CHAPTER 31
Mortality of the Bryozoan Pentapora fascialis in the Ligurian Sea
(NW Mediterranean) after Disturbance
S. Cocito and S. Sgorbini
ABSTRACT
Pentapora fascia/is (Pallas) population at Tina Island (Ligurian Sea, NW Mediterranean) underwent total and partial mortality. A severe sea-storm occurred in December 1993, caused near
total mortality of the colonies at the shallow station (11 m depth): seven out of eight colonies
were swept away. Total mortality was not size-selective and was independent of density. The
deeper station (22 m depth) resulted unaffected by the sea-storm. Partial mortality, i.e. necrosis
and loss of portions of colony,affected exclusively the largest colonies subjected to long lasting
epibiosis and high sedimentation. Changes in the morphological characters (large cross-sectional area and poor adhesion to the substratum), wich occurred in the largest colonies,
enhanced their susceptibility to dislodgement.
Introduction
Ecologists have long been concerned with the
processes that limit and regulate the distribution
and abundance of natural benthic populations,
and particular attention has been paid to the role
of processes that determine and regulate the
number of individuals in a population (Hughes
1990; Done and Potts 1992).
Sublittoral benthic populations are subjected
to disturbance. which may fluctuate from disruptive and episodic events to relatively discrete and
frequent events (Pickett and White 1985). The
effects of disturbance may differ according to its
intensity and periodicity resulting in total and
partial mortality.
The effect of a disruptive event, such as a seastorm, depends on the temporal and spatial
scales under consideration, the life histories of
the dominant species and its ability to recover
afterward through recolonization and regrowth
(Rogers 1993). Besides, the effect depends to a
large extent on colony morphology, size and vulnerability to wave or current impact. Especially
powerful storms are known to affect reefs in
tropical regions down to 20-35 m (e.g. Woodley et
al. 1981), but such storms are infrequent and have
never been reported for the Mediterranean Sea.
Istituto di Ricerca sulle Acque, ENEA, 19100 La Spezia. Italy
P.M. Faranda, L Guglielmo, G. Sperle (eds)
Mediterranean Ecosystems: Structures and Processes
@ Springer-Verlag Itaila 200 1
The effect of relatively discrete and periodical events, such as sedimentation and epibiosis.
affects the ability to capture food in suspension
feeders thus reducing growth rate (Stebbing
1971; Cocito and Ferdeghini 1998).
Comprehensive studies on the effects of
changes after disturbance are rare and need long
time-series data. Among these very few concern
bryozoans of temperate areas (Harvell et al. 1990;
Hughes 1990; Cocito et al. 1998a), and usually the
effects are mediated by competition for space
and food (Buss 1979; Frechette et al. 1992).
The major traits of the population biology of
Pentapora fascialis (Pallas)[Cheilostomatida:
Ascophorina], a large erect bryozoan dominating
a temperate rocky assemblage in the Ligurian Sea
(north-western Mediterranean), have been under
study since 1993 (Cocito et al. 1998b). We monitored over 4 years two stations at different depths
through underwater photography. During monitoring, in December 1993, an exceptionally severe
sea-storm struck the area, involving different
effects on the Pentapora fascialis population.
This paper reports on how disturbance
affected two stations situated at different depths.
Partial and total colony mortality were analysed
with respect to size, morphology, density of
colonies and time.
Mortality of the Bryozoan Pentapora fascialis in the Ligurian Sea
(NW Mediterranean) after Disturbance
S. Cocito and S. Sgorbini
ABSTRACT
Pentapora fascia/is (Pallas) population at Tina Island (Ligurian Sea, NW Mediterranean) underwent total and partial mortality. A severe sea-storm occurred in December 1993, caused near
total mortality of the colonies at the shallow station (11 m depth): seven out of eight colonies
were swept away. Total mortality was not size-selective and was independent of density. The
deeper station (22 m depth) resulted unaffected by the sea-storm. Partial mortality, i.e. necrosis
and loss of portions of colony,affected exclusively the largest colonies subjected to long lasting
epibiosis and high sedimentation. Changes in the morphological characters (large cross-sectional area and poor adhesion to the substratum), wich occurred in the largest colonies,
enhanced their susceptibility to dislodgement.
Introduction
Ecologists have long been concerned with the
processes that limit and regulate the distribution
and abundance of natural benthic populations,
and particular attention has been paid to the role
of processes that determine and regulate the
number of individuals in a population (Hughes
1990; Done and Potts 1992).
Sublittoral benthic populations are subjected
to disturbance. which may fluctuate from disruptive and episodic events to relatively discrete and
frequent events (Pickett and White 1985). The
effects of disturbance may differ according to its
intensity and periodicity resulting in total and
partial mortality.
The effect of a disruptive event, such as a seastorm, depends on the temporal and spatial
scales under consideration, the life histories of
the dominant species and its ability to recover
afterward through recolonization and regrowth
(Rogers 1993). Besides, the effect depends to a
large extent on colony morphology, size and vulnerability to wave or current impact. Especially
powerful storms are known to affect reefs in
tropical regions down to 20-35 m (e.g. Woodley et
al. 1981), but such storms are infrequent and have
never been reported for the Mediterranean Sea.
Istituto di Ricerca sulle Acque, ENEA, 19100 La Spezia. Italy
P.M. Faranda, L Guglielmo, G. Sperle (eds)
Mediterranean Ecosystems: Structures and Processes
@ Springer-Verlag Itaila 200 1
The effect of relatively discrete and periodical events, such as sedimentation and epibiosis.
affects the ability to capture food in suspension
feeders thus reducing growth rate (Stebbing
1971; Cocito and Ferdeghini 1998).
Comprehensive studies on the effects of
changes after disturbance are rare and need long
time-series data. Among these very few concern
bryozoans of temperate areas (Harvell et al. 1990;
Hughes 1990; Cocito et al. 1998a), and usually the
effects are mediated by competition for space
and food (Buss 1979; Frechette et al. 1992).
The major traits of the population biology of
Pentapora fascialis (Pallas)[Cheilostomatida:
Ascophorina], a large erect bryozoan dominating
a temperate rocky assemblage in the Ligurian Sea
(north-western Mediterranean), have been under
study since 1993 (Cocito et al. 1998b). We monitored over 4 years two stations at different depths
through underwater photography. During monitoring, in December 1993, an exceptionally severe
sea-storm struck the area, involving different
effects on the Pentapora fascialis population.
This paper reports on how disturbance
affected two stations situated at different depths.
Partial and total colony mortality were analysed
with respect to size, morphology, density of
colonies and time.