The French Atlantic Coasts
255
9.2.1 Production Mechanism of Green Tides
The term green tide generally refers to drifting Ulva concentrated along
sandy portions of the shoreline. The species involved occur also attached
to rocky and gravel substrates of the lower intertidal zone. These attached populations are the primary source of green tides on the coasts of
Brittany, but there is now evidence that they are not of preponderant
importance in the annual formation of the phenomenon. In fact, attached
populations of these species are not particularly abundant around affected areas. In addition, no whole individuals or basal parts with their
holdfast can be found in the floating green tide material, which would
indicate a recent removal from substrate and direct transport. It is likely
that material transfer from attached to free-living populations is small.
The bulk of the green tide production occurs in the floating phase,
through vegetative reproduction. In spring, the source of the green tide
is most probably a remnant of Ulva from the preceding year.
Mass development of green seaweeds in other coastal areas of Europe
and all over the world is generally attributed to an increase in the
nutrient concentrations in coastal waters. In most cases, green tides can
be directly linked to that of waters collected by drainage basins. The
most responsible nutrients are nitrogen and phosphorus compounds.
On the Brittany coast, enclosed areas, such as the inner parts of bays,
where water exchange with the sea is slow, are particularly sensitive to
green tide outbreaks (Menesguen and Salomon 1988; Piriou et al. 1991,
1993). Despite the high tidal amplitude of 6-9 m, residual drift of the
waterbody between tides is small, as shown in the Bay of St. Brieuc
(Menesguen and Salomon 1988). Drifting seaweed and nutrients are
therefore trapped together in these areas. Breaking swell, which usually
occurs in large intertidal areas with a low slope, probably takes part in
concentrating the seaweeds in shallow nearshore coastal waters. Keeping
the seaweeds in suspension in this enriched, mixed, easily warmed up
and well lighted environment favours rapid growth and vegetative multiplication through fragmentation.
Out of the eight species of Ulva that it is possible to identify around
the coast of Brittany (Hoeksema and Van den Hoek 1983), only one is
involved in large-scale green tide processes. The systematic position
of this species is not clear, and a new taxonomic nomenclature will
probably have to be put forward for it. Its predisposition to react to
eutrophicating conditions at the end of bays seems to be based on
particular biological and physical characteristics (Dion 1990).
Ulva species in general, and the Ulva form producing mass blooms on
Brittany coasts in particular, have a high photosynthetic potential, allowing efficient use of the summer light conditions for growth (Table 9.1).
255
9.2.1 Production Mechanism of Green Tides
The term green tide generally refers to drifting Ulva concentrated along
sandy portions of the shoreline. The species involved occur also attached
to rocky and gravel substrates of the lower intertidal zone. These attached populations are the primary source of green tides on the coasts of
Brittany, but there is now evidence that they are not of preponderant
importance in the annual formation of the phenomenon. In fact, attached
populations of these species are not particularly abundant around affected areas. In addition, no whole individuals or basal parts with their
holdfast can be found in the floating green tide material, which would
indicate a recent removal from substrate and direct transport. It is likely
that material transfer from attached to free-living populations is small.
The bulk of the green tide production occurs in the floating phase,
through vegetative reproduction. In spring, the source of the green tide
is most probably a remnant of Ulva from the preceding year.
Mass development of green seaweeds in other coastal areas of Europe
and all over the world is generally attributed to an increase in the
nutrient concentrations in coastal waters. In most cases, green tides can
be directly linked to that of waters collected by drainage basins. The
most responsible nutrients are nitrogen and phosphorus compounds.
On the Brittany coast, enclosed areas, such as the inner parts of bays,
where water exchange with the sea is slow, are particularly sensitive to
green tide outbreaks (Menesguen and Salomon 1988; Piriou et al. 1991,
1993). Despite the high tidal amplitude of 6-9 m, residual drift of the
waterbody between tides is small, as shown in the Bay of St. Brieuc
(Menesguen and Salomon 1988). Drifting seaweed and nutrients are
therefore trapped together in these areas. Breaking swell, which usually
occurs in large intertidal areas with a low slope, probably takes part in
concentrating the seaweeds in shallow nearshore coastal waters. Keeping
the seaweeds in suspension in this enriched, mixed, easily warmed up
and well lighted environment favours rapid growth and vegetative multiplication through fragmentation.
Out of the eight species of Ulva that it is possible to identify around
the coast of Brittany (Hoeksema and Van den Hoek 1983), only one is
involved in large-scale green tide processes. The systematic position
of this species is not clear, and a new taxonomic nomenclature will
probably have to be put forward for it. Its predisposition to react to
eutrophicating conditions at the end of bays seems to be based on
particular biological and physical characteristics (Dion 1990).
Ulva species in general, and the Ulva form producing mass blooms on
Brittany coasts in particular, have a high photosynthetic potential, allowing efficient use of the summer light conditions for growth (Table 9.1).