90
H. U. Riisgard and P. Kamermans
was sufficiently high to control the phytoplankton level in the eutrophicated
fjord, but wind-driven vertical mixing appeared to be a crucial factor for
realisation of the potential grazing impact. Wind speeds of <8 m S-1 were not
sufficient to mix the water column, and, therefore, phytoplankton depletion
occurred near the bottom. Because the wind speed seems to be below the
critical level during a considerable part of the year (about 70 % of the time in
March to August 1995), near-bottom phytoplankton depletion and switching from suspension feeding to an alternative feeding mode (predation,
scavenging, surface deposit feeding) may be a frequent phenomenon in the
Odense Fjord. From this it may be predicted that the time spent on suspension
feeding by N. diversocolor and its actual grazing impact may vary considerably between different localities, and be dependent on currents, wind and
wave action, and other local conditions. So far, no such knowledge exists for,
e. g., the Wadden Sea where this worm is very common.
4.3 Example II: Switching to Suspension Feeding
in Macoma balthica
P. KAMERMANS
The tellinid bivalve Macoma balthica is a common soft-bottom species along
both sides of the North Atlantic (Beukema and Meehan 1985; Kamermans et
al. 1999). Yonge (1949) classified all Macoma species as deposit feeders. Since
then, variability in feeding behaviour has been observed within this group. M.
balthica (L.) is now generally considered a facultative deposit/suspension
feeder (Bradfield and Newell 1961; Hughes 1969; Bubnova 1972; Rasmussen
1973; Hummel 1985a; Olafsson 1986; Thompson and Nichols 1988; Kamermans 1994b; Lin and Hines 1994; Peterson and Skilleter 1994; Skilleter and
Peterson 1994).
Deposit-feeding behaviour entails movements of the extended inhalant
siphon on the sediment surface and the burrow entrance while sucking in
material (e.g. Yonge 1949; Gilbert 1977). Defining suspension-feeding behaviour is more difficult. Some authors have suggested that M. balthica may
be suspension feeding when the inhalant siphon is short and immobile (Bradfield and Newell 1961; de Wilde 1975). On the other hand, Levinton (1991) and
Kamermans and Huitema (1994) suggested that this behaviour may be
resting. Other authors considered a rotating movement of the inhalant siphon
in the water column as suspension feeding (Rasmussen 1973; Hummel 1985a;
Olafsson 1986; Rosenberg 1993). However, Hulscher (1973) described the
rotating movement as a way to whirl up the top layer of the sediment in order
H. U. Riisgard and P. Kamermans
was sufficiently high to control the phytoplankton level in the eutrophicated
fjord, but wind-driven vertical mixing appeared to be a crucial factor for
realisation of the potential grazing impact. Wind speeds of <8 m S-1 were not
sufficient to mix the water column, and, therefore, phytoplankton depletion
occurred near the bottom. Because the wind speed seems to be below the
critical level during a considerable part of the year (about 70 % of the time in
March to August 1995), near-bottom phytoplankton depletion and switching from suspension feeding to an alternative feeding mode (predation,
scavenging, surface deposit feeding) may be a frequent phenomenon in the
Odense Fjord. From this it may be predicted that the time spent on suspension
feeding by N. diversocolor and its actual grazing impact may vary considerably between different localities, and be dependent on currents, wind and
wave action, and other local conditions. So far, no such knowledge exists for,
e. g., the Wadden Sea where this worm is very common.
4.3 Example II: Switching to Suspension Feeding
in Macoma balthica
P. KAMERMANS
The tellinid bivalve Macoma balthica is a common soft-bottom species along
both sides of the North Atlantic (Beukema and Meehan 1985; Kamermans et
al. 1999). Yonge (1949) classified all Macoma species as deposit feeders. Since
then, variability in feeding behaviour has been observed within this group. M.
balthica (L.) is now generally considered a facultative deposit/suspension
feeder (Bradfield and Newell 1961; Hughes 1969; Bubnova 1972; Rasmussen
1973; Hummel 1985a; Olafsson 1986; Thompson and Nichols 1988; Kamermans 1994b; Lin and Hines 1994; Peterson and Skilleter 1994; Skilleter and
Peterson 1994).
Deposit-feeding behaviour entails movements of the extended inhalant
siphon on the sediment surface and the burrow entrance while sucking in
material (e.g. Yonge 1949; Gilbert 1977). Defining suspension-feeding behaviour is more difficult. Some authors have suggested that M. balthica may
be suspension feeding when the inhalant siphon is short and immobile (Bradfield and Newell 1961; de Wilde 1975). On the other hand, Levinton (1991) and
Kamermans and Huitema (1994) suggested that this behaviour may be
resting. Other authors considered a rotating movement of the inhalant siphon
in the water column as suspension feeding (Rasmussen 1973; Hummel 1985a;
Olafsson 1986; Rosenberg 1993). However, Hulscher (1973) described the
rotating movement as a way to whirl up the top layer of the sediment in order