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The Mediterranean Beckons to Europe's Oceanographers

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Science  23 Jan 1998:
Vol. 279, Issue 5350, pp. 483-484
DOI: 10.1126/science.279.5350.483

The Mediterranean Sea is critical to the economies and environments of the 20 countries bordering it, but studies of its circulation and biology have been patchy and uncoordinated. Four years ago, the European Union launched a Mediterranean Targeted Program of joint research projects, and researchers met in Rome recently to discuss what they've learned in the program's first phase. They outlined a picture of an ocean that may be acutely sensitive to environmental change.

Too Much Salt in the Sea

Small and nearly isolated from the stabilizing influences of the waters beyond the Straits of Gibraltar, the Mediterranean is among the most vulnerable of the oceans to human activity. As results discussed at the Rome meeting showed, environmental change is affecting not only the Mediterranean's temperature and composition but even its grand circulation pattern—a change that could have global impacts.

Although many researchers suspect that the temperature of the world's oceans is increasing, only in the Mediterranean has a warming of deep waters, perhaps in response to the overall warming of the globe, been clearly detected. In measurements over the past 40 years, at depths of 2000 meters to 2600 meters in the northwest Mediterranean, Jean-Pierre Béthoux and his colleague Bernard Gentili of the Oceanography Observatory at Villefranche-sur-Mer, France, detected a 0.13-degree rise. They also found a signal that reflects environmental changes nearer to home: a small increase in salinity.

To account for the increase in salt levels, the two researchers analyzed changes in the sea's water budget in recent years. They found that the construction of key dams, notably the Aswan High Dam on the Nile in Egypt and a dam on the Ebro in Spain, has drastically reduced the flow of fresh water into the sea since the 1940s. Sea level might have dropped by 10 centimeters—if additional inflow of salty water from the Atlantic and from the even saltier Red Sea had not made up the deficit. That has led to a general increase in the salinity of Mediterranean seawater at most levels. And because salt boosts water density, a change in salinity can alter circulation patterns, says Béthoux.

Wolfgang Roether of the University of Bremen in Germany and his colleagues may have detected signs of those changes, in the eastern Mediterranean. They found hints that rising salinity has altered a vertical circulation pattern in which salty, dense surface water sinks into the depths. Previous studies had suggested that the process takes place mainly in the Adriatic Sea. But earlier this decade, the team detected a huge input of water from the Aegean region into the depths, probably because of the salinity increase.

Other researchers think that rising salt levels in a middle layer of Mediterranean water formed south of Greece pose a threat of another, more worrisome circulation change—this one in the Atlantic. This middle layer, called the Levantine Intermediate Water, spreads uniformly throughout the Mediterranean and forms around 80% of the water that leaves the sea through the lower levels at the Straits of Gibraltar. Saltier than the Atlantic, the former Mediterranean water flows west and influences the circulation of the North Atlantic. The salty plume helps shape the course of the Gulf Stream, which carries heat northward to Europe.

The increasing saltiness of the Mediterranean plume, some researchers fear, could somehow affect this interaction. Robert Johnson of the University of Minnesota believes that the saltier Mediterranean water might deflect the Gulf Stream westward toward the Labrador Sea, drastically cooling northern Europe. Eelco Rohling of the University of Southampton in the U.K. thinks the Mediterranean water could have the opposite effect, pushing the Gulf Stream farther toward Europe and turning up the heat there. Either way, says Béthoux, “the potentially hemispherewide effects highlight the worryingly broad impact that changes in the Mediterranean may have on climate.”

Well-Watered Desert

The eastern Mediterranean may not look like a desert, but to a marine biologist's eye, it is one of the most impoverished regions in the world ocean. At the Rome meeting, researchers reported that the scarcity of nutrients in the sea's eastern end has skewed its ecology, favoring bacteria, which are more efficient than larger organisms at exploiting the microscopic green algae—phytoplankton—at the base of the food chain.

Rising indicators.

Temperature and salinity at depths of 2000 to 2600 meters in the northwestern Mediterranean.

BÉTHOUX ET AL.

That could help explain why fishing grounds in the eastern Mediterranean are notoriously sparse. “Dominance of the lower part of the food web in the east [by bacteria] is of particular economic significance,” says Carol Turley of the Plymouth Marine Laboratory in the United Kingdom, leader of one group carrying out the studies.

Nutrients are scarce in the Mediterranean, compared with the rest of the world ocean, because the sea's main input comes from the surface waters in the Atlantic, which flow in through the Straits of Gibraltar. Atlantic plankton have already depleted these waters of nutrients, and the nutrient supply in the open sea declines further as the Atlantic water moves east, in spite of the nutrient-rich pollutants discharged by major rivers. Turley and her colleagues have found that the phytoplankton are on average only one-third as abundant in the Mediterranean's eastern basin as in the west. And bacteria consume a far greater proportion of this plant material in the eastern Mediterranean than in the west.

When Turley and colleagues analyzed phytoplankton and bacterial growth in shipboard experiments, they found that 55% of phytoplankton production flows to the microbial food web in the west; in the east, this figure rises to 85%. “The fierce competition for scarce nutrients favors the smaller organisms, which are able to utilize them most rapidly,” explains Frede Thingstad of the University of Bergen in Norway.

Because bacteria hog the phytoplankton, other organisms are disproportionately scarce. Fish production is just a third of that in the western basin. And because the bacteria also degrade organic detritus, the rain of organic particles into deep waters from the sunlit layers above is nine times lower than in the western basin, starving bottom-dwelling organisms. Their biomass is 46 times lower than in the western Mediterranean, according to Turley's analysis of experimental results.

At the National Center for Marine Research in Athens, Efstathios Balopoulos and his colleagues have traced a similar picture in the Aegean Sea, part of the eastern Mediterranean, finding that bacteria account for more than 56% of the organic particles in shallow layers. And because the bacteria consume nearly all the sinking waste matter, he found that the deep Aegean is one of the most meager habitats anywhere in the world ocean.

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