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Mediterranean Bioconstructions Along the Italian Coast 105

both coralligenous growth and survival. The invasion of the red alga
W. setacea, currently distributed along most of the Mediterranean basin
(Athanasiadis, 1997), leads to lower species richness than that is observed
at noncolonized sites (Piazzi et al., 2012). Similar effects have been docu-
mented for the other two algae (Cebrian et al., 2012; Gatti et al., 2015b).
In some areas, especially C. cylindracea has totally invaded the coralligenous
community, raising concerns for the conservation of this habitat (Meinesz
and Hesse, 1991; Piazzi et al., 2005). However, healthy gorgonian forests
made by P. clavata seem able to reduce the invasiveness of C. cylindracea
in coralligenous habitats (Ponti et al., 2018).
At shallow depth, C. cylindracea may overgrow the anthozoan C. caespitosa
(Gatti et al., 2015b; Kersting et al., 2014a; Kruz ˇi cet al., 2008). In the National
Park Mljet in Veliko Jezero (between 8 and 14m depth) approximately 150m
of coastline characterized by coral bank of C. caespitosa was partially affected by
this invasive alga (Kruz ˇi c et al., 2008). Similarly, C. cylindracea has occasionally
been observed to overgrow colonies of A. calycularis in Sicily (Fig. 10C).
Marine diseases, which often combine with climate change, represent
another biological threat driven by human activities. Globally, examples
of recent disease outbreaks with major impacts in marine ecosystems
include the Caribbean urchin die-off (Hughes, 1994), various coral diseases
(e.g. Aronson and Precht, 2001), lobster declines in the north Atlantic
(Glenn and Pugh, 2006) and marine mammal diseases (Kim et al., 2005).
It is presumed, even if not yet assessed, that diseases could represent a threat
for bioconstructions in general. Similar to the spread of NIS, disease out-
breaks can be driven by human activities such as aquaculture with its artifi-
cially dense populations, and shipping and ballast water transport, facilitating
the transport of disease vectors. Various synergistic stressors can weaken
populations’ disease resistance (Harvell et al., 2004), and other environmental
changes can enhance disease effects (e.g. Bruno et al., 2007). Marine diseases
are probably increasing in the ocean due to human activities (Harvell et al.,
1999), but documenting the human influence on disease outbreaks is difficult
due to lack of historical baselines.
Mucilages are another biological threat: algal aggregates and exudates can
cause severe damage to erect suspension feeders (mainly gorgonians) espe-
cially on coralligenous formations (Schiaparelli et al., 2007). These blooms
are still not well understood, but they are apparently caused by eutrophica-
tion. For instance, the benthic assemblages of the northern Adriatic biogenic
reefs were strongly affected by recurring anoxic crises and accumulation of
mucilaginous aggregates (Precali et al., 2005).