Prevedelli 1994), whose density increases dramatically (from a mean of 4.2 ± 5.9 to 35.9 ± 15
individualsl sample, Table 3). Besides N. incisa,
other tube dwelling polychaetes that can stabilise
the sediment with their tubes, exhibit a similar
trend. Among these are, Pectinaria (Lagis) koreni
that exhibits a strong increase from the first to
the second period and Owenia fusiformis that is
only present in the second period.
Among molluscs, the bivalve Corbula gibba,
reported as an indicator of sedimentary instability (Salen Picard 1981), dominates the communities of both the periods. Although C. gibba populations frequently exhibit great density fluctuations over the years, which are considered casual
(Curini Galletti 1987), we can note that its density in the second period is about two-fold that in
the first period. Moreover, the density of
Lumbrineris latreilli, a polychaete associated
with C. gibba as indicator of sedimentary instability, exhibits a similar trend.
Besides the above-mentioned variations -in
community composition and in species density,
all the results of community analyses support the
notion that, although the "point" diversity is not
different in the two periods, the level of biotic
variation, i.e. the amount of change of species
compositions across the samples, is lower in the
second as compared to the first period. This finding suggests that the amplitude of the zones in
which most species succeed in establishing
themselves, defined as realized zones (Pielou
1975) is increased from the first to the second
period, causing a fall in the species turnover rate
(13 diversity sensu Whittaker 1972).
Our results can be related to the fmrlings of
two sedimentological studies performed on the
same site and periods, aimed at detecting the
impact of platform activities on the surrounding
bottoms (Frascari and Bonvicini Pagliai 1986;
Frascari et al. 1994).
Both studies emphasize the role exerted per
se by the submerged structure that interacts with
waves and currents thus enhancing their energy
near the bottom. When circulation currents are
active, erosion surfaces have been detected as far
as 1 km from the platform, in their direction.
Storms interfere with the submerged structure
depending on their intensity and direction and
cause erosion and transport to great distances,
even outside the area of our sampling grid.
A classical generalization about infaunal
invertebrate distribution is that a clear associaEffect of Submerged Structures on Macrozoobenthos Diversity
373
tion between animals and specific sediment
types exist. According to this generalization,
infauna distribution should be determined by
only a few parameters, such as grain size, organic content, sediment stability and microorganisms (Gray 1974; Rhoads 1974; Peres 1982), In
addition to these traditional factors, the leading
role of hydrodynamic processes, such as the
boundary layer flow and sediment transport regimen, have been increasingly emphasized (Riedl
1971; Snelgrove and Butman 1994). Arguing with
the numerous studies that have correlated infaunal invertebrate distribution with single parameters of the sediment, Snelgrove and Butman
(1994) conclude that there is little evidence that
animal distribution is determined by any of the
classical above mentioned factors. They propose
a shift in focus towards understanding the relationship between organism distribution and the
dynamic sedimentary and hydrodynamic environment. Given that the bottom sediment
reflects the boundary-layer flow and sediment
transport regimen, animal distribution should
not pe determined by some particular aspect of
the sediment but by the same physical processes
that created that particular sedimentary environment.
In this context, many recent studies in larval
ecology are focused on the physical aspects of
larval dispersion and settlement. Laboratory
experiments and experimental field manipulations designed to evaluate the importance of bottom energy on larval distribution suggest that
the hydrodynamics influence benthic larvae distribution (Eckman 1979,1983; Kern and Taghon
1986; Savidge and Taghon 1988; Butman 1989).
Moreover, benthic flow may also redistribute settled individuals. This may be an important way
of dispersion for direct developers, and it has
been observed in some benthic species, such as
Mya arenaria and Macoma baltica (Emerson and
Grant 1991; Gunther 1991).
The patterns we have described for the communities of the two periods are consistent with
these views. In fact, concomitant with an increase
in bottom hydrodynamic energy, we detected a
sharp reduction of surface deposit feeders that
seemingly suffer from the erosion of their specific micro-habitat (Castelli and Prevedelli 1994),
and a new pattern of distribution evidencing an
increase of the realized zones of most faunal
invertebrates and a decrease of their spatial
turnover rate.
individualsl sample, Table 3). Besides N. incisa,
other tube dwelling polychaetes that can stabilise
the sediment with their tubes, exhibit a similar
trend. Among these are, Pectinaria (Lagis) koreni
that exhibits a strong increase from the first to
the second period and Owenia fusiformis that is
only present in the second period.
Among molluscs, the bivalve Corbula gibba,
reported as an indicator of sedimentary instability (Salen Picard 1981), dominates the communities of both the periods. Although C. gibba populations frequently exhibit great density fluctuations over the years, which are considered casual
(Curini Galletti 1987), we can note that its density in the second period is about two-fold that in
the first period. Moreover, the density of
Lumbrineris latreilli, a polychaete associated
with C. gibba as indicator of sedimentary instability, exhibits a similar trend.
Besides the above-mentioned variations -in
community composition and in species density,
all the results of community analyses support the
notion that, although the "point" diversity is not
different in the two periods, the level of biotic
variation, i.e. the amount of change of species
compositions across the samples, is lower in the
second as compared to the first period. This finding suggests that the amplitude of the zones in
which most species succeed in establishing
themselves, defined as realized zones (Pielou
1975) is increased from the first to the second
period, causing a fall in the species turnover rate
(13 diversity sensu Whittaker 1972).
Our results can be related to the fmrlings of
two sedimentological studies performed on the
same site and periods, aimed at detecting the
impact of platform activities on the surrounding
bottoms (Frascari and Bonvicini Pagliai 1986;
Frascari et al. 1994).
Both studies emphasize the role exerted per
se by the submerged structure that interacts with
waves and currents thus enhancing their energy
near the bottom. When circulation currents are
active, erosion surfaces have been detected as far
as 1 km from the platform, in their direction.
Storms interfere with the submerged structure
depending on their intensity and direction and
cause erosion and transport to great distances,
even outside the area of our sampling grid.
A classical generalization about infaunal
invertebrate distribution is that a clear associaEffect of Submerged Structures on Macrozoobenthos Diversity
373
tion between animals and specific sediment
types exist. According to this generalization,
infauna distribution should be determined by
only a few parameters, such as grain size, organic content, sediment stability and microorganisms (Gray 1974; Rhoads 1974; Peres 1982), In
addition to these traditional factors, the leading
role of hydrodynamic processes, such as the
boundary layer flow and sediment transport regimen, have been increasingly emphasized (Riedl
1971; Snelgrove and Butman 1994). Arguing with
the numerous studies that have correlated infaunal invertebrate distribution with single parameters of the sediment, Snelgrove and Butman
(1994) conclude that there is little evidence that
animal distribution is determined by any of the
classical above mentioned factors. They propose
a shift in focus towards understanding the relationship between organism distribution and the
dynamic sedimentary and hydrodynamic environment. Given that the bottom sediment
reflects the boundary-layer flow and sediment
transport regimen, animal distribution should
not pe determined by some particular aspect of
the sediment but by the same physical processes
that created that particular sedimentary environment.
In this context, many recent studies in larval
ecology are focused on the physical aspects of
larval dispersion and settlement. Laboratory
experiments and experimental field manipulations designed to evaluate the importance of bottom energy on larval distribution suggest that
the hydrodynamics influence benthic larvae distribution (Eckman 1979,1983; Kern and Taghon
1986; Savidge and Taghon 1988; Butman 1989).
Moreover, benthic flow may also redistribute settled individuals. This may be an important way
of dispersion for direct developers, and it has
been observed in some benthic species, such as
Mya arenaria and Macoma baltica (Emerson and
Grant 1991; Gunther 1991).
The patterns we have described for the communities of the two periods are consistent with
these views. In fact, concomitant with an increase
in bottom hydrodynamic energy, we detected a
sharp reduction of surface deposit feeders that
seemingly suffer from the erosion of their specific micro-habitat (Castelli and Prevedelli 1994),
and a new pattern of distribution evidencing an
increase of the realized zones of most faunal
invertebrates and a decrease of their spatial
turnover rate.