Sediment Dynamics by Bioturbating Organisms
141
the main bioturbators, reworking an amount of sediment equalling a layer of
6-7 cm per week. For the seven summer months this would mean a layer of
-2 m, to which an unknown amount of winter bioturbation has to be added.
The few data available suggest bioturbation activity to increase at least one
order of magnitude from the subarctic to the equator mainly due to the
activity of callianassid shrimps in tropical areas. Everywhere bioturbation is
higher in sheltered than in exposed coastal areas.
6.6 Changes in Historical Times
How interesting it would be to travel back in time some hundreds of years and
to study bioturbation in a pristine Wadden Sea! We would be able to observe
Gray whales and rays digging holes for food. Gray whales are extinct now in
the Atlantic Ocean, but once occurred in the Wadden Sea area (Van Deinse
and Junge 1937; De Smet 1981). Rays have been decimated mainly in the last
100 years due to fisheries (Walker and Heessen 1996). Before the eelgrass
Zostera marina disappeared from the Dutch Wadden Sea due to a still illunderstood "wasting disease" in the early 1930s (Polderman and Den Hartog
1975), large populations of Brent geese (Branta bernicla) grazed mainly on the
rhizomes of this species during winter, causing undoubtedly a considerable
bioturbation. Perhaps no tidal basin has been studied for such a long time as
the area near Sylt, where Mobius started research as early as 1869 (see Reise
1982). Reise observed considerable changes in macrobenthos; sessile taxa
suffered most (mainly from shrimp fisheries), and some new species arrived.
Together with these changes, the Wadden Sea biota of mammals, fish,
macrobenthic species and eelgrass vegetation, and bioturbation will also have
changed.
Man's influence on the marine environment has increased with time, this
also influences sediment dynamics. Fishing activity such as beam-trawling
and dredging for Arenicola and shellfish are also sources of sediment disturbance. As early as the end of the sixteenth century, Dutch fishermen asked
Prince William of Orange to place restrictions on the use of trawls, because
this would make the grounds rough and probably lower future catches (de
Groot 1984). Only recently has research clearly indicated the disastrous effects
of fishing activity, increasingly effecting the sea bottom (Lindeboom and de
Groot 1998), both the sediment and its macrobenthos, the so-called 'nontarget' species. Shellfisheries have become more and more dangerous for
benthic life. Piersma and Koolhaas (1997) warned or lasting changes due to
shellfisheries in the Wadden Sea. Bivalve-rich silty sediments in the lee of a
mussel-bank have been transformed by removal of the mussel-bank and
successive mechanical harvesting of cockles into a permanently sandy, lower-
141
the main bioturbators, reworking an amount of sediment equalling a layer of
6-7 cm per week. For the seven summer months this would mean a layer of
-2 m, to which an unknown amount of winter bioturbation has to be added.
The few data available suggest bioturbation activity to increase at least one
order of magnitude from the subarctic to the equator mainly due to the
activity of callianassid shrimps in tropical areas. Everywhere bioturbation is
higher in sheltered than in exposed coastal areas.
6.6 Changes in Historical Times
How interesting it would be to travel back in time some hundreds of years and
to study bioturbation in a pristine Wadden Sea! We would be able to observe
Gray whales and rays digging holes for food. Gray whales are extinct now in
the Atlantic Ocean, but once occurred in the Wadden Sea area (Van Deinse
and Junge 1937; De Smet 1981). Rays have been decimated mainly in the last
100 years due to fisheries (Walker and Heessen 1996). Before the eelgrass
Zostera marina disappeared from the Dutch Wadden Sea due to a still illunderstood "wasting disease" in the early 1930s (Polderman and Den Hartog
1975), large populations of Brent geese (Branta bernicla) grazed mainly on the
rhizomes of this species during winter, causing undoubtedly a considerable
bioturbation. Perhaps no tidal basin has been studied for such a long time as
the area near Sylt, where Mobius started research as early as 1869 (see Reise
1982). Reise observed considerable changes in macrobenthos; sessile taxa
suffered most (mainly from shrimp fisheries), and some new species arrived.
Together with these changes, the Wadden Sea biota of mammals, fish,
macrobenthic species and eelgrass vegetation, and bioturbation will also have
changed.
Man's influence on the marine environment has increased with time, this
also influences sediment dynamics. Fishing activity such as beam-trawling
and dredging for Arenicola and shellfish are also sources of sediment disturbance. As early as the end of the sixteenth century, Dutch fishermen asked
Prince William of Orange to place restrictions on the use of trawls, because
this would make the grounds rough and probably lower future catches (de
Groot 1984). Only recently has research clearly indicated the disastrous effects
of fishing activity, increasingly effecting the sea bottom (Lindeboom and de
Groot 1998), both the sediment and its macrobenthos, the so-called 'nontarget' species. Shellfisheries have become more and more dangerous for
benthic life. Piersma and Koolhaas (1997) warned or lasting changes due to
shellfisheries in the Wadden Sea. Bivalve-rich silty sediments in the lee of a
mussel-bank have been transformed by removal of the mussel-bank and
successive mechanical harvesting of cockles into a permanently sandy, lower-