CHAPTER 49
Recent Changes in Biodiversity in the Ligurian Sea
(NW Mediterranean): is there a Climatic Forcing?
C. Morri l and C.N. Bianchil
ABSTRACT
A few case studies from the Ligurian Sea provide examples of how climate change has influence
on marine biodiversity. This influence may be either direct (bryozoan population dynamics1coral
growth, sea-grass shoot density) or indirect, mediated by biotic interactions (bivalves in a sandy
bottom community) or marine currents (fish and rocky bottom communities/entering of southern species). The difficulty of distinguishing between anthropogenic and climatic actions on the
marine biota emphasises the need to study marine biodiversity in the context of the physical
setting and the relevant space and time scales.
Introduction
It has often been assumed, in the past, that
marine communities were naturally stable, and
that most changes (seasonal cycles apart) were
probably caused by man (Lewis 1996). Only in
the last decades the natural variability of marine
communities has been fully recognised, and its
relation with climate fluctuations hypothesised
(Bianchi 1997).
Angel (1991) suggested that monitoring biogeographic boundaries would give an unambiguous signal of climate change. Evidence in
that sense has been collected in recent years in
different regions of the globe (Barry et al. 1995;
Southward et al. 1995; Parker and Dixon 1998;
Bianchi et al. 1998a).
The Ligurian Sea, situated in the north-eastern corner of the western Mediterranean, is one
of the coldest areas of the Mediterranean Sea.
Accordingly, the biota of the Ligurian Sea are
characterised by a strong diminution of the subtropical elements and by a more marked presence of species of cold temperate waters (Rossi
1969). However, warm-water organisms have
recently increased their occurrence in the
Ligurian Sea (Bianchi and Morri 1993, 1994),
providing a clue for climate change (Francour et
al. 1994; Astraldi et al. 1995).
The aim of the present paper is to explore
existing data on change in the Ligurian Sea biota
in the search for further evidence for a climatic
influence. First, the Ligurian Sea climate will be
characterised in short and a climatic descriptor
identified. The time trend of the climatic
descriptor will be then compared with biological
changes for different marine communities, studied especially in recent years.
Ligurian Sea Climate
The Ligurian Sea is tightly connected with the
Gulf of Lion, which is subject to periodic intrusions of the northerly winds from the Rhone valley. They generate very energetic weather conditions, which are particularly severe in winter. The
major large-scale hydrodynamic feature
(Astraldi et al. 1994) is a well-defined cyclonic
circulation active year round, which contributes
to keeping the mean surface temperature lower
than that of the adjacent Tyrrhenian Sea.
The greater cooling of the Ligurian Sea
induces water and temperature losses, which in
turn draw a seasonal flux: of warmer Tyrrhenian
Sea waters to restore the hydrological budget
(Astraldi and Gasparini 1992). Analysing jointly
current regimes and invertebrate settlement off
I Dipartimento Territorio e Risorse. Universita di Genova, Corso Europa 26, 16132 Genova, Italy
2 Centro Ricerche Ambiente Marino, ENEA Santa Teresa, 19100 La Spezia. Italy
P.M. Faranda, L. Guglielmo, G. Spezie (eds)
Mediterranean Ecosystema: Structures and Processes
© Spdnger-Verlag Italia 2001
Recent Changes in Biodiversity in the Ligurian Sea
(NW Mediterranean): is there a Climatic Forcing?
C. Morri l and C.N. Bianchil
ABSTRACT
A few case studies from the Ligurian Sea provide examples of how climate change has influence
on marine biodiversity. This influence may be either direct (bryozoan population dynamics1coral
growth, sea-grass shoot density) or indirect, mediated by biotic interactions (bivalves in a sandy
bottom community) or marine currents (fish and rocky bottom communities/entering of southern species). The difficulty of distinguishing between anthropogenic and climatic actions on the
marine biota emphasises the need to study marine biodiversity in the context of the physical
setting and the relevant space and time scales.
Introduction
It has often been assumed, in the past, that
marine communities were naturally stable, and
that most changes (seasonal cycles apart) were
probably caused by man (Lewis 1996). Only in
the last decades the natural variability of marine
communities has been fully recognised, and its
relation with climate fluctuations hypothesised
(Bianchi 1997).
Angel (1991) suggested that monitoring biogeographic boundaries would give an unambiguous signal of climate change. Evidence in
that sense has been collected in recent years in
different regions of the globe (Barry et al. 1995;
Southward et al. 1995; Parker and Dixon 1998;
Bianchi et al. 1998a).
The Ligurian Sea, situated in the north-eastern corner of the western Mediterranean, is one
of the coldest areas of the Mediterranean Sea.
Accordingly, the biota of the Ligurian Sea are
characterised by a strong diminution of the subtropical elements and by a more marked presence of species of cold temperate waters (Rossi
1969). However, warm-water organisms have
recently increased their occurrence in the
Ligurian Sea (Bianchi and Morri 1993, 1994),
providing a clue for climate change (Francour et
al. 1994; Astraldi et al. 1995).
The aim of the present paper is to explore
existing data on change in the Ligurian Sea biota
in the search for further evidence for a climatic
influence. First, the Ligurian Sea climate will be
characterised in short and a climatic descriptor
identified. The time trend of the climatic
descriptor will be then compared with biological
changes for different marine communities, studied especially in recent years.
Ligurian Sea Climate
The Ligurian Sea is tightly connected with the
Gulf of Lion, which is subject to periodic intrusions of the northerly winds from the Rhone valley. They generate very energetic weather conditions, which are particularly severe in winter. The
major large-scale hydrodynamic feature
(Astraldi et al. 1994) is a well-defined cyclonic
circulation active year round, which contributes
to keeping the mean surface temperature lower
than that of the adjacent Tyrrhenian Sea.
The greater cooling of the Ligurian Sea
induces water and temperature losses, which in
turn draw a seasonal flux: of warmer Tyrrhenian
Sea waters to restore the hydrological budget
(Astraldi and Gasparini 1992). Analysing jointly
current regimes and invertebrate settlement off
I Dipartimento Territorio e Risorse. Universita di Genova, Corso Europa 26, 16132 Genova, Italy
2 Centro Ricerche Ambiente Marino, ENEA Santa Teresa, 19100 La Spezia. Italy
P.M. Faranda, L. Guglielmo, G. Spezie (eds)
Mediterranean Ecosystema: Structures and Processes
© Spdnger-Verlag Italia 2001