38
W. PuIs· T. Pohlmann· J. Siindermann
ern and the northern North Sea residual currents are typically in the order of 5
and 1 cm S-I, respectively. Maximum tidal currents are 1.2 m S-I. A comprehensive description of the hydrography of the North Sea is given by Otto et al.
(1990).
The suspended particulate matter (SPM) regime in the North Sea is described
by several authors, e.g. by Eisma and Irion (1988), Howarth et al. (1993) or Dyer
and Moffat (1998). SPM is supplied to the North Sea (a) from the adjacent seas,
above all the North Atlantic (mainly the Orkney-Shetland area) and the English
Channel via the Straits of Dover, (b) from cliff erosion, particularly along the
English coasts of East Anglia and Holderness, (c) from rivers and dumping, and
(d) from seafloor erosion. The major SPM flux out of the North Sea goes northward towards the North Atlantic via the Norwegian coastal current. Major SPM
sedimentation occurs in the Skagerrak and the Norwegian Trench.
In winter SPM consists of terrigeneous material plus detritus (10-20% by
weight). Typical (depth averaged) SPM concentrations during winter are 2-5 mg
1-1 and 0.2-1.0 mg 1-1 in the southern and the northern North Sea, respectively.
The approximate line separating the SPM regime of the North Sea into a northern and a southern part is between the Humber in the west and North Denmark
in the east. During summer, SPM concentrations in the northern North Sea may
increase significantly due to phytoplankton production with maximum phytoplankton concentrations in the order of 0.5 mg 1-1. In the southern North Sea
phytoplankton concentrations also increase to about 0.5 mg 1-1, but phytoplankton does not become dominant here because terrigeneous SPM (plus detritus)
concentrations are still in the order of 1-2 mg 1-1. The grain size of terrigeneous
SPM in the North Sea is typically in the clay fraction or in the fine silt fraction
(Eisma and Kalf 1979).
The temporal variability of SPM concentrations in the North Sea at a fixed site
and a fixed water depth is due to
- (tidal) advection of an SPM concentration gradient past the site, e.g. Konig et
al. (1994), Jones et al. (1994)
- cyclic resuspension and deposition by tidal currents, e.g. Jago et al. (1993)
- resuspension/erosion (and subsequent deposition) by storm induced waves
and currents (Green et al.1995)
- production and decay of phytoplankton, e.g. Reid et al. (1990)
- vertical diffusion and settling, e.g. Jones et al. (1994), PuIs et al. (1995).
The mostly pronounced variability of the SPM concentration in the shallow
southern North Sea is due to storm events. Konig et al. (1994), for instance,
measured an increase from 4 mg 1-1 to 20 mg 1-1 during a storm in the northern
part of the German Bight (sample depth: 10 m, total water depth: 20 m, bottom
sediment: sand).
The erosion of sediment during storms is an important transport mechanism
for particulate bound substances (contaminants) in the sediment. To give an example: the content of anthropogenic lead in the (fine) sediments of the German
Bight including the German Wadden Sea is estimated to be 35 000 tons (PuIs et
W. PuIs· T. Pohlmann· J. Siindermann
ern and the northern North Sea residual currents are typically in the order of 5
and 1 cm S-I, respectively. Maximum tidal currents are 1.2 m S-I. A comprehensive description of the hydrography of the North Sea is given by Otto et al.
(1990).
The suspended particulate matter (SPM) regime in the North Sea is described
by several authors, e.g. by Eisma and Irion (1988), Howarth et al. (1993) or Dyer
and Moffat (1998). SPM is supplied to the North Sea (a) from the adjacent seas,
above all the North Atlantic (mainly the Orkney-Shetland area) and the English
Channel via the Straits of Dover, (b) from cliff erosion, particularly along the
English coasts of East Anglia and Holderness, (c) from rivers and dumping, and
(d) from seafloor erosion. The major SPM flux out of the North Sea goes northward towards the North Atlantic via the Norwegian coastal current. Major SPM
sedimentation occurs in the Skagerrak and the Norwegian Trench.
In winter SPM consists of terrigeneous material plus detritus (10-20% by
weight). Typical (depth averaged) SPM concentrations during winter are 2-5 mg
1-1 and 0.2-1.0 mg 1-1 in the southern and the northern North Sea, respectively.
The approximate line separating the SPM regime of the North Sea into a northern and a southern part is between the Humber in the west and North Denmark
in the east. During summer, SPM concentrations in the northern North Sea may
increase significantly due to phytoplankton production with maximum phytoplankton concentrations in the order of 0.5 mg 1-1. In the southern North Sea
phytoplankton concentrations also increase to about 0.5 mg 1-1, but phytoplankton does not become dominant here because terrigeneous SPM (plus detritus)
concentrations are still in the order of 1-2 mg 1-1. The grain size of terrigeneous
SPM in the North Sea is typically in the clay fraction or in the fine silt fraction
(Eisma and Kalf 1979).
The temporal variability of SPM concentrations in the North Sea at a fixed site
and a fixed water depth is due to
- (tidal) advection of an SPM concentration gradient past the site, e.g. Konig et
al. (1994), Jones et al. (1994)
- cyclic resuspension and deposition by tidal currents, e.g. Jago et al. (1993)
- resuspension/erosion (and subsequent deposition) by storm induced waves
and currents (Green et al.1995)
- production and decay of phytoplankton, e.g. Reid et al. (1990)
- vertical diffusion and settling, e.g. Jones et al. (1994), PuIs et al. (1995).
The mostly pronounced variability of the SPM concentration in the shallow
southern North Sea is due to storm events. Konig et al. (1994), for instance,
measured an increase from 4 mg 1-1 to 20 mg 1-1 during a storm in the northern
part of the German Bight (sample depth: 10 m, total water depth: 20 m, bottom
sediment: sand).
The erosion of sediment during storms is an important transport mechanism
for particulate bound substances (contaminants) in the sediment. To give an example: the content of anthropogenic lead in the (fine) sediments of the German
Bight including the German Wadden Sea is estimated to be 35 000 tons (PuIs et