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International Journal of Marine Science 2013, Vol.3, No.20, 158-165
http://ijms.sophiapublisher.com
162
the plots. The species was not observed in the emerging
stretch of the
P. oceanica
meadow that forms the
barrier-reef.
2 Discussion
When Lenzi (1987) described the
P. oceanica
barrier
reef at Santa Liberata, besides the seagrasses
C.
nodosa
and
N. noltii
, he reported the macroalgae
Caulerpa prolifera
,
Cystoseira barbata
J. Ag.,
Halimeda tuna
Lamour,
Sphaerococcus coronopifolius
(Good.-Woodw.) C. Ag.,
Rytiphloea tinctoria
(Clem.)
C. Ag. and
Alsidium corallinum
C. Ag. This mixed
meadow collapsed, starting in 2003, and by 2005 not a
trace of it remained.
In the observations made between July and October
2005 and 2006 (Lenzi et al., 2007) and in those of the
present study (August 2011), none of the species listed
by Lenzi (1987), except
N. noltii
, was found. Since
2005, phytobenthic populations have been unstructured
and dead
Posidonia
-
mattes
have become covered with
thin algal mats consisting mainly of
J. rubens
and
Cladophora prolifera
. Another two species, never
previously observed along the southern Tuscan coast,
were found instead:
Caulerpa racemosa
and
Penicillus
capitatus
Lamarck (Bryopsidales, Chlorophyta). Both
species were observed on the dead
mattes
of
P.
oceanica
left by the mixed meadows, while
P.
capitatus
also colonized the sandy bottom, distributed
in isolated patches of a few square metres at most.
C.
racemosa
was widespread, its stolons entwined in the
thin algal mats. The explosion of this species between
2005 and 2006 in the shallow water of LB area
(Figure 1), is in line with this alga’s high phototolerance
(Raniello et al., 2006) and with the abundance of
organic detritus in this relatively quiet area, condition
favorable to growth according to Piazzi et al (2007).
Perhaps these two species had already been present for
some time, because it seems unlikely that they came
just a year after the perturbing event. They were
probably already present in microhabitats, escaping
previous, superficial observations.
Successive observations (Lenzi, not published) showed
rapid changes in
facies
, e.g. between June and
September 2008, there was a bloom of
Cladophora
sp.
that produced free-floating balls of pleustophytic thalli.
This species grows in height and forms pedunculated
tufts from which aegagropilous bodies detach.
Cladophora
balls formed beds in the deeper areas of
LB, where waves and currents carried them into the
open sea. While this still was an important development,
there were no further high blooms of
Cladophora
in
the following years.
Despite a significant overall decrease in biomass, the
phytobenthic community observed in 2011 seemed
better structured. This is also sustained by values of
the Ph:Sc ratio, which was highest in 2006, during
maximum "degradation”, and lowest in 2011, and by
the 27% increase in the number of algal species
present in the mat, with respect to 2005-2006. All the
species were still typical of PhIQ, PhIT, PhIH, SIQ,
SQ and SI.
A decrease in macroalgal biomass of about 65%
between 2006 and 2011 was mainly due to the loss of
relatively large macroalgae, such as
P. pavonica
, and
probably to dominance of
Cladophora
spp. with
respect to
J. rubens
in the mats (Table 1). The greatest
reduction in biomass was that of
C. racemosa
. The
drastic nature of the decrease in this species in 2011
was not evident from its cover values (Table 1)
because a small quantity of thalli covered most of the
bottom where it had spread in 2006. This species
seems to have stopped developing, and while it
remained widespread, underwent a drastic 95.7%
decrease in weight.
From this series of events, including the fact that
C.
racemosa
has not yet penetrated the meadows of
Posidonia oceanica
, we propose the following
explanation of the observations: 1) as a result of
disturbance, such as the excessive heating that
occurred in 2003 and/or increasing impact of bathing
from 2002 (Lenzi et al., 2007) to 2008, the plant
communities in LB underwent profound deterioration;
2) in the phase of impoverishment and destabilization
of the phytocoenoses, between 2005 and 2006,
C.
racemosa
showed massive development; 3) there
followed a phase of variability of phytobenthic
settlement, characterized by the development of
C.
racemosa
and
Cladophora
balls; 4) the recent period
seems to involve reorganization of plant community
structure, probably in the direction of restoration of
the typical phytobenthic community, with a sharp
decrease in the “invasive”
C. racemosa
, stability of
P.
capitatus
, a thermophilous species, and recovery of
the seagrass
N. noltii
.