DipTeRis, Department for the Study of the Territory and of its Resources, University of Genoa, Corso Europa 26, 16132 Genoa, Italy
SeaMap srl, Via Greto di Cornigliano 6R, 16152 Genova, Italy

Posidonia oceanica, the most important and abundant seagrass in the Mediterranean Sea, forms large
meadows from the sea surface down to 40 m. The depth of the lower limit of the meadows marks the
boundary between the infralittoral and the circalittoral zone, and is said to be normally set by light attenuation
underwater, while the role ofwatermovement has been little explored. In this paper, the position (i.e.
distance from the shoreline and depth) of P. oceanica meadow lower limits along the whole Ligurian
coastline (about 350 km) was related to the annual storm wave base. This depth represents the limit of
interaction between waves and seafloor and corresponds to L0/2, where L0 is the annual offshore wave
length. In all meadows, the lower limit has never been found deeper than the annual stormwave base, and
its depth (Zc) showed related to L0 according to the equation Zc¼0.32∙L0þ5.62. In the coastal tracts affected
by the least intense waves, the reduction of water movement with depth represents the most important
constraint to the seaward development of the meadow, whereas light availability plays a major role in
meadows affected by the most intense waves. The present study represents the first attempt at understanding
the role of hydrodynamic factors in setting the depth limit of seagrass meadows. If corroborated by
future research at other sites, this will have important implications for both basic and applied science, as it
would imply rethinking about the relative importance of water movement and light in seagrass depth
distribution, and could allow for a better estimate of the extent of meadow regression in anthropized areas.

Human use of ecosystem resources and services is increasing worldwide, generating pressures that alter
ecosystem structure, functioning and provision of services. Unexpected ecosystem change is becoming
frequent, and the complex ways through which multiple human pressures may interact leave conservation
practitioners and natural resource managers faced with high uncertainty. We developed a
geospatial approach for modeling the complex relationships between multiple human pressures and
coastal ecosystems status. This framework was then used to produce maps of the expected status of
marine coastal ecosystems resulting from variation in the cumulative human pressure. The geospatial
modeling approach we developed was tested on an emblematic study case requiring marine spatial planning,
i.e. a recently established marine protected area (MPA) that will have to coexist with the expansion
of a close commercial harbor. In the study case presented, our modeling approach was used to predict
the status of coastal ecosystems resulting from different management alternatives. Results showed that
should Port Authority support MPA in reducing human pressures in the area, coastal ecosystems would
not be expected to further deteriorate as a consequence of harbor expansion. Our approach proved effective
in modeling complex interaction among multiple pressures (e.g. synergisms) and predicting potential
future scenarios. The implementation of this approach into geographical information systems (GIS) allows
managers to represent the expected outcomes of their planned conservation efforts, thereby representing
an important decision-support tool for finding efficient management solutions in the face of complex
interactions and high uncertainty.

The video has been inserted in the “GEco” webpage.

GECO, an Innovation by SEAMap srl to sustain Environmental management

Side Scan Sonar (SSS) is a common method for mapping seagrass meadows that allows to define meadow extent and typology. Should sonogram interpretation be efficient, repeated mapping would allow evaluation of spatial meadow dynamics through time. Here, maps of Posidonia oceanica (L.) Delile meadows from western Liguria (NW Mediterranean Sea) produced in 1990, 1991 and 2006 through SSS surveys, were processed with GIS (Geographical Information System) and compared by means of diachronic analysis to establish concordances and discordances. While any difference between maps of 1990 and 2006 might provide information about change in seagrass cover with time, the availability of maps taken at only one year of distance (1991 vs 1990) offered the unique opportunity to evaluate the efficiency of SSS mapping, as the slow dynamics of P. oceanica makes significant change over such a short time scale unrealistic in absence of any large-scale environmental catastrophe. Results showed obvious differences in P. oceanica meadow distribution among the three periods in most of the areas. The largest differences were found between the maps of 1990 and 1991. This discrepancy was probably due to erroneous interpretation of small, fragmented shallow-water meadows on sonograms.

Errors in sonogram interpretation combine with inefficient positioning in old cartographies to blur the evaluation of change over large time scales (i.e. between 1990 and 2006). Extensive seatruthing based on a rigorous design is mandatory for efficient acoustic mapping of seagrass meadows.