Synthesis: Comparative Ecology of Sedimentary Shores
363
reworked annually. Expressing the magnitude of bioturbation in this way,
there is a notable increase from high to low latitudes. In cold to temperate
nearshore sediments, lugworms (Arenicolidae) are dominant bioturbators
and these rework some 5-40 cm annually, depending on the length of their
active season. At warm temperate to tropical shores, large decapod crustaceans take over, particularly callianassid shrimps, which rework some 50 to
more than 100 cm annually.
Effects of large invertebrate bioturbators, lugworms and callianassid
shrimps on the associated fauna are both positive and negative (Flach and
Tamaki, Chap. 7). In the European Wadden Sea, lugworm burrows provide an
attractive habitat for some meiofauna, an amphipod and a scale worm
species. In southern Japan, callianassid burrowing benefits a mobile isopod
near the surface and a snake eel in the burrow. Both of the large deposit
feeders decrease sediment stability. This negatively affects various smaller
infauna. In the Wadden Sea, tube-building amphipods and spionid polychaetes, in Japan a trochid snail and small polychaetes. Different species of
bioturbators from distant tidal flats apparently have very similar effects and
the basic processes are the same.
In southern England, erosion occurs at the edge of salt marshes, where in
earlier times sediment accretion prevailed with a prograding pioneer vegetation of salt marsh plants. Experiments show that invertebrates, particularly
the ragworm Nereis diversicolor, playa key role in the sedimentary processes
at the upper shore (Hughes, Chap. 8). These worms graze on the microphytobenthos, on filamentous algae and on seeds and seedlings of the vascular
plants. This indirectly reduces shear strength of the sediment and may
enhance erosion. At level surfaces within the marsh, further net accretion will
be impeded. Alongside salt marsh creeks, a positive feedback is initiated by
grazing on the sediment binding and accreting algae. Erosion is facilitated
and further increased because creeks become wider, are filled with more
water, and this increases tidal flow dynamics and hence erosion. More wading
birds would help to decrease the invertebrates and this may halt the erosional
process. Maintaining salt marshes in front of a dyked land may prove very
difficult in the face of sea-level rises.
Not only invertebrates, but also rays, ducks, geese, flamingoes, walrus and
Gray whales among others contribute to sediment disturbances (Cadee,
Chap. 6). Their feeding pits range from tens of centimeters up to 4 m in length.
Gray whales stay with their young in coastal lagoons and hence were among
the most susceptible to overkill by early whalers. They have become extinct
from the shores of the Atlantic and are presently confined to the eastern North
Pacific coasts. These baleen whales sieve benthic fauna out of the sediment.
This causes considerable bioturbation and creates a very patchy habitat for
the benthos.
363
reworked annually. Expressing the magnitude of bioturbation in this way,
there is a notable increase from high to low latitudes. In cold to temperate
nearshore sediments, lugworms (Arenicolidae) are dominant bioturbators
and these rework some 5-40 cm annually, depending on the length of their
active season. At warm temperate to tropical shores, large decapod crustaceans take over, particularly callianassid shrimps, which rework some 50 to
more than 100 cm annually.
Effects of large invertebrate bioturbators, lugworms and callianassid
shrimps on the associated fauna are both positive and negative (Flach and
Tamaki, Chap. 7). In the European Wadden Sea, lugworm burrows provide an
attractive habitat for some meiofauna, an amphipod and a scale worm
species. In southern Japan, callianassid burrowing benefits a mobile isopod
near the surface and a snake eel in the burrow. Both of the large deposit
feeders decrease sediment stability. This negatively affects various smaller
infauna. In the Wadden Sea, tube-building amphipods and spionid polychaetes, in Japan a trochid snail and small polychaetes. Different species of
bioturbators from distant tidal flats apparently have very similar effects and
the basic processes are the same.
In southern England, erosion occurs at the edge of salt marshes, where in
earlier times sediment accretion prevailed with a prograding pioneer vegetation of salt marsh plants. Experiments show that invertebrates, particularly
the ragworm Nereis diversicolor, playa key role in the sedimentary processes
at the upper shore (Hughes, Chap. 8). These worms graze on the microphytobenthos, on filamentous algae and on seeds and seedlings of the vascular
plants. This indirectly reduces shear strength of the sediment and may
enhance erosion. At level surfaces within the marsh, further net accretion will
be impeded. Alongside salt marsh creeks, a positive feedback is initiated by
grazing on the sediment binding and accreting algae. Erosion is facilitated
and further increased because creeks become wider, are filled with more
water, and this increases tidal flow dynamics and hence erosion. More wading
birds would help to decrease the invertebrates and this may halt the erosional
process. Maintaining salt marshes in front of a dyked land may prove very
difficult in the face of sea-level rises.
Not only invertebrates, but also rays, ducks, geese, flamingoes, walrus and
Gray whales among others contribute to sediment disturbances (Cadee,
Chap. 6). Their feeding pits range from tens of centimeters up to 4 m in length.
Gray whales stay with their young in coastal lagoons and hence were among
the most susceptible to overkill by early whalers. They have become extinct
from the shores of the Atlantic and are presently confined to the eastern North
Pacific coasts. These baleen whales sieve benthic fauna out of the sediment.
This causes considerable bioturbation and creates a very patchy habitat for
the benthos.