landscape on hard substrates. When these communities are well conserved and
structured, up to four strata of vegetation (arboreal, shrub, caespitose, and basal or
incrusting) similar to terrestrial forests can be differentiated. The seagrasses, with a
significantly lower species richness and abundance, present a more uniform structural pattern. However, they constitute communities of great importance for the
ecology and coastal sandy dynamics because seagrass meadows are habitats with
high diversity, altering the geomorphology of seabottoms extensively and, interestingly, working as carbon sinks (especially, due to the biomass of rhizomes and
roots).
In the marine environment, the presence of seaweeds and seagrass communities is
restricted to the photic zone where irradiance levels allow photosynthesis to surpass
respiration (Ros et al. 1989; Romero et al. 2004). Their distribution is also strongly
conditioned by environmental factors that have an important vertical gradient, giving
rise to distributions in more or less apparent horizontal bands, which may be
characterized by the presence or absence of certain species. The amplitude of these
bands is very variable and depends on the rate with which environmental factors
vary. Close to the surface, the variations are very rapid, and therefore the bands are
narrower and better defined. At greater depths, the transitions are more indistinct and
may not appear completely clear, with frequent transition zones and mixed communities. The result of all these gradients and biological factors is a zonation of stripes
at different levels. The phytal system (where seagrasses and seaweeds can proliferate) includes, from the surface to depths, the following levels: supralittoral,
eulittoral, infralittoral, and circalittoral (Pérès 1982; Ros et al. 1989; Romero et al.
2004). The marine communities in which different species are grouped are named in
reference to the type of substratum they colonize, hydrodynamism to which they are
subjected, degree of illumination to which they are exposed, and the dominant
species. However, it must be highlighted that in the Alboran Sea, an additional
West-East (Atlantic-Mediterranean) gradient is superimposed due to the particular
oceanographic conditions, as described in Sect. 8.3.
8.2 Origin and History of the Seaweeds and Seagrasses
from the Alboran Sea
Four elements underlie the origin and diversity of the marine macrophyte flora of the
Alboran Sea today: origin and geological history of the Mediterranean Sea as a
whole, Atlantic influence due to its proximity to the Strait of Gibraltar, antiquity of
the region, and the influence of recent human activities.
It can be considered that the flora of the Alboran Sea had its origin more than
100 million years ago during the Cretaceous, when the separation of the European
and African plates allowed the connection of very different masses of waters from
the Tethys Ocean and the primitive Indo-Pacific Ocean, approximately 120 million
years ago. In this period, the Tethys was an ocean populated by numerous
248
A. Flores-Moya et al.
structured, up to four strata of vegetation (arboreal, shrub, caespitose, and basal or
incrusting) similar to terrestrial forests can be differentiated. The seagrasses, with a
significantly lower species richness and abundance, present a more uniform structural pattern. However, they constitute communities of great importance for the
ecology and coastal sandy dynamics because seagrass meadows are habitats with
high diversity, altering the geomorphology of seabottoms extensively and, interestingly, working as carbon sinks (especially, due to the biomass of rhizomes and
roots).
In the marine environment, the presence of seaweeds and seagrass communities is
restricted to the photic zone where irradiance levels allow photosynthesis to surpass
respiration (Ros et al. 1989; Romero et al. 2004). Their distribution is also strongly
conditioned by environmental factors that have an important vertical gradient, giving
rise to distributions in more or less apparent horizontal bands, which may be
characterized by the presence or absence of certain species. The amplitude of these
bands is very variable and depends on the rate with which environmental factors
vary. Close to the surface, the variations are very rapid, and therefore the bands are
narrower and better defined. At greater depths, the transitions are more indistinct and
may not appear completely clear, with frequent transition zones and mixed communities. The result of all these gradients and biological factors is a zonation of stripes
at different levels. The phytal system (where seagrasses and seaweeds can proliferate) includes, from the surface to depths, the following levels: supralittoral,
eulittoral, infralittoral, and circalittoral (Pérès 1982; Ros et al. 1989; Romero et al.
2004). The marine communities in which different species are grouped are named in
reference to the type of substratum they colonize, hydrodynamism to which they are
subjected, degree of illumination to which they are exposed, and the dominant
species. However, it must be highlighted that in the Alboran Sea, an additional
West-East (Atlantic-Mediterranean) gradient is superimposed due to the particular
oceanographic conditions, as described in Sect. 8.3.
8.2 Origin and History of the Seaweeds and Seagrasses
from the Alboran Sea
Four elements underlie the origin and diversity of the marine macrophyte flora of the
Alboran Sea today: origin and geological history of the Mediterranean Sea as a
whole, Atlantic influence due to its proximity to the Strait of Gibraltar, antiquity of
the region, and the influence of recent human activities.
It can be considered that the flora of the Alboran Sea had its origin more than
100 million years ago during the Cretaceous, when the separation of the European
and African plates allowed the connection of very different masses of waters from
the Tethys Ocean and the primitive Indo-Pacific Ocean, approximately 120 million
years ago. In this period, the Tethys was an ocean populated by numerous
248
A. Flores-Moya et al.