quadrant of the flume. At this high current velocity clams were experiencing continuous bombardment of the siphons with sand
grains and some would keep them constricted at the tip while others would simply close their valves. On several occasions the inhalant siphons were observed to eject sediment
higher than 5 cm into the water column. At the end of the experimental runs, strings of mucus bound sediment particles i.e. material rejected as pseudofeces were observed on
the sediment surface.
3.2. Effect of current velocity on algal cell depletion in the water column Vertical profiles of cell concentrations in the water column at different free-stream
velocities are shown in Fig. 3. In all flume runs with clams present and at all current velocities, the maximum cell concentration occurred at a height of 20 cm above the bed.
Consequently, all cell concentrations are expressed as a percentage of the cell concentration at 20-cm height. The results presented in Fig. 3 illustrate that algal
depletion near the bed was more evident at the lower current velocities, especially at 0.6
21
and 3 and 8 cm s . Cell concentration profiles at the three lower speeds were all
significantly different from the controls t-test, P , 0.01, 0.05 and 0.01 respectively. At higher current velocities the cell concentration profiles were not significantly different
P . Sobral, J. Widdows J. Exp. Mar. Biol. Ecol. 245 2000 111 –125
117
Fig. 3. Vertical profiles of algal cell concentrations mean6S.E. after depletion by Ruditapes decussatus j
21
at different current velocities cm s in relation to control algal cell concentrations x. Results are
expressed as the percentage of the cell concentration at 20-cm height.
118 P
. Sobral, J. Widdows J. Exp. Mar. Biol. Ecol. 245 2000 111 –125
from the control profiles, reflecting the lower clearance rates and the greater vertical mixing.
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At the lowest current velocity measured 0.6 cm s the algal cell depletion was
greatest at 10 cm above the sediment surface Fig. 3 but this was less pronounced at 3
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cm s and was beginning to disappear by 8 cm s
. Algal cell depletion at 5 and at
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10-cm height at 0.6 and 3 cm s is significantly different from the cell concentrations at
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15 cm t-test, P , 0.01 for 10 cm at 0.6 cm s and P , 0.05 for the others.
At higher current velocities differences in cell concentrations at 5 cm and 15 cm are not significant. Algal cell concentrations found at 10 cm are significantly different t-test,
P , 0.05 from the cell concentrations found higher 15 and 20 cm in the water column
21
at all current velocities except at 17 cm s where differences where not significant due
to higher variability Fig. 3.
4. Effect of turbidity on clearance rate and particle-size selection
