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J. Romero et al.
6 )lmol dm -3 of soluble reactive phosphorus, which indicates high uptake rates at unusually high phosphorus concentrations. Nutrient addition after a starvation period can increase the total phosphorus content
of the plant by a factor of 10, while the nitrogen content is only increased
by a factor of 2. It is likely that this species benefits from very short
episodes of high phosphorus concentration (Vidal 1991).
In short, conditions for major green macro algal accumulation occur
only close to the discharge channels, which provide the nutrients needed
for fast growth, and in the inner part of the bay, where water movement
is more restricted and waters and shallow ( < 1 m depth). From observations in the Fanger Bay similar results can be concluded (Fig. 12.2).
However, these proliferations do not appear to be much more frequent or with higher biomasses than in the recent past. We have no
reliable data on the biota of the bay before 1986, but it seems (on the
basis of aerial photographs) that no changes have occured in seagrass
cover during the last 15 years. As stated by Niell et al. (Chap. 10, this
Vol.), algal cover interferes with light adsorption by seagrasses; since
Cymodocea nodosa has a light compensation point of 10 - 35 )lE m -2 S-1
(Perez and Romero 1992), a cover of two Ulva blades would reduce the
light intensity at the canopy level enough to cause the disappearance of
the seagrass. Thus, we conclude that the system, which is in equilibrium
between the three main groups of primary producers, has been resistant
to recent changes in nutrient inputs. Such changes have not been dramatic, although the water quality of the Ebro River has deteriorated in
the last 30 years, but the rice culture has remained more or less unchanged since the beginning of the century. However, care must be taken
in not increasing nutrient loads or decreasing the exchange rate between
the bay and the open sea. An imbalance in the present ratio of the
productivity of phytoplankton, rooted macrophytes and macro algae
could drive the system to severe eutrophic events, either through phytoplankton blooms or mass development of green algae.
12.4 Green Tides in the Mediterranean:
A Conceptual Framework and Future Goals
In summary, excessive growth of green macro algae does not occur unless
at least three conditions are met: (1) high nutrient loading; (2) limited
freshwater dilution; (3) suitable (hard) substrata. For massive proliferation, two additional conditions are needed; (4) calm waters which, apart
J. Romero et al.
6 )lmol dm -3 of soluble reactive phosphorus, which indicates high uptake rates at unusually high phosphorus concentrations. Nutrient addition after a starvation period can increase the total phosphorus content
of the plant by a factor of 10, while the nitrogen content is only increased
by a factor of 2. It is likely that this species benefits from very short
episodes of high phosphorus concentration (Vidal 1991).
In short, conditions for major green macro algal accumulation occur
only close to the discharge channels, which provide the nutrients needed
for fast growth, and in the inner part of the bay, where water movement
is more restricted and waters and shallow ( < 1 m depth). From observations in the Fanger Bay similar results can be concluded (Fig. 12.2).
However, these proliferations do not appear to be much more frequent or with higher biomasses than in the recent past. We have no
reliable data on the biota of the bay before 1986, but it seems (on the
basis of aerial photographs) that no changes have occured in seagrass
cover during the last 15 years. As stated by Niell et al. (Chap. 10, this
Vol.), algal cover interferes with light adsorption by seagrasses; since
Cymodocea nodosa has a light compensation point of 10 - 35 )lE m -2 S-1
(Perez and Romero 1992), a cover of two Ulva blades would reduce the
light intensity at the canopy level enough to cause the disappearance of
the seagrass. Thus, we conclude that the system, which is in equilibrium
between the three main groups of primary producers, has been resistant
to recent changes in nutrient inputs. Such changes have not been dramatic, although the water quality of the Ebro River has deteriorated in
the last 30 years, but the rice culture has remained more or less unchanged since the beginning of the century. However, care must be taken
in not increasing nutrient loads or decreasing the exchange rate between
the bay and the open sea. An imbalance in the present ratio of the
productivity of phytoplankton, rooted macrophytes and macro algae
could drive the system to severe eutrophic events, either through phytoplankton blooms or mass development of green algae.
12.4 Green Tides in the Mediterranean:
A Conceptual Framework and Future Goals
In summary, excessive growth of green macro algae does not occur unless
at least three conditions are met: (1) high nutrient loading; (2) limited
freshwater dilution; (3) suitable (hard) substrata. For massive proliferation, two additional conditions are needed; (4) calm waters which, apart