Detection and mapping of "blue carbon" reservoir storage in Posidonia oceanica dead matte in front of an extensive industrial area

The endemic Mediterranean seagrass Posidonia oceanica is the only marine phanerogam able to form matte, a biogenic structure representing important long-term “blue carbon” reservoir. Natural and/or anthropogenic factors can lead to the formation of a regressive structure, called “dead matte”, resulting from the disappearance of living shoots in the upper part of the matte. In the framework of TETIDE project (PON01_03112, www.progettotetide.it) a multiple sensors acoustic survey -multibeam sonar (MBS), side scan sonar (SSS), sub-bottom profiler (SBP) -was carried out in a portion of the Gulf of Augusta (Sicily, Southern Ionian Sea). The aim of investigation was to estimate the extent of dead mattethat are present in this area due to pollution from industrial activities over the last several decades. The investigated area covers a surface of about 141 ha from 3.5 m to 21.5 m of depth. MBS and SSS showed an irregular seabed morphology with rock, sand and dead mattestructures delimited by walls up to 2 m high. Dead mattecovers about 58 ha, corresponding to 41% of seabed. The seismo-acoustic data, recorded by SBP, allowed to estimate the thickness of the matteand then to calculate its volume per unit surface area of seabed. The volume of mattevaries from 0.83 m3/m2to 2.68 m3/m2with a mean value of 1.46 m3/m2. The total seabed surface affected by industrial activities concerns an area of about 3,000 ha. Considering the percentage of dead mattecoverage and its volume per unit surface area, a total volume of 18•106m3was estimated. From literature data, the amount of carbon storage inside the matteis 0.034 tons/m3on average, thus we extrapolated that total carbon accumulated is about 612 tons. These “blue carbon” reservoirs should be put into preserving in order to contrast their erosion and the consequent potentially massive CO2release into the atmosphere.