Geoscience Reference
In-Depth Information
provided replacement of some facies in given communities. Assemblages of
Ceram-
ium ciliatum
var.
robustum
and
Cladophora
sp. recorded previously
37
in photophilic
community on the rocks were mostly replaced by facies of
Acetabularia acetabulum,
Jania rubens,
Padina pavonica
and,
in some zones, by
Laurencia obtusa.
21
In recent
years, some of these assemblages have been replaced by
Caulerpa prolifera
.
Two main mechanisms operate in these processes: the increase in the coloniza-
tion rates of marine species (both in larval or juvenile stages and by migration of
adults) and a reduction in salinity and the mitigation of extreme temperatures, thus
permitting the establishment of allochthonous species to the lagoon conditions
38,39
(
see Chapter 5 for a more detailed discussion).
Some of the most important changes affecting the physiography and functioning
of the Mar Menor took place at the benthic meadow level.
Cymodocea nodosa
(Ucria)
Ascherson,
Zostera marina,
and
Z. nana
meadows dominated the Mar Menor bottoms
before 1970
28,40
with scant mats of
Posidonia oceanica
(Linnaeus) Delile found in
some small areas. In 1980, however, it was possible to find only a few mats of
Posidonia
close to some of the small islands to the south and close to the mouth of the El Estacio
Channel.
9,29
At present, the benthic vegetation on the soft bottoms of the Mar Menor
mainly consists of monospecific
Caulerpa prolifera
(Forskal) Lamouroux meadows
on muddy and some rocky bottoms, covering more than 80% of the bottoms, favoring
high levels of organic matter in sediment and low oxygen concentration. Scarce patches
of
Cymodocea nodosa
are now restricted to shallow sandy bottoms and more or less
dense spots of
Ruppia cirrhosa
(Petagna) Grande remain in the shallowest and calm
Spreading of the
Caulerpa prolifera
at the expense of the
C. nodosa
monospecific
meadows has been progressive since the opening of El Estacio Channel, starting in
the north basin with intermediate states of mixed
Cymodocea
-
Caulerpa
meadows.
41
Two related processes seems to be involved in this change: (1) changes in environ-
mental conditions of the lagoon, mainly moderation of salinity and extreme temper-
atures, permitting the entrance of the algae
Caulerpa prolifera
and (2) perhaps
increasing stress in sediment and the increase of nutrients in water, in a second phase,
thus giving competitive advantage to the algae over the seagrass
Cymodocea nodosa
.
Before the opening of El Estacio Channel,
C. nodosa
was reported at all depths
in the Mar Menor.
28,29,40,41
In the 1990s it was present above the 3.3-m isobath
estimated as the critical depth for this species in this ecosystem at that time.
42
After
the opening of the channel, salinity dropped from 50-60 to 42-45 and the lowest
temperatures were usually higher than 11
C.
Caulerpa
prolifera
is sensitive to both
salinity and temperature and cannot withstand temperatures below 10
°
C,
43
temperature
readings frequently reached in winter before the enlargement of El Estacio. It has
almost continuous growth throughout the year and a high capacity to generate vege-
tatively a new thallus from any fragment swept away by the water, resulting in a high
colonization rate. It seems that most probably
C. prolifera
entered the lagoon after
the opening of the El Estacio Channel, when salinity and temperature were no longer
limiting factors, and settled progressively, mainly on muddy bottoms. Instability of
sediment also provides macroalgae a competitive advantage over rooted vegetation.
It seems that in the Mar Menor lagoon, both instability of sediment (dredging and
pumping of sand, increase in sedimentation rates, increase in organic matter content) and
°
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