142
G.C.Cadee
lying and higher dynamic intertidal flat, where bivalves can no longer settle
and maintain their populations. New developments in mechanical harvesting
of bivalves also enable collecting of deep-living species such as Ensis directus.
In the North Sea shell-fishery is now able to fish 1 ha per hour down to a
maximum depth of 40 m, scraping and sieving the upper 30 cm of the bottom
(Leopold 1999). This is not bioturbation but habitat destruction by man. What
will be left of the North Sea ecosystem in the twenty-first century?
6.7 Conclusions
Sediment reworking by organisms occurs in almost all coastal areas. Under
very high-energy conditions, where few infaunal organisms can survive,
bioturbation is negligible. Estimates of sediment mixing depth can be obtained, e. g., from vertical 2l0Pb distributions, but they give the combined
effects of physical and biological sediment reworking and thus underestimate
bioturbation. Structures in sediment cores can be used as an archive of the
relative role of physical and biological sediment disturbance. It is not only
deposit feeders and all organisms crawling on, or dwelling in, the sediment
that cause bioturbation, but also predators (including birds, rays and the Gray
whale) on infaunal macrobenthos, and herbivores such as geese and dugong
feeding on sea grass rhizomes. Bioturbation in the coastal zone increases one
order of magnitude from the sub-arctic towards the tropics, particularly due
to the rich and diverse (sub)tropical crustacean fauna. In temperate areas
bioturbation shows seasonal variation; for its quantification, year-round
studies are necessary. Little is known of seasonal variation in tropical areas. In
the Wadden Sea, bioturbation has considerably changed in historical times:
Gray whales and rays have disappeared due to fishing activities; geese feeding
on rhizomes of seagrass have disappeared with the disappearance of Zostera
due to a "wasting disease". Human impact on the sediment has increased due
to trawling and mechanical harvesting of shellfish and lugworms, now
disturbing the top 30 cm of the sediment. This long-term and still-increasing
human impact continues to change the Wadden Sea ecosystem, and such is
the future of many over-fished coastal areas in the world.
Acknowledgements. The author is very grateful for critical remarks by referees Herman
Michaelis and Akio Tamaki. This is NIOZ publication nr. 3486.
G.C.Cadee
lying and higher dynamic intertidal flat, where bivalves can no longer settle
and maintain their populations. New developments in mechanical harvesting
of bivalves also enable collecting of deep-living species such as Ensis directus.
In the North Sea shell-fishery is now able to fish 1 ha per hour down to a
maximum depth of 40 m, scraping and sieving the upper 30 cm of the bottom
(Leopold 1999). This is not bioturbation but habitat destruction by man. What
will be left of the North Sea ecosystem in the twenty-first century?
6.7 Conclusions
Sediment reworking by organisms occurs in almost all coastal areas. Under
very high-energy conditions, where few infaunal organisms can survive,
bioturbation is negligible. Estimates of sediment mixing depth can be obtained, e. g., from vertical 2l0Pb distributions, but they give the combined
effects of physical and biological sediment reworking and thus underestimate
bioturbation. Structures in sediment cores can be used as an archive of the
relative role of physical and biological sediment disturbance. It is not only
deposit feeders and all organisms crawling on, or dwelling in, the sediment
that cause bioturbation, but also predators (including birds, rays and the Gray
whale) on infaunal macrobenthos, and herbivores such as geese and dugong
feeding on sea grass rhizomes. Bioturbation in the coastal zone increases one
order of magnitude from the sub-arctic towards the tropics, particularly due
to the rich and diverse (sub)tropical crustacean fauna. In temperate areas
bioturbation shows seasonal variation; for its quantification, year-round
studies are necessary. Little is known of seasonal variation in tropical areas. In
the Wadden Sea, bioturbation has considerably changed in historical times:
Gray whales and rays have disappeared due to fishing activities; geese feeding
on rhizomes of seagrass have disappeared with the disappearance of Zostera
due to a "wasting disease". Human impact on the sediment has increased due
to trawling and mechanical harvesting of shellfish and lugworms, now
disturbing the top 30 cm of the sediment. This long-term and still-increasing
human impact continues to change the Wadden Sea ecosystem, and such is
the future of many over-fished coastal areas in the world.
Acknowledgements. The author is very grateful for critical remarks by referees Herman
Michaelis and Akio Tamaki. This is NIOZ publication nr. 3486.