Geoscience Reference
In-Depth Information
2001; Department of Trade and Industry 2001),
which are localized hotspots of contamination by
hydrocarbons and a range of other compounds.
Contamination tends to be limited to within
a distance of about 500 m from most offshore
platforms, but in some areas hydrocarbon-
contaminated sediments extend to 2 - 8 km from
the platforms (OSPAR Commission 2000). In
other areas of the North Sea, dispersal has
occurred, preventing the formation of such cut-
tings piles. Effects on benthic communities have
been observed out to 3-5 km, although since
cuttings discharges were prohibited, the zones
of impact have decreased.
1961 dam, and the variable effect of oceano-
graphic processes is unknown, given the relatively
high input of land-derived sediment. Regarding
the potential effects of dams on the South Otago
shelf-coast system, Carter (1986) considers that
the key is whether the sediment being supplied
by a river has historically (i) become entrained
within the inner-shelf transport system, in which
case there would be further impacts, or (ii) has
tended to bypass the nearshore zone, in which
case the effect would be to reduce sediment trans-
port on the mid-shelf and reduce sand supply to
the adjacent shelf and coastline to the north.
10.3.2.2 Trawling
10.3.2 Anthropogenic disturbances on shelf
sedimentation
Globally, trawling may be the most intensive
of anthropogenic disturbances to the sea bed.
An estimated area equivalent to all the world's
continental shelves is trawled every 2 years
(Watling & Norse 1998), and in the North Sea,
total trawling effort is equivalent to the whole
North Sea being trawled at least seven times
a year. On the regional and local scale, how-
ever, trawling is very patchy. As an example,
the Dutch beam-trawl fleet visits some areas
of the North Sea over 400 times per year but
other areas not at all (Rijnsdorp et al. 1998;
Trimmer et al. 2005), and there is also signific-
ant within-year variation in overall trawling
effort. Trawling may mix and resuspend sur-
face sediments (those down to depths of a few
centimetres), can disturb sediment down to a
decimetre or more, can release nutrients into
the water column and can influence the benthic
biology ( Jennings & Kaiser 1998; Kaiser &
De Groot 2000), particularly surface dwellers.
A changed benthic biology can alter sediment
biogeochemistry because of changes in bio-
turbation and bio-irrigation. At some heavily
trawled sites in the North Sea, biogeochemical
processes in the upper layers of sediment, both
oxic and anoxic, may be unaffected by trawl-
ing in the long-term (Trimmer et al. 2005), but
in underlying anoxic sediments, mineralization
via sulphate reduction may be stimulated by the
extra disturbance, at least in areas where tidal
energy is weak.
10.3.2.1 Dammed rivers
The practice of building dams on rivers to secure
water supplies for humans or for hydroelectric
power plants has the general effect of decreasing
sediment supply down rivers, particularly of
bedload, with potential effects on the coastal
and nearshore zones (see also Case Study 8.3).
On the open shelf there is a decreasing relative
effect with distance down the transport path,
because shelf processes influence the river-borne
sediment supply. For the South Otago shelf,
New Zealand (section 10.2.3.3), over the past
9600 years, sediment input to the shelf has been
around 2.1
10
6
tyr
−1
of bedload, of which
around half accumulates in the shelf-sand
wedge and half is transported northwards to
adjacent shelf regions (Carter 1986). Virtually
all the 2.3
×
10
6
tyr
−1
of suspended load received
by the shelf is transported northwards along the
shelf or offshore to the adjacent continental
slope. On one of the three main supplying catch-
ments, the Roxburgh Dam built in 1961 on the
Clutha River traps 0.6
×
10
6
tyr
−1
of bedload,
which is around 30% of the total bedload
supply to the entire South Otago shelf - this is
likely to have affected sediment transport and/
or the development of the Holocene inner-shelf
sediment body. It is unknown whether the
coast-shelf sediment regime has adjusted to the
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