350
E. Cecere et aI.
showed high cover values (data not reported).
whose thallus is a tightly clumped aggregate of
upright and lateral branches (Hay 1981).
Moreover, the upward migration of deep water
species was also observed.
The detection of these changes would have
not been possible without the availability of past
data. This sort of comparison is not very frequent in the literature (but see Wilkinson and
Tittley 1979; Cecere et al. 1991 and Nielsen 1991,
for marine benthic flora), despite the importance
of long-term studies advocated for many years
(see references in Strayer et al. 1986, Likens
1989). However. even though the importance of
time-series data is unquestionable, such observations are not sufficient to detect the causes of
the changes and it is necessary to integrate the
descriptive approach with both the experimental
and the theoretical one (Tilman 1989).
Therefore, time series should be used to generate
hypotheses to be tested experimentally (Coull
1985): this is the only approach useful to reveal
the reasons for changes in natural populations.
On the basis of collected data we advance
some hypotheses to explain the observed
changes. The most evident was the disappearance
of some Fucales which are very sensitive and
characteristic of the photophilic communities on
hard substrata in unpolluted Mediterranean
waters (Perrera and Giaccone 1986). Such a disappearance might be due to different factors, acting separately or in synergy: eutrophication,
chemical pollution, reduction of light penetration. sedimentation, fishery activities, overgrazing by sea urchins. In the studied area there is no
evidence of an increase in eutrophication. on the
contrary in the last decades in the Southern
Adriatic a nutrient lowering was observed
(Vukadin et al. 1988; Zore-Armanda et al. 1991).
Along the Gargano coast there are no industrial
plants, even though the effects of chemical pollution can not be excluded; in fact, the general circulation of the Adriatic Sea is cyclonic, with a surface current flowing southwards along the Italian
coast. which. especially in winter, consists of cold
and fresh water coming from the Po river and
other streams (La Violette and Gacic 1990). A
reduction in light penetration occurred from the
sixties to the eighties in both coastal and open
waters of the Southern Adriatic Sea, as reported
by Morovie and Domijan (1991) who recorded a
decrease in the euphotic zone. In the Middle
Adriatic Sea there is no evidence of increased
sedimentation. even though Gargano coastal
waters are presumably affected by sediment
deposition and movement, as suggested by the
large quantities of sediment observed both when
sampling and washing the samples before examination. We can exclude the possibility that other
human activities such as fishery or anchoring
could affect the sampled area because of its proximity to the coast line. Finally, during both sampling campaigns the sea urchin density was
rather low « 1 m- 2 ) in all the sampling stations
leading to the exclusion of their grazing as playing an important role along the Gargano coast.
Therefore. the most probable hypothesis to
explain the upward migration of deep water
species and the disappearance of some species of
Cystoseira and Sargassum. is the reduction in
light penetration due to the high sedimentation
rate. This is also in agreement with the increase
in richness of turf-forming species. In fact. it is
known from the literature that the turf morphology makes seaweeds more resistant to abrasion
(Hay 1981), making turf-forming algae generally
more abundant in habitats stressed by sediment
(Littler et al. 1983; Airoldi et al. 1995) which is
trapped in the algal matrix (Stewart 1983).
Moreover, the change from canopy- to turf-dominated communities also occurred in Cystoseiradominated rockpools as a result of the experimental removal of canopy species (Benedetti
Cecchi and Cinelli 1992).
Furthermore, the decrease in Ulvaceae might
be due to nutrient lowering as well as to the outcompetition by turf-forming species. In fact,
algal turfs may preclude the settlement of other
algal spores both by pre-empting the substrate
by vegetative propagation (Sousa 1979), and by
trapping the sediment which fills the space
among axes (Sousa et al.1981).
Similar changes in the composition of the
benthic algal flora also occurred in the nearby
Tremiti Islands, where both reduction in light
penetration and sedimentation have also been
supposed to be the most probable causes of the
observed changes (Cormaci and Furnari 1999;
Cormaci et al. this volume).
Conclusions
Over the last three decades a change in benthic
algal flora composition of the Gargano coast has
occurred, mainly consisting of: (1) a reduction in
E. Cecere et aI.
showed high cover values (data not reported).
whose thallus is a tightly clumped aggregate of
upright and lateral branches (Hay 1981).
Moreover, the upward migration of deep water
species was also observed.
The detection of these changes would have
not been possible without the availability of past
data. This sort of comparison is not very frequent in the literature (but see Wilkinson and
Tittley 1979; Cecere et al. 1991 and Nielsen 1991,
for marine benthic flora), despite the importance
of long-term studies advocated for many years
(see references in Strayer et al. 1986, Likens
1989). However. even though the importance of
time-series data is unquestionable, such observations are not sufficient to detect the causes of
the changes and it is necessary to integrate the
descriptive approach with both the experimental
and the theoretical one (Tilman 1989).
Therefore, time series should be used to generate
hypotheses to be tested experimentally (Coull
1985): this is the only approach useful to reveal
the reasons for changes in natural populations.
On the basis of collected data we advance
some hypotheses to explain the observed
changes. The most evident was the disappearance
of some Fucales which are very sensitive and
characteristic of the photophilic communities on
hard substrata in unpolluted Mediterranean
waters (Perrera and Giaccone 1986). Such a disappearance might be due to different factors, acting separately or in synergy: eutrophication,
chemical pollution, reduction of light penetration. sedimentation, fishery activities, overgrazing by sea urchins. In the studied area there is no
evidence of an increase in eutrophication. on the
contrary in the last decades in the Southern
Adriatic a nutrient lowering was observed
(Vukadin et al. 1988; Zore-Armanda et al. 1991).
Along the Gargano coast there are no industrial
plants, even though the effects of chemical pollution can not be excluded; in fact, the general circulation of the Adriatic Sea is cyclonic, with a surface current flowing southwards along the Italian
coast. which. especially in winter, consists of cold
and fresh water coming from the Po river and
other streams (La Violette and Gacic 1990). A
reduction in light penetration occurred from the
sixties to the eighties in both coastal and open
waters of the Southern Adriatic Sea, as reported
by Morovie and Domijan (1991) who recorded a
decrease in the euphotic zone. In the Middle
Adriatic Sea there is no evidence of increased
sedimentation. even though Gargano coastal
waters are presumably affected by sediment
deposition and movement, as suggested by the
large quantities of sediment observed both when
sampling and washing the samples before examination. We can exclude the possibility that other
human activities such as fishery or anchoring
could affect the sampled area because of its proximity to the coast line. Finally, during both sampling campaigns the sea urchin density was
rather low « 1 m- 2 ) in all the sampling stations
leading to the exclusion of their grazing as playing an important role along the Gargano coast.
Therefore. the most probable hypothesis to
explain the upward migration of deep water
species and the disappearance of some species of
Cystoseira and Sargassum. is the reduction in
light penetration due to the high sedimentation
rate. This is also in agreement with the increase
in richness of turf-forming species. In fact. it is
known from the literature that the turf morphology makes seaweeds more resistant to abrasion
(Hay 1981), making turf-forming algae generally
more abundant in habitats stressed by sediment
(Littler et al. 1983; Airoldi et al. 1995) which is
trapped in the algal matrix (Stewart 1983).
Moreover, the change from canopy- to turf-dominated communities also occurred in Cystoseiradominated rockpools as a result of the experimental removal of canopy species (Benedetti
Cecchi and Cinelli 1992).
Furthermore, the decrease in Ulvaceae might
be due to nutrient lowering as well as to the outcompetition by turf-forming species. In fact,
algal turfs may preclude the settlement of other
algal spores both by pre-empting the substrate
by vegetative propagation (Sousa 1979), and by
trapping the sediment which fills the space
among axes (Sousa et al.1981).
Similar changes in the composition of the
benthic algal flora also occurred in the nearby
Tremiti Islands, where both reduction in light
penetration and sedimentation have also been
supposed to be the most probable causes of the
observed changes (Cormaci and Furnari 1999;
Cormaci et al. this volume).
Conclusions
Over the last three decades a change in benthic
algal flora composition of the Gargano coast has
occurred, mainly consisting of: (1) a reduction in