Thematic maps

On the basis of its statutory aims of protecting and enhancing the natural features, which require a high level of knowledge of the marine area, the managing body of the MPA has over the last few years fielded various projects to achieve a complete, up-to-date and exhaustive mapping of the entire protected area, according to the extent identified by the founding Decree of 21/10/2009. This approach has also been incorporated into the monitoring plans, on the basis that the first step in developing an appropriate monitoring plan is to have an accurate map of the seabed and the main populations in the marine area.

The monitoring activity is strategic since the Secche della Meloria are important both from a nature point of view, with their distinctive morphology (e.g. with the distinctive “basins” formations) and their Mediterranean marine ecosystem, which is valuable in terms of its extent and diversity, and from a socio-economic point of view, as they host various human activities, from commercial and recreational fishing, to yachting, diving, environmental education and other socio-recreational activities, which mainly involve local communities.

Within this framework, the studies carried out were aimed at a detailed investigation and survey of the morphology of all the seabeds currently falling within the MPA, with the priority objective of describing as accurately as possible all their characteristics, supplementing and implementing knowledge already available.

In order to be able to implement effective protection and conservation actions in a marine reserve, it is essential to be aware of the populations present in the area, the relative configuration of the seabed and, above all, to be familiar with the biocoenoses present and their relative abundance. To achieve all these objectives, the first step is therefore to obtain a complete and detailed bionomic map of the area together with accurate bathymetric mapping.

The current size of the Secche Meloria MPA is approximately 92 square kilometres, which necessitates the use of indirect survey methods such as those now offered by the most advanced digital scanning technologies, including Side-Scan-Sonar and Multibeam. This approach made it possible to investigate the large area in question within a set timeframe, but has also required subsequent in-depth work to refine and digitally process the data and, most importantly, several spot checks by means of direct in-situ surveys, in order to unambiguously associate the signals obtained at an instrumental level (backscatter) with the actual biocoenosis and substrates present on the seabed. Numerous video and photo surveys were therefore conducted in all areas of the MPA. The hydrographic and geophysical surveys conducted within the area were aimed at the following main objectives:
– to capture the bathymetric patterns of the seabeds;
– to identify the biocoenoses present (including valuable biocoenoses such as Coralligenous and Posidonia oceanica) in order to quantify their economic value by identifying the ecosystem services they can provide to the local community;
– to provide an image of the seabed with a sufficiently high resolution to highlight sedimentological variations and sandy or pebbly areas not colonised by organisms.

The work summarised above was followed by rendering in georeferenced mapping of the detailed bathymetry of the entire stretch of sea included within the MPA (remember that this is a stretch of open sea stretching from over 3 nautical miles to 5 mn from the coast). This was followed by the processing and production of the bionomic chart, which is the final result of a series of activities structured and organised in successive stages:

  1. acquisition of morpho-bathymetric and backscatter data through appropriate surveys;
  2. ground verification and processing of the acquired data;
  3. sampling of plant species on the seabed and production of videos and images of the seabed at specific points identified following the analysis of the acquired backscatter data, with checks at sea at significant sampling points;
  4. final interpretation of all acquired data, both from multibeam and sampling, and production of the bionomic chart using appropriate GIS software.

The different phases of the study were divided into two main phases of morpho-bathymetric data collection and processing based on the various protection zones in the MPA. The first phase of the investigation was conducted from 2015 to 2016 and covered zones A and B3 (about 30% of the study); the second phase was completed in 2018–2019 and saw the acquisition and processing of data related to zones B1, B2 and C (about 70% of the study). All the data thus collected and processed resulted in the final production of the bionomic map, completed in 2020.

Survey area
The survey area was the entire extent of the Secche della Meloria MPA, as defined by the Ministry of the Environment Decree of 21/10/2009 establishing and zoning this marine protected area. The protected area is located off the coast of Livorno, and involves the municipalities of Livorno and Pisa.

The current extent of the MPA is approximately 5.4 Nm (10 km) in width in an east-west direction and approximately 5 Nm (9.2 km) in length in a north-south direction, with an approximately trapezoidal perimeter shape, ranging from approximately 2.15 to 7.5 Nm (4–14 km) from the coast of Livorno. This configuration covers an open stretch of sea of more than 91 km2.

 

Fig. 1 Secche della Meloria MPA survey area: areas A and B3 were mapped in the first survey phase

The survey also required a number of checks outside the exact boundaries of the MPA; therefore, in the end, a total area of approximately 95 km2 was covered, which also included a few deviations and the need for several stages of checks in certain places.
The survey also took into account data collected during an initial survey phase carried out in 2015–2016 within the current perimeters of zones A and B3.

 

Instrument set-up

The field work for the morpho-bathymetric survey was carried out using a specially equipped nautical vessel, adapted for the operational requirements. Since the working area has a bathymetry that varies from a few metres to around 50 m, it was decided to use a fast vessel with versatile and easy-to-use features; it therefore consisted of a dinghy with a rigid keel, with an overall length of 7 m and a maximum beam width of 2.54 m.

All the necessary instruments were installed on this vessel, and spaces for carrying out inspection and sampling dives were set up. To enable the installation of the multibeam and to use it in the most effective way, while ensuring maximum safety at sea, a special stainless steel support pole was built and attached to a bulwark of the craft with a movable system. The multibeam transceiver was fixed with brackets to one end of the pole, and the GPS antenna was placed at the other end. The device was set relative to the centre of gravity of the dinghy, as required for multibeam activation, and the following offset parameters were therefore entered: 0.09m from the starboard side, -0.85m from the bow vertical, -2.23m relative to the centre axis of the vessel (Fig. 2).

 

Fig. 2 Multibeam support structure (left) and installation diagram with offset parameters (right)

The pole, fixed in this way to a metal structure that also hooked onto the hull structure, supported the boom on which the survey instruments (multibeam and GPS) were installed; this boom protruded on the starboard side of the vessel and sat 1.65 m above sea level. The pole-arm structure was designed in such a way as to also allow rotation, through a lock/unlock pin, which also enabled it to move from a horizontal to a vertical position.

After removing the GPS antenna, the horizontal position is used during sailing so that the multibeam rests against the hull out of the water, thus allowing navigation even at high speeds without the risk of damage or loss of instruments. The vertical position, with the multibeam in the water and the GPS antenna installed, was used during all instrumental morpho-bathymetric survey operations (Fig. 3).

During the monitoring activities, the entire pole-arm structure system was also secured by two tensioned cables, also fixed to the edge of the dinghy’s bulwark, so as to keep it safe in the event of any shocks or accidents as well as to minimise vibrations and thus optimise the quality of the survey data.

At the end of the daily operations, the multibeam and GPS antenna were removed and stored on board in a secure container to avoid any damage either by accident or misuse.

 

Fig. 3. Horizontal position of the pole during sailing and vertical position during data acquisition

 

Due to its specific configuration, to acquire bathymetric data from the multibeam, the waves must not be higher than 50 to 80 cm. The sea surveys were therefore always carried out in good weather (mainly in the spring and summer months) and, moreover, always in dry, sunny weather, as the boat did not have a cabin, so the computers and electronic scientific instruments on board could not be exposed to the elements.

All field operations were carried out by specialist scientific personnel, who had already carried out surveys of this type, including Dr Antonio Petrizzo and Dr Aleksandra Kruss of CNR-ISMAR, assisted by CIBM’s scientific technical staff: Dr Carlo Ceccarelli, Dr Lorenzo Pacciardi, Dr Marco Pertusati and Dr Matteo Oliva.

Fig. 4. multibeam echosounder Norbit iWBMSh STX

The instrument used was an Echosounder Norbit iWBMSh STX multibeam (Fig. 4). This specific multibeam was chosen both for the quality of the data produced and for its compactness and lightness, essential factors for working easily on a dinghy and for enabling it to be installed and removed quickly if necessary.

The multibeam was integrated with an Applanix Ocean-Master inertial navigation system, supported by a pair of high-performance GNSS receivers, which enable precise georeferencing of data, providing information on latitude, longitude, altitude, roll, pitch, heading, heave and ensuring time synchronisation. This system is also compatible with Real Time Kinematic (RTK) correction, which guarantees centimetre-level accuracy of horizontal positioning.

The system was also accompanied by two probes for measuring the speed of sound, necessary for accurate electromagnetic beamforming. A sensor was integrated into the multibeam and captured data continuously.

A second SVP (Sound Velocity Profiler) probe, an AML Smart-X, provided sound velocity profiles throughout the water column and, due to its design, was always used when the vessel was stationary. The SVP probe was normally lowered at the beginning of the day and, if necessary, whenever the water temperature and salinity changed significantly (e.g. when the survey area changed or there was a significant change in depth or in the presence of river discharges, visible from the colour and composition of the water).

The speed of the vessel during the survey varied from about 4 knots, the minimum limit below which it is no longer possible to steer the boat properly, to a maximum of 7 knots, the maximum limit beyond which degradation in data quality begins.

Software

During the entire work process, it was necessary to use various pieces of software, some specific to the data acquisition phase, others specific to the data analysis and post-processing phase.
During the data acquisition phase, the following software was used:

  • QINSy (Quality Integrated Navigation System) by QPS (Quality Positioning Services): hydrographic data acquisition software for recording bathymetry, backscatter and navigation data.
  • Lefebure NTRIP (Network Transport of RTCM data over IP) client: enables real-time RTK correction for positioning by receiving data via the Internet from a special service that provides positioning data from a network of ground stations. The NetGEO service – the nationwide network of Permanent GNSS Stations set up by Geotop, which is equipped with all GPS+GLONASS receivers and is capable of providing real-time and post-processing services – was used.

For the processing of the acquired data, the following software was used:

  • QPS Qimera for processing bathymetric and backscatter data.
  • CARIS HIPS AND SIPS by Teledyne for processing bathymetric and backscatter data.
  • POSPac by Applanix for any post-processing of RTK data.

Data acquisition

The first steps required for data acquisition were the installation and calibration of the instruments, which in both phases of the survey took an average of four to five working days, depending on weather and sea conditions, as it is always necessary to carry out the appropriate checks at sea. Data acquisition took about three months in the first campaign and about six months in the second campaign.

The instrumentation did not suffer any significant malfunctions during any of the survey campaigns, apart from checking settings and resetting if necessary, especially during periods of prolonged use.

For example, in approximately 164 hours of data acquisition, 800 transects were carried out and approximately 30 km2, or 35% of the entire area, was mapped at a depth of between 1.5 and 20 m.

 

Fig. 5. Transects carried out during monitoring activities in 2018 (in green) and 2019 (in yellow); other areas (not shown) were surveyed in the first campaign in 2015–2016

Fig. 5. Transetti effettuati durante le attività di monitoraggio del 2018 (parte in verde) e del 2019 (parte in giallo), le altre aree (non indicate) sono state rilevate nella prima campagna 2015-2016

In general, the survey was always organised in such a way as to sail with the waves either at the bow or stern, in order to minimise roll and ensure the highest quality of the data acquired. When the weather conditions were most favourable, work was preferably carried out in the most offshore part of the MPA with the deepest bathymetry, whereas in the presence of wave motion (within the maximum allowed range of 80 cm wave height), surveys were carried out in the part further down the coast, which is the most sheltered from the wind and has the least swell.

The surveys were carried out with the multibeam with frequency set at 400 KHz and in “equidistant mode” with swath-angle set between 110° and 140° to ensure homogenous coverage and to optimise the quality of the data, also in relation to wave conditions.

During the monitoring activity, SVP profiles were also acquired along the water column and sampled at several points in the study area. These samples were necessary to obtain accurate sound velocity values for the correction of the multibeam data.

Bathymetry mapping
For the production of bathymetry mapping of the entire seabed of the MPA, the data collected during all the acquisition campaigns were analysed and processed using Caris HIPS & SIPS 10.4 software, following standard procedures involving the conversion of data, the application of tidal correction, the creation of surfaces, the cleaning of spurious data and the application of refraction coefficients to correct sound velocity errors. Where necessary, RTK positioning data corrections were applied using the software POSPac.

The data acquired in 2018–2019 were interfaced with the data from the first survey phase, which took place in 2015–2016, and resulted in the final bathymetry mapping of the entire MPA.

The new bathymetry has a decimetre-level scale of detail, and for greater ease/immediacy of reading, the various depth ranges have been shown in different colours, providing a highly representative image of the morphology of the seabed in the final graphics.