P . Sobral, J. Widdows J. Exp. Mar. Biol. Ecol. 245 2000 111 –125
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2. Materials and methods
Clams were collected at the sampling site in Ria Formosa at low tide, and allowed to
acclimatise to laboratory conditions temperature 2008C, salinity 32‰ at the Plymouth Marine Laboratory PML, UK, where all the experiments were performed. Clams were
3
maintained in a system containing 3 m of recirculating seawater and they were fed with
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a culture of Isochrysis galbana at a rate of approximately 3000 cells ml .
2.1. Effect of current velocity Current velocities were generated using an annular flume Fig. 1, for details see
Widdows et al. 1998a. The flume has a 10-cm wide circular channel of 64-cm outer diameter, a maximum water depth of 35 cm, and a maximum volume of 60 l. The
rotating smooth drive plate on the water surface can create free stream velocities up to
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80 cm s . Water can be sampled at up to seven different heights above the bed via
1-mm bore tubes connected to syringes. A small electromagnetic EM current sensor Valeport 800–175 was used to
quantify the free-stream velocities at 10 cm height and velocity-height profiles 2 to 16 cm above the bed from which to estimate the shear stress on the bed Pa. These
vertical profiles were also confirmed by video tracking of neutrally buoyant particles close to the bed Widdows, unpublished observations.
Sediment was introduced in the flume to a depth of 5 cm and the clams allowed to bury overnight prior to each experiment. The sandy sediment 364-mm mean diameter
615 S.E. was collected from Whitsand Bay, Cornwall, and washed several times under running water.
Four groups n 5 13 clams per group; mean length 34.660.2 mm, n 5 52 were buried in the sediment within the quadrant just upstream of the 1-mm bore sampling
tubes for measuring the vertical profile in the water column. Algal cells were introduced
Fig. 1. A Annular flume used to study the effects of current velocity on the clearance rates of Ruditapes decussatus. B Experimental setup view across the flume channel.
114 P
. Sobral, J. Widdows J. Exp. Mar. Biol. Ecol. 245 2000 111 –125
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into the flume in order to obtain a concentration of approximately 15 000 cells ml . The
feeding rates of clams were then measured at six current velocities of 0.6, 3, 8, 17, 34
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and 36 cm s equivalent to bed shear stresses of 0.001, 0.02, 0.11, 0.89, 1.68 and 3.8
Pa, respectively. In each experimental run, 30 min after addition of the algal cells, two water samples
separated by a 2-min interval, were collected for algal cells counts at three different heights 2, 10 and 20 cm above the sediment, and this was repeated every 30 min for 2
h. Cells were counted with a Coulter Counter Model D, equipped with a 140-mm aperture tube. Clearance rates, or the volume of water cleared of suspended particles,
were calculated from the exponential decline in algal cell concentrations for details see Widdows et al., 1998a. This was based on the concentrations at the upper height, which
reflected the longterm mixed algal cell concentrations in the flume. Samples from lower levels reflected small scale cell depletion at lower current velocities.
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Controls, without clams, were run with the addition of algal cells, at 0.6 cm s and at
21
24 cm s . Algal settlement in the controls was very low equivalent to a clearance rate
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of , 0.25 l h and was subtracted from clearance rate values for each group of clams.
Clearance rates values were standardised to an animal of 0.3 g dry tissue mass. During each experimental run at a specific current velocity water samples were
collected from all six heights 2, 5, 10, 15, 20 and 25 cm above the sediment. This provided a vertical profile of cell concentrations in order to determine algal cell
depletion in the water column in relation to current velocity. Control counts without
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animals, for these profiles were again made at free stream velocities of 0.6 cm s and
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24 cm s .
2.2. Turbidity To test the influence of turbidity on the clearance rate of R
. decussatus, three SPM
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concentrations, 10, 100 and 300 mg l were used. These were obtained by adding fine
surface mud mean diameter 12 mm to seawater to make up the required SPM concentration. Mud was collected from the banks of the Lynher river, at Whacker Quay,
Cornwall, and then held at 58C and in the dark until required for the experiment. SPM concentrations were monitored with an optical back-scatter probe OBS-3 DA
Instrument, previously calibrated for the same mud. Calibration was achieved by fitting a regression line to the relationship between OBS readings volts and the concentration
21
of suspended material mg l in samples taken for gravimetric analysis on GF C
filters. The turbidity experiments were performed in a closed system consisting of sixteen 2-l
beakers each containing one clam, plus one control beaker without an animal. These were placed on a multi-point magnetic stirrer. A mud suspension was added to each
beaker to the required concentration and turbidity level monitored with the OBS sensor. Algal cells were also added to the fine sediment to provide a concentration of 10 000
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cells ml . Particles were kept in suspension by continuous stirring with a magnetic
stirrer follower placed in a small Petri dish to avoid disturbing the clams and resuspending faecal material.
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
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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
