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THE GIRDLE of shallow water that stretches from the beach to the edge
of the continental shelf could be the key to the planet’s survival in the
changing climate. This narrow ribbon of sea makes up only 10 per cent of
the area of the oceans and perhaps only 0.5 per cent of their volume. But
it is a vital zone: it is the most productive part of the sea, and so it
plays a crucial part in the global carbon cycle.

The coastal zone is also the most used and abused part of the oceans.
We mine it for fish and fossil fuels; we pollute it with sewage and other,
more noxious, pollutants; and we still like to sit by it, swim in it and
build our homes and hotels by it. On the whole, the coastal zone has tolerated
most of the abuse. But there are growing signs that it has had enough.

Last summer, tourists fled the beaches of the Adriatic in the face of
a stinking green slime – a bloom of diatoms in a cloud of mucus, encouraged
in part by the massive outflow of nutrients from Italy’s rivers. The year
before, Norway mounted its biggest peacetime naval operation – towing salmon
cages out to sea, out of the path of a bloom of toxic algae that was spreading
along the coasts of Denmark and Sweden.

1988 was also the year when huge numbers of dolphins died along the
East Coast of the US. Some biologists suspect that the appearance of a poisonous
dinoflagellate might have been to blame. All around the world, red tides,
glutinous green slimes and filthy looking froths of yellow foam, caused
by the colonial alga Phaeocystis, have become more frequent.

This is a warning, said Peter Liss, of the University of East Anglia,
at a recent Dahlem conference in Berlin. It is a sign that we ought to treat
the ocean margins with care. Blooms are a sign of increasing eutrophication,
a process well known in lakes. Large additions of nutrients change the productivity
of the water and encourage certain species of algae to proliferate at the
expense of others, altering the balance of the community. After the bloom
dies, the bacteria that break down the huge population of algae proliferate
and consume the oxygen in the water.

In extreme cases the bacteria use up so much of the oxygen that other
organisms suffocate – sometimes causing mass mortality of fish in spectacular
‘fish kills’. The seas, much larger and open to the flushing effects of
the ocean, have tolerated massive inputs of nutrients, but there are signs
that eutrophication is spreading.

Globally, agricultural runoff, industrial and human waste and so on
add at least as much nutrient to the seas as do natural processes. And there
is evidence that both the frequency and the scale of algal blooms has increased
along with the nutrients. The increase in nutrients and associated changes
in algal communities could alter the way that carbon cycles in the coastal
zone.

Eutrophication can have alarming consequences, but it is just one of
the results of changing the chemistry of coastal waters. Toxic pollutants
– heavy metals, pesticides, hydrocarbons and radionuclides – are accumulating
in the sediments of coastal seas. They can stay buried in sediments, out
of harm’s way, or they can be devastating. Tributyl tin or TBT, the toxic
component of most popular antifouling paints until recently, is poisonous
to marine life in tiny concentrations. At parts per trillion it induces
in whelks a phenomenon called imposex, in which female whelks change into
males.

This is another warning shot, says Ed Goldberg of Scripps Institution
of Oceanography in California. ‘If TBT works on this sort of scale, what
else does?’ he asks. The chemicals industry makes many new compounds each
year that are alien to nature. Many of them escape to the environment, yet
few have been studied in the aquatic environment.

Chemical changes to coastal waters produce some of the most immediate
and obvious effects. The concern is how far we can continue adding chemicals
before the system reaches the point of no return and instead of just steadily
growing worse – more toxic or more eutrophic – flips into a new state, out
of control.

Liss calls these additions to the system ‘chemical bombs’, which have
the potential to cause disaster. Anoxia is an example of a process involving
a flip from one state to another: initially the concentration of oxygen
declines, but once it reaches a critical low, the whole chemistry and ultimately
the biology of the water changes – with spectacular results such as fish
kills.

Changes to the physical nature of the coastline are clear to see, but
their knock-on effects in the coastal zone may be less obvious. Both engineering
works on the coast and the removal of coastal habitats such as mangroves
and salt marshes alter patterns of circulation and the deposition and movements
of sediments in inshore waters.

More important still, management of the great rivers – changing their
flow and withdrawing much of their water en route to the sea – has a huge
knock-on effect at the coast. The water near the shore becomes more saline.
Many inshore species, particularly those in mangroves and estuaries need
brackish, rather than full-strength sea water. Some species, such as shrimps,
oysters, mussels and certain fish that migrate between salt and freshwater
need brackish water at some time in their life cycles.

Reducing the flow of water from reservoirs also impairs the flushing
capacity of the river. Damming the Nile killed off the traditional sardine
fishery in the eastern Mediterranean, which depended on the influx of nutrients
and subsequent blooming of algae off the mouth of the river. Like the Nile,
the Indus is almost entirely diverted for irrigation of agricultural land.
The Danube, Rhine and Colorado are even more thoroughly harnessed.

According to Yousseff Halim, of the University of Alexandria in Egypt,
by the year 2000 about two-thirds of the world’s total flow of water to
the ocean margins will be controlled by dams. If climate changes and the
world warms significantly, and many prime agricultural lands begin to dry
out, this proportion is likely to increase. In the past 30 years, the reduction
in river discharges worldwide is equivalent to a drop in sea level of 0.7
millimetres a year.

Other processes upstream are also felt in the coastal zone. Turning
forested land to fields increases the load of nutrients, organic matter
and trace metals in the water. The load of silt deposited in the coastal
zone may have increased tenfold since the early days of agriculture. An
increase in the amount of silt suspended in coastal waters can push the
most productive zone offshore, where the water is clearer, changing the
nature of the coastal communities.

In the next century, the ocean margins will come under increasing pressure
from human activities, in turn putting pressure on human health. As populations
grow, the outflow of waste will grow – sewage in particular. The latest
report from the United Nations Environment Programme (UNEP) on the state
of the marine environment pinpoints the spread of enteric viruses and bacteria
as one of the most important problems in the coastal zone.

Oil and gas extraction are also likely to spread as countries develop.
Coastal engineering will increase as new resorts are built and coastal defences
spring up to protect low-lying nations from the rising sea.

One activity that is burgeoning in coastal waters and is likely to spread
to developing countries is marine fish farming. Mariculture depends on clean
water, but is itself highly polluting. Excess food falls from fish cages
and encourages eutrophication. At the same time, pesticides intended to
rid the fish of parasites kill other invertebrates in the surrounding area.

More invidious still is the application of antibiotics to control bacterial
disease. No one knows what effect such drugs have on the bacterial flora
of local waters. Worse, such chemicals, which are potent in tiny concentrations,
will be carried around in coastal currents.

Many of these activities conflict, which will lead to some self-regulation
by those involved. But what one country decides to do in its coastal waters
will affect its neighbours along the coast too. The UNEP report on the marine
environment concludes that the margins of the sea ‘are affected by man almost
everywhere, and encroachment on coastal areas continues worldwide’. If unchecked,
the report says, ‘this trend will lead to global deterioration in the quality
and productivity of the marine environment’.

‘The ocean is a world resource and there must be some limit to what
you can do to it,’ says Goldberg. The only effective way to ensure that
coastal seas remain healthy is to manage them. ‘We can’t achieve a pristine
environment however much we want it, we must accept that it will be managed,’
said John Steele, of the Woods Hole Oceanographic Institution in Massachusetts.
Within 50 years, says Goldberg, the whole coast will be managed, except
for a few areas set aside as reserves.

But management by individual countries will not be enough. What is needed
is an International Clean Ocean Act, according to Ken Hsu, of the Institute
of Geology in Zurich in Switzerland. Such an act would have internationally
recognised powers to regulate what individual nations do in their own coastal
zone.

Whoever is responsible faces a difficult task. ‘The assumption is that
if we deal with problems on a local scale, global problems will go away,’
said Tom Church, of the University of Delaware. But ocean scientists still
know little about the processes taking place in the water and sediments
– even in nearshore waters. They need to pin down the physics and geochemistry
of the margins, and how they affect the biology. They need to measure natural
fluxes of carbon, nitrogen and sulphur, before they can say how human activities
are disturbing the workings of the system – and how resilient it is.

With so little knowledge about the natural processes at the ocean margins,
it is almost academic to consider what will happen as the climate changes.
But the coastal zone plays an important part in mediating climate change.
As the most productive part of the ocean, it absorbs huge amounts of carbon
dioxide. Just how much of that it locks away is the subject of much debate.

Some geochemists calculate that 750 million tonnes of anthropogenic
carbon may be buried in shelf sediments, about 15 per cent of all the carbon
emitted by human activities. ‘The shelf is a site where large amounts of
carbon could be secreted away without anyone knowing about it,’ said Henry
Blackburn, of the University of Aarhus in Denmark. ‘These sinks are in the
end the most important sinks,’ said Chris Martens of the University of North
Carolina. ‘That’s why we make the case that the coastal margins are important
in the global carbon cycle.’

Oceanographers and climatologists have made some tentative guesses at
what might happen to the ocean margins as climate changes. As global temperature
increases the atmosphere will have a greater capacity to hold water. The
hydrological cycle will be more intense. More rain will leach more nutrients
from the soil and will weather rocks and soil more effectively, washing
out inorganic chemicals, and flushing organic material, fertilisers and
pesticides from the soil.

If the sea level rises, erosion of the coast will wash still more sediments
into the sea. Most of these will come from low-lying coastal flats and swamps
that are rich in organic material and dissolved nutrients.

At the same time, climatologists believe that there will be a much smaller
difference in the temperatures at the equator and the poles, which will
reduce the intensity of meridional winds (north-south). One result will
be a change – probably a reduction – in the intensity of upwelling, so that
the nutrients that are normally transported from deep water stay in the
depths. Good models are needed in order to say what will happen in the coastal
zone – but modellers need to have an idea about what will happen there in
order to improve their models.

Changes in patterns of wind can also affect coastal waters, but meteorologists
are not sure whether winds will increase, or decrease. If winds increase,
the normal layers of water of different densities will be better mixed,
drawing up nutrients from greater depths and increasing productivity and
perhaps helping to mop up more carbon dioxide. On the other hand, if the
wind is weaker, the opposite will be true.

Eutrophication might consume more carbon dioxide, but it might also
lead to the death of reefs and communities that live on the seabed, and
which sequester carbon in the form of calcium carbonate. ‘Eutrophication
will switch off production of inorganic carbon in shallower waters, accelerating
the dissolution of carbonate back into the water,’ maintains Fred Mackenzie
of the University of Hawaii. Inorganic carbon must be included in models
of carbon cycling, he stressed.

Indirect effects of climate change include possible changes in the patterns
of ocean circulation and shifts in the paths of major currents. ‘The combination
of habitat loss, chemical modification of the ocean and shifts in climate
will lay stress on populations of organisms of types we don’t know much
about,’ says Patrick Holligan, of the Plymouth Marine Laboratory.

Whatever steps are taken to control activities in the coastal zone –
whether nationally or internationally – engineers and scientists should
already be looking further ahead, says Philip O’Kane, of the Centre for
Water Resources Management in Dublin, Ireland. They must begin to design
entirely new technologies, clean, closed-cycle technologies and ways to
recycle waste, he suggests. ‘The path to a sustainable biosphere is to radically
redesign production so that it becomes ‘environmentally benign’ – neither
using up its resources nor contaminating it with waste products.’

This article is based on the Dahlem Conference on Ocean Margin Processes
in Global Change, held in Berlin, 18-22 March 1990. The results will be
published later this year by John Wiley & Sons.