92
H.U. Riisgard and P. Kamermans
such as high current flow, low food availability on the sediment surface and
high predation pressure on siphons or whole individuals.
4.3.2 Current Velocity
Levinton (1991) studied the effect of high current velocity on the feeding
behaviour of three Pacific Macoma species. In a flume experiment, a decrease
in deposit-feeding behaviour was observed with an increase in water velocity.
The deposit-feeding radius was used as a measure of deposit-feeding behaviour. This was the distance from the siphon hole to the tip of the depositfeeding siphon. He suggested that the drag on the siphon was preventing deposit feeding at high velocities. At velocities between 10.5 and 12 cm S-1 protruding siphons quivered and then flopped over. At higher velocities, the
bivalves did not show their siphons above the sediment surface which suggests suspension feeding, or non-feeding. However, evidence of an actual
switch to suspension feeding was not provided. Peterson and Skilleter (1994)
studied feeding behaviour in Macoma balthica in relation to current velocities in the field. Two identical sets of M. balthica were introduced into two
adjacent field sites with different flow regimes in a tributary of the Neuse
River estuary, North Carolina. Flow velocity was measured with a current
meter. The amount of sediment present in the mantle cavity of the bivalves
was used as a measure of deposit-feeding activity. The site with a mean
current velocity of 20 cm S-1 showed lower levels of deposit feeding than the
site with a mean current velocity of 5 cm S-I. Again, the data indicate a reduction in deposit feeding, but do not provide evidence of a switch to suspension
feeding.
4.3.3 Food Availability
In the Wadden Sea, Hummel (1985a) compared the algal species composition
of the stomach contents of Macoma balthica with the algal composition of
simultaneously collected plankton and surface sediments. The algal species
composition from the stomach was then used as an indicator of feeding
behaviour. Results indicated that the bivalves on the tidal flats took twice as
much food originating from the overlying water than from the bottom. This
behaviour was related to the high chlorophyll concentrations in the water. Lin
and Hines (1994) demonstrated in a laboratory experiment that an increase in
the amount of food in the water to a chlorophyll a concentration of 751lg 1-1
resulted in an increase in the proportion of M. balthica individuals that
showed suspension-feeding behaviour. In this case, suspension-feeding
behaviour was defined as holding the inhalant siphon straight up and
H.U. Riisgard and P. Kamermans
such as high current flow, low food availability on the sediment surface and
high predation pressure on siphons or whole individuals.
4.3.2 Current Velocity
Levinton (1991) studied the effect of high current velocity on the feeding
behaviour of three Pacific Macoma species. In a flume experiment, a decrease
in deposit-feeding behaviour was observed with an increase in water velocity.
The deposit-feeding radius was used as a measure of deposit-feeding behaviour. This was the distance from the siphon hole to the tip of the depositfeeding siphon. He suggested that the drag on the siphon was preventing deposit feeding at high velocities. At velocities between 10.5 and 12 cm S-1 protruding siphons quivered and then flopped over. At higher velocities, the
bivalves did not show their siphons above the sediment surface which suggests suspension feeding, or non-feeding. However, evidence of an actual
switch to suspension feeding was not provided. Peterson and Skilleter (1994)
studied feeding behaviour in Macoma balthica in relation to current velocities in the field. Two identical sets of M. balthica were introduced into two
adjacent field sites with different flow regimes in a tributary of the Neuse
River estuary, North Carolina. Flow velocity was measured with a current
meter. The amount of sediment present in the mantle cavity of the bivalves
was used as a measure of deposit-feeding activity. The site with a mean
current velocity of 20 cm S-1 showed lower levels of deposit feeding than the
site with a mean current velocity of 5 cm S-I. Again, the data indicate a reduction in deposit feeding, but do not provide evidence of a switch to suspension
feeding.
4.3.3 Food Availability
In the Wadden Sea, Hummel (1985a) compared the algal species composition
of the stomach contents of Macoma balthica with the algal composition of
simultaneously collected plankton and surface sediments. The algal species
composition from the stomach was then used as an indicator of feeding
behaviour. Results indicated that the bivalves on the tidal flats took twice as
much food originating from the overlying water than from the bottom. This
behaviour was related to the high chlorophyll concentrations in the water. Lin
and Hines (1994) demonstrated in a laboratory experiment that an increase in
the amount of food in the water to a chlorophyll a concentration of 751lg 1-1
resulted in an increase in the proportion of M. balthica individuals that
showed suspension-feeding behaviour. In this case, suspension-feeding
behaviour was defined as holding the inhalant siphon straight up and