Marine: Cleaner Seas report
Natural Processes
Life in the sea is dominated by the way in which the sea currents move and the way in which the sea water separates into different layers ('stratification'). The main input into UK waters is from the Gulf Stream, which feeds into the North Sea through the English Channel and, even more, round the north of Scotland. Studies of water movements in the North Sea show that there is a long-term anti-clockwise circulation. On average it takes about 36 months for water in the German Bight to leave the North Sea, while water in the northern North Sea remains there for only about six months. In the shallower southern North Sea, fresh waters from rivers mix thoroughly with the salt water, but further north, the river water tends to remain as a distinct layer above the saltier sea water below. This can retain contaminants in the surface water and may stimulate unwanted growths of algae and so reduce the flow of oxygen to seabed life.
Surface current system of UK waters: studies of water movements show that there is a long-term anti-clockwise circulation in the North Sea. On average it takes about 36 months for water in the German Bight to leave the North Sea, whereas water in the northern North Sea remains for only six months.
Fish and shellfish
Fish and shellfish are the creatures we first think of when we consider
the inhabitants of the sea. The waters of the North East Atlantic support
potentially the most productive fisheries in the world and have done so
for centuries, but commercial fish species represent only a small part
of the marine ecosystem.
Benthos and plankton
Critical to the health of the sea are seabed (benthic) organisms and free-floating
microscopic plants and animals (plankton).
Life on the seabed (benthos)
There are estimated to be more than 15,000 benthic species in our seas
reflecting the wide range of environmental conditions around our coasts.
These can be indicators of environmental stress, but they do undergo natural
variations which are often hard to distinguish from the effects of human
activities.
Considerable research is continuing into this area. Present evidence shows that the effects of human industrial activities can only be detected in developed estuaries or specific locations such as dumping sites and oil platforms. These are very carefully monitored for harmful effects.
Schematic Section of the North Sea SW to NE
Layers of water: in the shallower southern waters of the North
Sea, fresh and salt waters mix together. In the northern North Sea, the
river water tends to remain as a distinct layer above the saltier sea
water below. This layer may retain contaminants in the surface water,
stimulating the growth of algae and so reducing the flow of oxygen to
seabed life.
Plankton
Plankton comprises a diverse assembly of free-floating plants (phytoplankton)
and animals (zooplankton) ranging in size from less than 0.0001mm to 60mm.
It constitutes a major source of food for the marine ecosystem. In recent
years there have been marked changes in plankton populations in UK coastal
waters and over much of the North East Atlantic.
Plankton blooms (rapid increases in the populations of certain species) have been observed around the UK coastline for hundreds of years. While most are not harmful, some plankton blooms can have an adverse effect on wild and farmed fish and shellfish, and benthic communities. Such blooms have been associated with a combination of climatic conditions and excessive levels of nutrients (eutrophication). However, there is insufficient evidence to confirm a link between blooms and land-based inputs of nutrients in UK waters.
Some blooms of naturally occurring plankton species are associated with the production of algal or bacterial toxins which, when concentrated in certain shellfish species, can lead to syndromes such as Paralytic Shellfish Poison and Diarrhetic Shellfish Poison. Regular monitoring gives early warning of this problem and protects public health.
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Birds
Britain's varied coast provides breeding and overwintering sites for more
than 70 species of waders, wildfowl and seabirds. Many of the sites are
of inter-national importance such as St Kilda, the Isle of May, the Wash
and the Mersey estuary.
Concern has arisen over variations in the breeding success of seabirds on some of the Northern Isles. There were signs in the late 1980s that puffins and Arctic terns, amongst others, were not finding enough prey, mainly sandeels, to feed their young. However, in the 1990s the situation has improved.
Mammals
The main marine mammals found in UK coastal waters are common and grey
seals, cetaceans (the harbour porpoise, dolphins and whales) and the otter
(mainly in the Hebrides and Shetland Islands).
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In 1988 common and grey seals were affected by a virus. The death of over 2,500 (mainly common) seals was reported and the cause identified as phocine distemper virus. As with most mammals, seals can be affected by very high levels of pollutants, but there is no evidence that pollution contributed to the outbreak of this disease. There has been no recurrence of the virus, but follow up studies continue.
Populations have recovered significantly with the 1996 population survey estimates indicating a minimum population of about 30,000 common and 112,000 grey seals in the United Kingdom.
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Wherever possible contaminant analyses are carried out to identify contaminant burdens in the animals. Unlike seals, whose populations are regularly monitored, when ashore, by the Sea Mammal Research Unit, it is extremely difficult to obtain population data about small cetaceans. DETR, MAFF (The Ministry of Agriculture, Fisheries and Food), SOAEFD (The Scottish Office Agriculture, Environ-ment and Fisheries Department) and their agencies are continuing to seek ways of monitoring cetacean populations and the causes of death of individuals.
Page last modified:
01 March 2005
Page published: 21 September 1988