Sediment Dynamics by Bioturbating Organisms
139
1990). Geese feeding on Scirpus rhizomes are only active in the St. Lawrence
estuary tidal marshes twice a year when they visit this area during their
migration (Dionne 1985). Gray whales feed mainly in the summer season, so
their sediment reworking by suction-sieving for their food crustaceans is
seasonal (Johnson and Nelson 1984; Weitkamp et al.I992). Rays off California
showed a strong seasonality in hole digging (VanBlaricom 1982).
This strong seasonality influences sediment properties such as stability
(erodability). Intertidal sediments are stabilised by exopolymers (mucopolysaccharides) produced by diatoms during locomotion between the sediment
grains (e.g. Underwood and Paterson 1993; Chap. 5). Disturbing this diatom
population by sediment reworking or feeding on these diatoms therefore
might destabilise the sediment. This indeed has been observed in the Dollard
estuary (Dutch Wadden Sea; Kornman and De Deckere 1998). Benthic diatom
growth in April increased sediment erosion resistance as compared with the
winter situation, but in May the increased population of the amphipod
Corophium volutator caused diatoms to decrease and erodability to increase
again, both by their feeding on diatoms and by bioturbation. Quite interestingly, Daborn et al. (l993) published a paper that may almost be seen as a
sequel. On a tidal flat in the Bay of Funday they observed a decrease in bioturbation and grazing pressure on diatoms, due to the feeding of migratory
birds on Corophium volutator in August: the diatoms could increase again
and, with the mucopolysaccharides they produced, the stabilisation of the
sediment increased. We might complete the seasonal cycle of sediment stability by assuming again lower values in winter due to lower diatom densities
(Underwood and Paterson 1993) and a higher frequency of sediment disturbance by strong winds.
To study the relative importance of physical forces and bioturbation on
sediment transport, short-term in situ studies are insufficient (McManus
1998); they will miss this seasonal variation. However, in the sediments, effects
of bioturbation and physical forces remain visible. The study of sediment
cores from tidal flats and near-shore sediments gives us an insight into the
relative importance of energy level and bioturbation in sediment dynamics,
as shown by Reineck (l963, 1976, 1977). Howard (l975) indicates how bioturbation traces can be used to study depositional regimes in older
sediments.
6.5 Latitudinal Variation
Bioturbation by the total community cannot be estimated in a simple way.
Darwin's elegant method of measuring the sinking velocity of particles could
work on land. In marine (and fresh water) environments it measures the
139
1990). Geese feeding on Scirpus rhizomes are only active in the St. Lawrence
estuary tidal marshes twice a year when they visit this area during their
migration (Dionne 1985). Gray whales feed mainly in the summer season, so
their sediment reworking by suction-sieving for their food crustaceans is
seasonal (Johnson and Nelson 1984; Weitkamp et al.I992). Rays off California
showed a strong seasonality in hole digging (VanBlaricom 1982).
This strong seasonality influences sediment properties such as stability
(erodability). Intertidal sediments are stabilised by exopolymers (mucopolysaccharides) produced by diatoms during locomotion between the sediment
grains (e.g. Underwood and Paterson 1993; Chap. 5). Disturbing this diatom
population by sediment reworking or feeding on these diatoms therefore
might destabilise the sediment. This indeed has been observed in the Dollard
estuary (Dutch Wadden Sea; Kornman and De Deckere 1998). Benthic diatom
growth in April increased sediment erosion resistance as compared with the
winter situation, but in May the increased population of the amphipod
Corophium volutator caused diatoms to decrease and erodability to increase
again, both by their feeding on diatoms and by bioturbation. Quite interestingly, Daborn et al. (l993) published a paper that may almost be seen as a
sequel. On a tidal flat in the Bay of Funday they observed a decrease in bioturbation and grazing pressure on diatoms, due to the feeding of migratory
birds on Corophium volutator in August: the diatoms could increase again
and, with the mucopolysaccharides they produced, the stabilisation of the
sediment increased. We might complete the seasonal cycle of sediment stability by assuming again lower values in winter due to lower diatom densities
(Underwood and Paterson 1993) and a higher frequency of sediment disturbance by strong winds.
To study the relative importance of physical forces and bioturbation on
sediment transport, short-term in situ studies are insufficient (McManus
1998); they will miss this seasonal variation. However, in the sediments, effects
of bioturbation and physical forces remain visible. The study of sediment
cores from tidal flats and near-shore sediments gives us an insight into the
relative importance of energy level and bioturbation in sediment dynamics,
as shown by Reineck (l963, 1976, 1977). Howard (l975) indicates how bioturbation traces can be used to study depositional regimes in older
sediments.
6.5 Latitudinal Variation
Bioturbation by the total community cannot be estimated in a simple way.
Darwin's elegant method of measuring the sinking velocity of particles could
work on land. In marine (and fresh water) environments it measures the