208
C. Hily and M. Bernard
populations by overgrazing young plants. This process induced a visible decreasing
of fucoid cover rates on the shores (both Fucus vesiculosus and Ascophyllum
nodosum). Because fucoids are engineer species, this process induced a shift in
mediolittoral habitats. We experimented in the field the control exerted on macroalgae by limpets (unpublished data): we removed all individuals of Patella vulgata
from 5 × 5 m quadrats (n = 5) during one year. Changes in the intertidalscape were
visible three weeks after removing with the appearance of a dense cover of green
opportunistic macroalgae (Enteromorpha spp.) (Fig. 13.1a, 13.1b). Three months
later, this first step was followed by a phase of Fucoids settlement (mainly Fucus
serratus and few Ascophyllum nodosum and Fucus vesiculosus). This succession
was easily visible at the intertidalscape: (1) bare rock with barnacles and limpets; (2)
full cover of green algae cover; (3) mixed green and brown algae; (4) brown algae
dominant cover. This was a visual example of the consequences of an anthropogenic
disturbance (the removing of grazers) which induced an opportunistic peak of green
algae, further replaced by a fucoid cover through a facilitation process.
The development of green algae (Entheromorpha spp., Ulva spp.) is well known to
detect and characterize stress and disturbance of the benthic ecosystems, particularly
under high nutrient input (Ménesguen and Piriou 1995; Cloern 2001). Consequently,
it seems interesting to look for a use of green opportunistic algae as a bioindicator
of disturbance for ecosystems at the intertidalscape scale.
13.6 Green Opportunistic Algae (Enteromorpha spp., Ulva
spp.) Cover to Characterize Disturbances on the Rocky
Shores
Firstly, it must be underlined that we shall consider only the green opportunistic algae
(Enteromorpha spp., Ulva spp.) fixed on the rock substratum and not those which
are lying on the sediment or the drifted algae which are beached on the shore. Such
beaching events can be very impressive in some sites of Brittany where eutrophication
induce a very high green macroalgae production (Fig. 13.3).
Secondly the natural conditions under which green opportunistic macroalgae
can develop must be identified to avoid errors of interpretation. Surface of the
substratum must be free of dense macroalgal or faunal beds. Proliferations are facilitated on rocks and boulders located under a high flux of dissolved organic matter
(Bellan and Bellan-Santini 1972; Raffaelli et al. 1998; Neto et al. 2011). They occur
very often at the output of freshwater effluents, sewages and rivers on the shore,
their high tolerance to low salinities unlike most of the other species of the shore,
is an asset for the colonization. As a consequence, most of the natural situations
where the green opportunistic algae are common in dense cover on the shore, are
on areas under freshwater flows from small rivers and resurgences. Pebbles which
are frequently overturned by waves at the high mediolittoral level of beaches can be
also colonized by these algae during their period of stability between two successive
storms. Tides pools of the high mediolittoral are also habitats which are naturally and
C. Hily and M. Bernard
populations by overgrazing young plants. This process induced a visible decreasing
of fucoid cover rates on the shores (both Fucus vesiculosus and Ascophyllum
nodosum). Because fucoids are engineer species, this process induced a shift in
mediolittoral habitats. We experimented in the field the control exerted on macroalgae by limpets (unpublished data): we removed all individuals of Patella vulgata
from 5 × 5 m quadrats (n = 5) during one year. Changes in the intertidalscape were
visible three weeks after removing with the appearance of a dense cover of green
opportunistic macroalgae (Enteromorpha spp.) (Fig. 13.1a, 13.1b). Three months
later, this first step was followed by a phase of Fucoids settlement (mainly Fucus
serratus and few Ascophyllum nodosum and Fucus vesiculosus). This succession
was easily visible at the intertidalscape: (1) bare rock with barnacles and limpets; (2)
full cover of green algae cover; (3) mixed green and brown algae; (4) brown algae
dominant cover. This was a visual example of the consequences of an anthropogenic
disturbance (the removing of grazers) which induced an opportunistic peak of green
algae, further replaced by a fucoid cover through a facilitation process.
The development of green algae (Entheromorpha spp., Ulva spp.) is well known to
detect and characterize stress and disturbance of the benthic ecosystems, particularly
under high nutrient input (Ménesguen and Piriou 1995; Cloern 2001). Consequently,
it seems interesting to look for a use of green opportunistic algae as a bioindicator
of disturbance for ecosystems at the intertidalscape scale.
13.6 Green Opportunistic Algae (Enteromorpha spp., Ulva
spp.) Cover to Characterize Disturbances on the Rocky
Shores
Firstly, it must be underlined that we shall consider only the green opportunistic algae
(Enteromorpha spp., Ulva spp.) fixed on the rock substratum and not those which
are lying on the sediment or the drifted algae which are beached on the shore. Such
beaching events can be very impressive in some sites of Brittany where eutrophication
induce a very high green macroalgae production (Fig. 13.3).
Secondly the natural conditions under which green opportunistic macroalgae
can develop must be identified to avoid errors of interpretation. Surface of the
substratum must be free of dense macroalgal or faunal beds. Proliferations are facilitated on rocks and boulders located under a high flux of dissolved organic matter
(Bellan and Bellan-Santini 1972; Raffaelli et al. 1998; Neto et al. 2011). They occur
very often at the output of freshwater effluents, sewages and rivers on the shore,
their high tolerance to low salinities unlike most of the other species of the shore,
is an asset for the colonization. As a consequence, most of the natural situations
where the green opportunistic algae are common in dense cover on the shore, are
on areas under freshwater flows from small rivers and resurgences. Pebbles which
are frequently overturned by waves at the high mediolittoral level of beaches can be
also colonized by these algae during their period of stability between two successive
storms. Tides pools of the high mediolittoral are also habitats which are naturally and