262
P. Dion and S. Le Bozec
development of Enteromorpha sp. is rapid in June in the intertidal zone,
where populations are attached to the substratum, partly in the Zostera
beds. During this period, the size of the plants increases, until they are
torn off from their substrata, then invading the water body where they
continue to grow free-floating. This phenomenon usually ends in October. Such proliferation can lead to various detrimental effects including
anoxia and fish mortality.
Another green alga has become the dominant species in the bay since
1989; it was identified as Monostroma obscurum (Auby et al. 1994). The
period of maximum biomass development is between January and April, i.e.
before the bloom or Enteromorpha sp. The vegetative propagation of the
unattached thalli is probably the principal way of producing large biomasses, in the subtidal channels and in the shallow water of the intertidal
zone. During other periods, significant quantities of algae can be maintained
in the water, and they do not completely disappear in autumn and winter.
For a few years, the filamentous red alga Centroceras clavatum was also
observed to produce blooms sporadically in the Bay of Arcachon.
Concerning Monostroma at least, nitrogen is considered the limiting nutrient.
9.3.2 Eutrophication and Salt Marshes
Salt marshes of the French Atlantic coast cover about 30000 ha from the
Gulf of Morbihan (southern Brittany) to the Bay of Arcachon (Aquitaine)
and extend mainly in the Regions Pays de Loire and Poitou-Charentes
(Fig. 9.7). They consist of complex hydraulic systems of channels and
ponds, which used to be exploited by the salt industry in the past and are
partly utilized for shellfish aquaculture today.
The natural productivity of these systems is very high and macro algal
blooms occur frequently in the ponds. Ulva spp. and Enteromorpha spp.
are the main species responsible for the mass blooms, but other green
seaweeds (Cladophora spp., Ulothrix sp., Chaetomorpha spp.), as well as
brown algae (Colpomenia peregrina, some members of Ectocarpaceae) or
red seaweeds (Gracilaria verrucosa) also invade the ponds depending on
the localities, the seasons and the ponds' management procedures. Such
macro algal blooms can cause dystrophic conditions in the ponds, but
also may compete for nutrients with phytoplankton, the food base for
shellfish aquaculture. The removal of the seaweeds uses up a considerable proportion of the working time and efforts of the aquaculturists.
The development of eutrophication and pollution by trace elements
has recently received attention, particularly in areas where intensive agriculture has spread to the buffer wet zone besides the salt marshes.
P. Dion and S. Le Bozec
development of Enteromorpha sp. is rapid in June in the intertidal zone,
where populations are attached to the substratum, partly in the Zostera
beds. During this period, the size of the plants increases, until they are
torn off from their substrata, then invading the water body where they
continue to grow free-floating. This phenomenon usually ends in October. Such proliferation can lead to various detrimental effects including
anoxia and fish mortality.
Another green alga has become the dominant species in the bay since
1989; it was identified as Monostroma obscurum (Auby et al. 1994). The
period of maximum biomass development is between January and April, i.e.
before the bloom or Enteromorpha sp. The vegetative propagation of the
unattached thalli is probably the principal way of producing large biomasses, in the subtidal channels and in the shallow water of the intertidal
zone. During other periods, significant quantities of algae can be maintained
in the water, and they do not completely disappear in autumn and winter.
For a few years, the filamentous red alga Centroceras clavatum was also
observed to produce blooms sporadically in the Bay of Arcachon.
Concerning Monostroma at least, nitrogen is considered the limiting nutrient.
9.3.2 Eutrophication and Salt Marshes
Salt marshes of the French Atlantic coast cover about 30000 ha from the
Gulf of Morbihan (southern Brittany) to the Bay of Arcachon (Aquitaine)
and extend mainly in the Regions Pays de Loire and Poitou-Charentes
(Fig. 9.7). They consist of complex hydraulic systems of channels and
ponds, which used to be exploited by the salt industry in the past and are
partly utilized for shellfish aquaculture today.
The natural productivity of these systems is very high and macro algal
blooms occur frequently in the ponds. Ulva spp. and Enteromorpha spp.
are the main species responsible for the mass blooms, but other green
seaweeds (Cladophora spp., Ulothrix sp., Chaetomorpha spp.), as well as
brown algae (Colpomenia peregrina, some members of Ectocarpaceae) or
red seaweeds (Gracilaria verrucosa) also invade the ponds depending on
the localities, the seasons and the ponds' management procedures. Such
macro algal blooms can cause dystrophic conditions in the ponds, but
also may compete for nutrients with phytoplankton, the food base for
shellfish aquaculture. The removal of the seaweeds uses up a considerable proportion of the working time and efforts of the aquaculturists.
The development of eutrophication and pollution by trace elements
has recently received attention, particularly in areas where intensive agriculture has spread to the buffer wet zone besides the salt marshes.