258
P. Dion and S. Le Bozec
possibility would be to collect enough seaweeds from those already in the
water at the start of the proliferation period to prevent an outbreak of
green tides. However, this does not appear to be very feasible from a
technical and economic point or view. In addition, storage of collected
seaweed masses is an environmental problem in itself. Treatments such
as methanization or composting are now technically perfected (Briand
1989), but it is difficult to reach an industrial breakthrough.
At the present state of scientific progress, direct application of any
chemical or microbiological method, which would lead to growth inhibition or destruction of the green tide-forming plants, is not ecologically
acceptable.
After curative means, preventive means have to be considered. Of the
primary environmental factors controlling growth and mass development of the seaweeds, light and temperature, of course, cannot be altered. Undoubtedly, the only realistic preventive means to reduce green
tides is to lower nutrient levels in sensitive coastal areas. Three different
aproaches are possible, at least theoretically.
1. Physical Means. Collecting or rerouting nutrient-loaded waste water
has already been used on different scales to prevent coastal zone
pollution. This solution would be theoretically possible in some
locations of Brittany (e.g. Bay of Lannion), where river discharge is
comparatively small and could be canalized or piped out of the
sensitive zone. In the Bay of St. Brieuc, rerouting of tidal channels
would probably help decrease green tides to a certain extent (Piriou
et al. 1991).
Generally speaking, physical solutions have the disadvantage that they
can only be applied in particular cases. In addition, they require major
investments, and they do not decrease, but only transfer the pollution
problem (even if diluted) from one area to another, with unpredictable
long-term effects.
2. Preventive Means of the Biological Type. These could, for example,
include the cultivation of economically valuable seaweeds that would
compete with Ulva for nutrients, or the settlement of macrophyte
swamps or wetlands, the purifying effect of which is well known (cf.
Reddy and Smith 1987). Their disadvantage is that they would require
large land or sea areas, which are rarely available along the Brittany
coast.
The possibility of using wetlands in Brittany was first considered
(in 1990) for the Fremur river (Departement of Cotes d'Armor), which
has at least a potential of 80 ha for the wetland settlement. The estimated cost for such a project, however, was discouraging for the local
P. Dion and S. Le Bozec
possibility would be to collect enough seaweeds from those already in the
water at the start of the proliferation period to prevent an outbreak of
green tides. However, this does not appear to be very feasible from a
technical and economic point or view. In addition, storage of collected
seaweed masses is an environmental problem in itself. Treatments such
as methanization or composting are now technically perfected (Briand
1989), but it is difficult to reach an industrial breakthrough.
At the present state of scientific progress, direct application of any
chemical or microbiological method, which would lead to growth inhibition or destruction of the green tide-forming plants, is not ecologically
acceptable.
After curative means, preventive means have to be considered. Of the
primary environmental factors controlling growth and mass development of the seaweeds, light and temperature, of course, cannot be altered. Undoubtedly, the only realistic preventive means to reduce green
tides is to lower nutrient levels in sensitive coastal areas. Three different
aproaches are possible, at least theoretically.
1. Physical Means. Collecting or rerouting nutrient-loaded waste water
has already been used on different scales to prevent coastal zone
pollution. This solution would be theoretically possible in some
locations of Brittany (e.g. Bay of Lannion), where river discharge is
comparatively small and could be canalized or piped out of the
sensitive zone. In the Bay of St. Brieuc, rerouting of tidal channels
would probably help decrease green tides to a certain extent (Piriou
et al. 1991).
Generally speaking, physical solutions have the disadvantage that they
can only be applied in particular cases. In addition, they require major
investments, and they do not decrease, but only transfer the pollution
problem (even if diluted) from one area to another, with unpredictable
long-term effects.
2. Preventive Means of the Biological Type. These could, for example,
include the cultivation of economically valuable seaweeds that would
compete with Ulva for nutrients, or the settlement of macrophyte
swamps or wetlands, the purifying effect of which is well known (cf.
Reddy and Smith 1987). Their disadvantage is that they would require
large land or sea areas, which are rarely available along the Brittany
coast.
The possibility of using wetlands in Brittany was first considered
(in 1990) for the Fremur river (Departement of Cotes d'Armor), which
has at least a potential of 80 ha for the wetland settlement. The estimated cost for such a project, however, was discouraging for the local