Switching Between Deposit and Suspension Feeding in Coastal Zoobenthos
93
relatively still in the water column. Olafsson (1986) observed differences in
feeding mode between M. balthica populations from two contrasting habitats
in the Baltic Sea. Individuals from both populations were placed in aquariums
under conditions that were similar to those at the site of collection. Individuals from a muddy site with stagnant water showed predominantly
deposit-feeding behaviour, while suspension-feeding behaviour was observed
in individuals from a sandy site with moderate currents. In this study, suspension-feeding behaviour was defined as holding the siphon relatively still
and straight up in the water as well as swirling it around in a circular fashion.
Olafsson (1986) suggested that suspension feeding was a response to
increased food availability in the water column in the sandy habitat with
moderate currents. A cage experiment in the Baltic Sea showed densitydependent growth in the population in the muddy habitat, but not in the
sandy habitat (Olafsson 1986). These results support deposit feeding in the
muddy habitat and suspension feeding in the sandy habitat.
On a local scale, food limitation is expected to occur more readily in
deposit feeders than in suspension feeders, because for suspension feeders
food is continuously supplied by the passing water (Levinton 1972). Growth
reduction of suspension feeders occurs only at very high densities of suspension feeders such as cockle and mussel banks (Jensen 1992; Kamermans
1993). One can ask why the deposit-feeding Macoma balthica of the muddy
site in the Baltic, that were experiencing growth limitation, did not switch to
suspension feeding. Experiments in outdoor flow-through basins carried out
by Kamermans et al. (1992) showed density-dependent growth in M.
balthica, but not in the suspension feeder Cerastoderma edule. In this
experiment, the sediment was sandy and the fresh seawater was constantly
supplied, but density-dependent growth was found which suggests depositfeeding behaviour. Following Olafsson's suggestion, suspension-feeding
behaviour, and thus no density-dependent growth, was expected in M.
balthica in the basins. The continuation of deposit feeding may be explained
by the low food availability in the water column in the basins. During the
basin experiment chla levels in the water ranged between 2 and 20 flg 1-1,
which is much lower than the switching level of 75 flg 1-1 observed by Lin and
Hines (1994). Olafsson (1986) does not provide data on food availability in
the water column at the two sites where he performed the cage experiments.
Collectively, these observations indicate that food supply in the water
column was probably much lower at the muddy site than at the sandy site,
which may explain the deposit-feeding behaviour of M. balthica at the
muddy site.
93
relatively still in the water column. Olafsson (1986) observed differences in
feeding mode between M. balthica populations from two contrasting habitats
in the Baltic Sea. Individuals from both populations were placed in aquariums
under conditions that were similar to those at the site of collection. Individuals from a muddy site with stagnant water showed predominantly
deposit-feeding behaviour, while suspension-feeding behaviour was observed
in individuals from a sandy site with moderate currents. In this study, suspension-feeding behaviour was defined as holding the siphon relatively still
and straight up in the water as well as swirling it around in a circular fashion.
Olafsson (1986) suggested that suspension feeding was a response to
increased food availability in the water column in the sandy habitat with
moderate currents. A cage experiment in the Baltic Sea showed densitydependent growth in the population in the muddy habitat, but not in the
sandy habitat (Olafsson 1986). These results support deposit feeding in the
muddy habitat and suspension feeding in the sandy habitat.
On a local scale, food limitation is expected to occur more readily in
deposit feeders than in suspension feeders, because for suspension feeders
food is continuously supplied by the passing water (Levinton 1972). Growth
reduction of suspension feeders occurs only at very high densities of suspension feeders such as cockle and mussel banks (Jensen 1992; Kamermans
1993). One can ask why the deposit-feeding Macoma balthica of the muddy
site in the Baltic, that were experiencing growth limitation, did not switch to
suspension feeding. Experiments in outdoor flow-through basins carried out
by Kamermans et al. (1992) showed density-dependent growth in M.
balthica, but not in the suspension feeder Cerastoderma edule. In this
experiment, the sediment was sandy and the fresh seawater was constantly
supplied, but density-dependent growth was found which suggests depositfeeding behaviour. Following Olafsson's suggestion, suspension-feeding
behaviour, and thus no density-dependent growth, was expected in M.
balthica in the basins. The continuation of deposit feeding may be explained
by the low food availability in the water column in the basins. During the
basin experiment chla levels in the water ranged between 2 and 20 flg 1-1,
which is much lower than the switching level of 75 flg 1-1 observed by Lin and
Hines (1994). Olafsson (1986) does not provide data on food availability in
the water column at the two sites where he performed the cage experiments.
Collectively, these observations indicate that food supply in the water
column was probably much lower at the muddy site than at the sandy site,
which may explain the deposit-feeding behaviour of M. balthica at the
muddy site.