134
G.C.Cadee
ceans), or surface sediments to deeper layers (inverted conveyer-belt species,
e.g. the polychaete Scolelepis see Wohlenberg 1937). They may also mix the
surface sediment layer by surface deposit feeding (Yoldia limatula, see
Rhoads 1963) or grazing (the gastropod Hydrobia ulvae). Deep layers may be
mixed by deposit feeders that bulldoze horizontally through subsurface
sediments (e.g. heart urchins, Howard et al. 1974). Funnel feeders feed at
depth and surface sediment sinks to their feeding depth via a funnel. This
works best in loose sediments. One of the best-known examples is Arenicola
marina (Wells 1966; Riisgard and Banta 1998). This is also a good example
that deposit feeders do not always keep to one mode of feeding. The funnel
feeder Arenicola may also feed on deep sediments (Richter 1924; Rijken 1979).
Gardening (stimulation of microbial growth by irrigation or bioturbation)
has been proposed to enrich the sediment eaten (Hylleberg 1975). Riisgard
and Banta (1998) demonstrated that Arenicola cannot make a living as true
filter feeders, as proposed by Kruger (1959). For other examples of versatile
deposit feeders, see Cadee (1984) and Riisgard and Kamermans (Chap. 4).
Callianassid crustaceans rework more sediment per surface area than other
bioturbators (see compilations in Cadee 1976; Lee and Swartz 1980).
McGinitie (1934) estimated that a population of Callianassa californiensis reworked annually an amount of sediment equalling a layer of over 1 m. Myrick
and Flessa (1996) observed the same callianassid to burrow to a depth of
1.15 m on the tidal flat of Bahia la Choya, northern Gulf of California, Mexico,
where the population reworked a layer of 56 cm annually.
Whereas most sediment transport by bioturbators is in a vertical direction,
their activity may also increase horizontal sediment transport by waves and
currents: In the Danish Wadden Sea, Wesenberg-Lund (1905) already observed erosion by the incoming tide of Arenicola faecal casts deposited on the
tidal flat during low tide, bringing fine particles into suspension. Much later
Rhoads and Young (1970), in a now 'classical' paper, noted that intensive
bioturbation by deposit feeders in Buzzards Bay produced a fluid sediment
surface that was easily resuspended by low-velocity tidal currents, thereby
preventing establishment of suspension feeders.
6.3.3 Larger Predators and Grazers
Recently, more attention is being paid to feeding holes made by predators on
benthic macrofauna or by grazers on plant rhizomes. It is now realised that
they may also produce considerable sediment disturbance. Side-scan sonar
revealed up to 4m long, 2m wide and OAm deep holes in the surface sediments of the Bering Sea (Johnson and Nelson 1984). They could be related to
Gray whales (Eschrichtius robustus) that feed there on benthic tube-building
ampeliscid amphipods. Abundant amphipod tubes commonly coalesce to
G.C.Cadee
ceans), or surface sediments to deeper layers (inverted conveyer-belt species,
e.g. the polychaete Scolelepis see Wohlenberg 1937). They may also mix the
surface sediment layer by surface deposit feeding (Yoldia limatula, see
Rhoads 1963) or grazing (the gastropod Hydrobia ulvae). Deep layers may be
mixed by deposit feeders that bulldoze horizontally through subsurface
sediments (e.g. heart urchins, Howard et al. 1974). Funnel feeders feed at
depth and surface sediment sinks to their feeding depth via a funnel. This
works best in loose sediments. One of the best-known examples is Arenicola
marina (Wells 1966; Riisgard and Banta 1998). This is also a good example
that deposit feeders do not always keep to one mode of feeding. The funnel
feeder Arenicola may also feed on deep sediments (Richter 1924; Rijken 1979).
Gardening (stimulation of microbial growth by irrigation or bioturbation)
has been proposed to enrich the sediment eaten (Hylleberg 1975). Riisgard
and Banta (1998) demonstrated that Arenicola cannot make a living as true
filter feeders, as proposed by Kruger (1959). For other examples of versatile
deposit feeders, see Cadee (1984) and Riisgard and Kamermans (Chap. 4).
Callianassid crustaceans rework more sediment per surface area than other
bioturbators (see compilations in Cadee 1976; Lee and Swartz 1980).
McGinitie (1934) estimated that a population of Callianassa californiensis reworked annually an amount of sediment equalling a layer of over 1 m. Myrick
and Flessa (1996) observed the same callianassid to burrow to a depth of
1.15 m on the tidal flat of Bahia la Choya, northern Gulf of California, Mexico,
where the population reworked a layer of 56 cm annually.
Whereas most sediment transport by bioturbators is in a vertical direction,
their activity may also increase horizontal sediment transport by waves and
currents: In the Danish Wadden Sea, Wesenberg-Lund (1905) already observed erosion by the incoming tide of Arenicola faecal casts deposited on the
tidal flat during low tide, bringing fine particles into suspension. Much later
Rhoads and Young (1970), in a now 'classical' paper, noted that intensive
bioturbation by deposit feeders in Buzzards Bay produced a fluid sediment
surface that was easily resuspended by low-velocity tidal currents, thereby
preventing establishment of suspension feeders.
6.3.3 Larger Predators and Grazers
Recently, more attention is being paid to feeding holes made by predators on
benthic macrofauna or by grazers on plant rhizomes. It is now realised that
they may also produce considerable sediment disturbance. Side-scan sonar
revealed up to 4m long, 2m wide and OAm deep holes in the surface sediments of the Bering Sea (Johnson and Nelson 1984). They could be related to
Gray whales (Eschrichtius robustus) that feed there on benthic tube-building
ampeliscid amphipods. Abundant amphipod tubes commonly coalesce to