Page 3 - Bioconstructions_2018

Mediterranean Bioconstructions Along the Italian Coast 63

1. INTRODUCTION

Marine bioconstructions, or bioherms (Schuhmacher and Zibrowius,
1985), are elevated structures made of living benthic organisms (bio-
constructors) that overgrow the remnants (usually skeletons or shells) of their
predecessors. Bioconstructions of various shapes and sizes are found along
depth gradients ranging from the sea surface to the deep sea; some are
ephemeral and last only a few seasons or years, whereas others persist for cen-
turies or even millennia.
Bioconstructors modify primary (i.e. geological) substrates and provide
secondary (i.e. biogenic) substrates for new bioconstructors and for non-
bioconstructors who simply inhabit them. Darwin (1842) first described
the formation of tropical coral reefs, but biogenic formations are also present
in nontropical seas. Erect benthic organisms are habitat formers since they
modify primary substrates with their presence, but most of them do not build
permanent formations and must renew with each generation ( Jones et al.,
1994). Bioconstructions, instead, are the result of centuries or even millennia
of biological activities: their destruction can be almost irreversible, so they
require the greatest attention in any conservation measures.
The European Union (EU) Habitats Directive 92/43/EEC (European
Community, 1992) identified just a few marine habitats deserving protec-
tion at Community level, including Posidonia oceanica (Linnaeus) Delile,
1813 meadows and “reefs”. The category “reefs” comprises both biogenic
and geogenic formations that arise from the sea bed (European Commission,
2007, 2013). Rhizomes of Posidonia meadows grow on each other, entrap
sediments and elevate the substrate, thus creating a bioconstruction type
which might be referred to as being a simple subcategory of “reefs”. Both
Posidonia meadows and biogenic “reefs”, as bioconstructions, promote high
levels of biodiversity and enhance ecosystem functioning.
In the Mediterranean Sea, the richest European basin in terms of biodi-
versity, conservation priorities through the Natura 2000 network have been
mostly devoted to Posidonia meadows (e.g. Boudouresque et al., 2009), with
limited initiatives towards other types of bioconstructions. As a result, our
understanding of the status and trends of most Mediterranean bio-
constructions is inadequate and often confined to few species, whose role
is more presumed than quantitatively demonstrated. Recent studies show
that human pressures are having critical effects on the distribution of various
types of bioconstructions, causing their degradation, fragmentation and loss,