Long term data series on Mediterranean Sea temperature, nutrients and hydrology changes
303
cm over the whole Mediterranean from dams across the Nile and Ebro rivers), to the
increase of evaporation (2 cm) and to the decrease of precipitation (3 cm), a decrease
observed around the whole sea (Bradley et al., 1987) and estimated to about 10% in
the north-western basin over the 1940-1995 period (Bethoux et al., 1998).
In order to increase the accuracy of previous estimates, it would be necessary
either to have a better estimate of climate change at the Mediterranean scale
(precipitation, air temperature), or to know the true water exchanges across the strait
of Gibraltar. For instance, meteorological data over the north western coast give air
temperature trends of23x I 0·3C yl over the 1960-1995 period. With this value, we
may calculate a corresponding sea surface temperature trend of 20xl0·' C yl, and
the resulting estimate of the increase in greenhouse effect is 2.2 W m· 2 instead of the
previous 1.7 W m o2 , calculated with respective trends of 7.3x10 3 C yl for surface
temperature and 8Ax I 003C yol for air temperature (Bethoux and Gentili, 1998).
2.2 Phosphate and Nitrate Increase, Silicate Stagnation
Analysis of available chemical data shows an increase of phosphate concentrations
in western deep water since the early sixties, an increase of nitrate since the
seventies, and a concomitant constancy of silicate concentration. Increases of
nutrient in deep waters mean increases of surface input via the atmospheric and
terrestrial inputs. Via the water circulation, the 0.5% per year increase for phosphate
and nitrate concentration may be linked to a 3% per year increase of surface inputs
(Bethoux et al., 1998), resulting from increasing inhabitants and industrial,
agricultural and urban activities around the sea, mainly since the early sixties
(UNEP, 1988).
Increases of phosphate and nitrate concentration in deep waters follow similar
increases in the rivers, like the Rhone and Po rivers. They are signatures of
increasing biological new production in the surface layer, without apparent problem
except at some hot spots of coastal eutrophication. Inversely, increase of human
activity do not increase the silicate load and even there may be a decrease due to the
dams on rivers. Nevertheless, increasing phosphate and nitrate concentration and
constant silicate concentration means a change in the molar ratio P:N: Si, i.e., a
probable change in the plankton distribution. Due to a rather long residence time of
deep waters in the eastern Mediterranean, nutrient ratio measured in the seventies
may represent a quasi-steady state prior to the anthropic effect. The Si:P molar ratio
was equal to 32 in the eastern basin, when in the western Mediterranean it was
equal to 24 in early 70s and 21 in 1994. From an important diatom community,
there is probably a shift towards non siliceous community, i.e., flagellates and
dinoflagellates. The plankton change will affect the upper level of biological
community, the anchovies and sardines, representing about 40% of the fishery,
subsist on copepods grazing the diatoms. Inversely, flagellates and dinoflagellates
promote gelatinous ecosystem, without fishery interest. Previous studies of ecosystem
change due to human activity concerned estuaries of great rivers (Rhine, Mississippi,
303
cm over the whole Mediterranean from dams across the Nile and Ebro rivers), to the
increase of evaporation (2 cm) and to the decrease of precipitation (3 cm), a decrease
observed around the whole sea (Bradley et al., 1987) and estimated to about 10% in
the north-western basin over the 1940-1995 period (Bethoux et al., 1998).
In order to increase the accuracy of previous estimates, it would be necessary
either to have a better estimate of climate change at the Mediterranean scale
(precipitation, air temperature), or to know the true water exchanges across the strait
of Gibraltar. For instance, meteorological data over the north western coast give air
temperature trends of23x I 0·3C yl over the 1960-1995 period. With this value, we
may calculate a corresponding sea surface temperature trend of 20xl0·' C yl, and
the resulting estimate of the increase in greenhouse effect is 2.2 W m· 2 instead of the
previous 1.7 W m o2 , calculated with respective trends of 7.3x10 3 C yl for surface
temperature and 8Ax I 003C yol for air temperature (Bethoux and Gentili, 1998).
2.2 Phosphate and Nitrate Increase, Silicate Stagnation
Analysis of available chemical data shows an increase of phosphate concentrations
in western deep water since the early sixties, an increase of nitrate since the
seventies, and a concomitant constancy of silicate concentration. Increases of
nutrient in deep waters mean increases of surface input via the atmospheric and
terrestrial inputs. Via the water circulation, the 0.5% per year increase for phosphate
and nitrate concentration may be linked to a 3% per year increase of surface inputs
(Bethoux et al., 1998), resulting from increasing inhabitants and industrial,
agricultural and urban activities around the sea, mainly since the early sixties
(UNEP, 1988).
Increases of phosphate and nitrate concentration in deep waters follow similar
increases in the rivers, like the Rhone and Po rivers. They are signatures of
increasing biological new production in the surface layer, without apparent problem
except at some hot spots of coastal eutrophication. Inversely, increase of human
activity do not increase the silicate load and even there may be a decrease due to the
dams on rivers. Nevertheless, increasing phosphate and nitrate concentration and
constant silicate concentration means a change in the molar ratio P:N: Si, i.e., a
probable change in the plankton distribution. Due to a rather long residence time of
deep waters in the eastern Mediterranean, nutrient ratio measured in the seventies
may represent a quasi-steady state prior to the anthropic effect. The Si:P molar ratio
was equal to 32 in the eastern basin, when in the western Mediterranean it was
equal to 24 in early 70s and 21 in 1994. From an important diatom community,
there is probably a shift towards non siliceous community, i.e., flagellates and
dinoflagellates. The plankton change will affect the upper level of biological
community, the anchovies and sardines, representing about 40% of the fishery,
subsist on copepods grazing the diatoms. Inversely, flagellates and dinoflagellates
promote gelatinous ecosystem, without fishery interest. Previous studies of ecosystem
change due to human activity concerned estuaries of great rivers (Rhine, Mississippi,