Although most scleractinian reef-forming corals occur in
tropical regions and in shallow water, there is a group of
scleractinian corals which can exist in water between 4 and 12 °C
and at depths from c 50 m to over 2,000 m. These corals do not have
symbiotic algae but are still able to form a hard skeleton. These
corals form colonies and can aggregate into patches and banks which
may be described as reefs. The most common cold-water coral is
Lophelia pertusa which has a global distribution but is
most common in the north-east Atlantic. Other cold-water coral
species include Madrepora oculata and Solenosmilia
variabilis and patches of coral often include more than one
species (Rogers 1999; Long et al. 1999).
In the UK, Lophelia pertusa has been found frequently
in small colonies from north of the Shetlands to the far west of
Rockall with the majority of the findings from Rockall westwards
(Wilson 1979). However, in some areas, larger colonies of coral
have been found, e.g. Hatton Bank, where sidescan sonar has
identified features c 30 m high as possibly being coral reefs, and
the Rockall Plateau (Long et al. 1999).
The most exciting discovery in UK waters so far is the Darwin
Mounds fields which were found using remote sensing techniques in
May 1998 during surveys funded by the oil industry and steered by
the industry-government group the Atlantic Frontier Environment
Network (AFEN) (Masson and Jacobs 1998). They have been further
investigated in June 1998 (Bett 1999), August 1999 (Bett &
Jacobs 2000) and twice during summer 2000 (B. Bett, pers. comm.).
Instruments deployed during the studies have included sidescan
sonar, stills and video cameras and piston corers.
The mounds are located at a depth of about 1000m in the
north-east corner of the Rockall trough, immediately south of the
Wyville-Thomson Ridge (see figure 1 below). The mounds, named after
the research vessel Charles Darwin, cover an area of
approximately 100 km² and contain some hundreds of mounds in two
main fields (referred to as Darwin Mounds East and Darwin Mounds
West. Other mounds are scattered at much lower densities in nearby
areas. Each of the mounds is approximately 100 m in diameter and 5
m high. Most of the mounds are also distinguished by the presence
of an additional feature visible on the side-scan sonar referred to
as a 'tail'. The tails are of a variable extent and may coalesce,
but are generally a teardrop shape and are orientated south-west of
the mound.
Figure 1: The
location of the Darwin Mounds in UK offshore waters.
The mounds are comprised mostly of sand, interpreted as 'sand
volcanoes'. These features are caused when fluidised sand
'de-waters' and the fluid bubbles up through the sand,
pushing the sediment up into a cone shape. Sand volcanoes
are common in the Devonian fossil record in UK, and in seismically
active areas of the planet. In this case, tectonic activity is
unlikely; some form of slumping on the south-west side of the
Wyville-Thomson Ridge being a more likely cause. The tops of the
mounds have living stands of Lophelia and blocky rubble
(interpreted as coral debris) which are visible in the photos
below.
The mounds also support significant
populations of the xenophyophore Syringammina
fragilissima. This is a large (15 cm diameter) single-celled
organism that is widespread in deep waters, but occurs in
particularly high densities on the mounds and the tails. The corals
themselves provide a habitat for various species of larger sessile
or hemi-sessile invertebrates such as sponges and brisingiids.
Various fish have been observed, but not apparently at
significantly higher densities than the background environment
(Bett & Jacobs 2000).
Photos taken from 1999 survey
of the Darwin Mounds showing live coral, dead coral rubble and some
associated fauna.
Images courtesy of Brian Bett, Southampton Oceanographic
Centre. Copyright © DTI
The mound-tail feature of the Darwin Mounds is apparently
unique globally. The mounds are also unusual in that
Lophelia appears to be growing on sand rather than a hard
substratum. Prior to research on the mounds in 2000, it was thought
that Lophelia required a hard substratum for
attachment.
Cold-water corals are vulnerable to damage from towed fishing
gear. Even the very large stands off Norway have been impacted
(Fosså et al. 1999), and Norway has consequently
designated its most important reefs as reserves and banned
towed-gear fisheries from within them. The largest reef is some 100
km from land, and thus is one of the first protected areas in
European waters outside territorial waters. The Darwin Mounds are
also vulnerable, and evidence of new damage was visible over about
a half of the Darwin Mounds East during summer 2000. This damage
was visible as smashed coral strewn on the seabed. A trawler was
operating nearby during the surveys. Given that Lophelia
appears to need (or favour) the elevation provided by the sand
volcanoes for growth in this area, it seems likely that this damage
will be permanent.
The Darwin Mounds are now protected by a permanent ban
on bottom trawling through an EC regulation. The UK is in the
process of designating the site as a Special Area of Conservation
for Reef habitat. Further information can be found by
following the links below.
Related External Links:
References:
Bett, BJ & Jacobs,
CL (2000) RRS Charles Darwin cruise 119C leg B, 13
August - 24 September, 1999. White Zone (WhiZ) Environmental
survey: Seabed survey of the deep waters to the north and west of
Shetland. Southampton Oceanography Centre Cruise report.
Report to the UK Department of Trade and Industry.
Fosså, JH, Furevik, DM, Mortensen, PB & Hovland, M
(1999) Effects of bottom trawling on Lophelia deep water
coral reefs on the continental shelf of Norway. Poster and
abstract in ICES/SCOR Symposium on Ecological Effects of Fishing.
Montpellier, France, 15-19 March 1999.
Long, D, Roberts, JM & Gillespie, EJ (1999)
Occurrences of Lophelia pertusa on the Atlantic
margin. British Geological Survey Technical Report
WB/99/24
Masson, DG & Jacobs, CL (1998) RV Colonel Templar
cruises 01 and 02/98, 22 Apr - 18 May, 20 May - 18 Jun 1988.
TOBI surveys of the continental slope north and west of
Scotland. Southampton Oceanography Centre Cruise Report. AFEN
UKCS 17th Round Atlantic Margin Environmental Survey Data
CD-ROM.
Rogers, AD (1999) The biology of Lophelia
pertusa (Linnaeus 1758) and other deep-water reef-forming
corals and impacts from human activities. International Review
of Hydrobiology, 84(4) 315-406
Wilson JB (1979) The distribution of the coral
Lophelia pertusa (L.) [L. prolifera
(Pallas)] in the North East Atlantic. Journal of the Marine
Biological Association of the UK, 59,
149-164